The string theory crackup, continued
Peter Woit
Anyone interested in what is happening in string theory
might want to take a look at a new preprint whose
co-authors include two prominent senior string theorists:
http://www.arxiv.org/ps/hep-th/0309170
Their title is "Is there a string theory landscape?" and
they begin with the statement "String Theory has always been
plagued by a plethora of solutions which do not describe
the real world" and go on to note that in recent years
new such solutions have appeared ("flux compactifications").
Some estimate that there are at least 10^1000 families of
such solutions. The authors go
on to note that some string theorists are now claiming that
string theory might somehow still be salvaged as a theory
with predictive value by invoking an anthropic principle.
Whatever mysterious process magically picks out the
solution we find ourselves in, it must be capable of
supporting life, and maybe solutions of this kind actually
predict something. The authors conclude that this is not
the case, that: "many features of anthropically selected flux
compactifications are likely to disagree with experiment".
The string theory community seems to be cracking up into
two pieces:
1. Those who argue that string theory basically
can't be used to ever predict anything, but since you
thus can't show it is wrong, you should believe
it anyway, invoking some deus ex machina that determines
everything that isn't explained by the standard model.
2. Those who argue "Never, never, never, never give up!"
(see David Gross's summary talk at Strings 2003), which
seems to mean "our colleagues in camp 1 aren't doing science
anymore, but even though we don't have a glimmer
of an idea about how to ever get a prediction out of string
theory, we have far too much invested in it to admit
defeat".
By the way, in case one has heard that string theory
is an incredibly sophisticated mathematical formalism
that requires years of mathematical training to follow,
this new preprint has few equations and high school math
is all that is needed to understand most of them, except
perhaps for an ODE or two that you might need a first-year
calculus class to understand. As with the rest of this
kind of literature, while the math is straightforward, the
physics is obscure beyond belief.
Robert C. Helling
<wo...@cpw.math.columbia.edu> wrote:
Let me put this a bit into perspective although I am sure there are
much more decisive and eloquent comments being written at Harvard and
maybe U Texas at the moment ;-)
> Their title is "Is there a string theory landscape?" and
> they begin with the statement "String Theory has always been
> plagued by a plethora of solutions which do not describe
> the real world"
Indeed, 'vacuum selection' is a problem of string theory, but this is
not new: At least at the perturbative level and semiclassically, there
is plethora of of supersymmetric ground states with a variety of low
energy excitation spectra. One is for example 10 or eleven dimensional
flat Minkowski space, one that is definitely not realized in our
universe.
If for example you restrict yourself to states that look 3+1
dimensional at low energies (and let's forget about branes at the
moment) even if you have no physical motivation to do that there are
many different ways of choosing the six dimensional (Calabi-Yau) or
seven dimensional (G2 holonomy) manifold that the compact dimensions
are curled up to. An in addition to a huge (possibly infinite) number
of topological (ie often discrete) choices you must make, these
manifolds have continuous parameters (like volumes and sizes of their
'geometrical features'). So there is an infinite number of
possibilities and only one can be realized in our world!
Even worse, none of them has the standard model as its low energy
theory! At least we don't know any. The main reason for that is our
world as you know is not supersymmetric at low energies at least. And
all the states I mentioned above are presumably supersymmetric. It's
not that there cannot be any non-supersymmetric states, but we don't
have the tools yet to analyze them reliably. But we are making
progress: By now, I would say there is a very good understanding of
theories with N=4 super-symmetry. The AdS/CFT correspondence has helped
a lot in that respect. N=2 susy theories are under good control as
well, for example using the work of Seiberg and Witten or various
brane realizations. While N=4 is very restrictive (no real quantum
corrections to many things), N=2 leaves much more choice when it comes
to quantum effects and matter couplings. Still, N=2 theories cannot
have chiral fermions and there are arguments that tell you that there
is no confinement in N=2.
Currently, there is a lot of work going on on N=1 theories. There is
some new understanding at least of holomorphic quantities like the
super-potential which allows you to understand the vacuum structure
(question everybody: What exactly (I mean physically) can we learn
from W without knowing the Kaehler potential?). These theories are
nearly realistic, they have both confinement and chiral fermions, it's
just that they are supersymmetric and thus not 100% realistic. But we
know how to obtain N=1 theories as sting theory ground states (we had
a thread about that here some months ago in which I made the claim
that the different approaches, M/G2, intersecting D6 branes, Het/CY
are all related by dualities, I still don't know whether that's true
in some strict sense, esp the case of Het/CY being M/G2).
But the important thing are the continuous parameters: As there are
parameters, there is an infinity of choices! (And along with parameters
come massless scalar fields that are not observed in nature).
But remember, we haven't broken susy, yet. It is believed that
breaking susy generically produces potentials for these scalars and
thus make nearly all of the vacua unstable. Unfortunately, these
potentials also generically tend to be exponentials so their minima
are at infinite values of the scalars corresponding to the extreme
corners of the space of compactification manifolds. But these studies
are very difficult and there might well be balancing effects (like
cosh is a sum of exponentials but has a minimum at finite parameter).
> and go on to note that in recent years
> new such solutions have appeared ("flux compactifications").
> Some estimate that there are at least 10^1000 families of
> such solutions.
You are misreading this bit: These flux compactifications are a step
forward: You can have p-form gauge fields in these theories and you
can have magnetic and electric fluxes on the cycles of your
compactification manifold. But these fluxes obey some Dirac type
quantization condition (like electric charges in the presence of
magnetic monopoles) meaning that certain integrals of the field
strength over certain submanifolds have to be integers.
This means that the size of those submanifolds can not be any real,
continuous number anymore but it has to be an integer (in the
appropriate units). Thus the infinity of choices has been replaced by
a discrete (and possibly finite, although large number) of choices.
> The authors go
> on to note that some string theorists are now claiming that
> string theory might somehow still be salvaged as a theory
> with predictive value by invoking an anthropic principle.
This brings us to the question of what we can expect from a
fundamental theory: I like to make the analogy to an engineering vs a
physics theory: An engineer can build bridges and machines and all
sorts of nice things. And he calculates using books of tables of
properties if his building blocks: Sizes of screws and stiffness of
steal for example. His theory is very practical but has many
parameters, namely all those numbers from his books. A physicist might
have a different view on a rod of steel: He sees that its made up of
of a collection of crystals of various metal atoms and if he is very
strong he might even be able to calculate the stiffness from his
knowledge of atomic properties and how the rod was made. His theory
will possibly contain many fewer parameters and he can predict the
numbers in the engineer's book. However, he will never be able to
predict the size of the screws from atomic properties, because in the
microscopic theory that will depend on the initial conditions of where
the atoms are and how many of the iron atoms in the world you use to
make the screw. If you like, the physicist has many solutions to his
microscopic equations corresponding to different screws. Which of
those is realized is a matter of choice.
So the more fundamental a theory gets, the more parameters are shifted
from being parameters of the theory to being parameters of the initial
condition or solution or state (these terms are interchangeable
here). This doesn't mean that the fundamental theory cannot be more
predictive that the engineering theory: For example, the fundamental
theory might tell you why its impossible to have very long very very
thins screws of steel that don't break. Even if you have a lot of
choice in your choice of state, the fundamental theory might predict
patterns in the macroscopic things that are possible. For example
(using lattice calculations) QCD is more or less able to predict the
meson and baryon masses. In the end, string theory might be able to
predict super-symmetry below a TeV and thus a 'light' Higgs or extra
dimensions or four macroscopic dimensions or three generations or
number of colors = number of generations (there are some models with
such predictions) but it is unlikely to be able to predict the number
of hairs on your or my head (although mine become fewer every day so I
have to assume to reach a a fixed-point at the boundary of number of
hairs moduli space in finite time).
However, this might not be acceptable for theory of everything, as
in the end there is only one universe and we cannot have different
runs with different initial conditions. So, you could argue that a TOE
should be able to predict the number of hairs and the sizes of all the
screws in the golden gate bridge. And I doubt that string theory will
ever be able to do that. Am I a string theory sceptic?
> Whatever mysterious process magically picks out the
> solution we find ourselves in, it must be capable of
> supporting life, and maybe solutions of this kind actually
> predict something. The authors conclude that this is not
> the case, that: "many features of anthropically selected flux
> compactifications are likely to disagree with experiment".
I'm not a fan of the anthropic principle but it might be true that the
condition that there should be someone possible to observe a
particular choice of initial conditions restricts the choice of
vacua. It's terribly hard to make this more precise as we only know
what should not happen if you want to have humans that are very
similar to us.
> 1. Those who argue that string theory basically
> can't be used to ever predict anything, but since you
> thus can't show it is wrong, you should believe
> it anyway, invoking some deus ex machina that determines
> everything that isn't explained by the standard model.
No, that's not true. As I argued above, this camp finds it likely that
you cannot predict everything. There might well be a lot of structure
or patterns to predict that are not yet predicted by standard model
physics for example.
> By the way, in case one has heard that string theory
> is an incredibly sophisticated mathematical formalism
> that requires years of mathematical training to follow,
> this new preprint has few equations and high school math
> is all that is needed to understand most of them, except
> perhaps for an ODE or two that you might need a first-year
> calculus class to understand. As with the rest of this
> kind of literature, while the math is straightforward, the
> physics is obscure beyond belief.
So, what's the problem? The papers of Banks and company recently have
been discussions at some meta-level that indeed do not contain much
maths (and some people might argue that the contend of substance is
similar) but, just as an example, the papers where the flux
compactifications are worked out (and you are supposed to have looked
at them to appreciate Banks et al's comments) contain some more
formulas. Again, you are confusing 'there are' and 'all': There are
papers in string theory that contain some deep and profound maths but
there are as well papers that are easily accessible as far as the
math is concerned.
Robert
--
.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oO
Robert C. Helling Department of Applied Mathematics and Theoretical Physics
University of Cambridge
print "Just another Phone: +44/1223/766870
stupid .sig\n"; http://www.aei-potsdam.mpg.de/~helling
Lubos Motl
Dear Peter,
> Anyone interested in what is happening in string theory
> might want to take a look at a new preprint whose
> co-authors include two prominent senior string theorists:
>
> http://www.arxiv.org/abs/hep-th/0309170
Tom Banks was my advisor. He is a very nice man and he is interested in
the most conceptual questions of string theory, especially those that are
related to cosmology. However I think that he would agree that it is not
fair to pick their paper with Michael Dine and Elie Gorbatov as a
description of "what is happening in string theory nowadays". Let us admit
that most string theorists think that most of his recent proposals are a
bit weird.
Even if he disagreed, it is just a matter of fact that there are many
other, usually more specific directions that string theorists study
nowadays, and they are also likely to get more citations.
It is true that some recent results and proposals re-opened the question
of the anthropic principle in theoretical physics. This is a question that
always divided the scientific community into two subcommunities - it is
not just string theory community that is affected.
String theory is a democratic community of independent thinkers, so one
should not expect that everyone will agree about everything, especially if
the question is about the amount of things that we will know about the
world in the far future.
We don't know for sure whether we will ever be able to predict certain
properties of the Universe, or whether they are gonna remain an
unexplainable result of historical coincidences in the evolution of the
Universe (or multiverse) or even a result of some random divine choices.
I believe that we will have a reasonable chance to answer all these
questions, as we did with all others in the past, if we assume that these
questions may be answered, and together with my colleagues we renamed the
anthropic principle to the "anthropic lack of principles". But it is just
not guaranteed that we will succeed and everyone realizes that.
By the way, Tom Banks and Mike Dine (I probably don't know Elie Gorbatov)
are also not the biggest proponents of the anthropic principle. In fact,
our older paper
http://arxiv.org/abs/hep-th/0007206
may be viewed as criticism of the anthropic principle in general, and I
know very well what Tom always used to think about these general matters.
> 2. Those who argue "Never, never, never, never give up!"
> (see David Gross's summary talk at Strings 2003),
Yes, I certainly belong to this "Churchill's" category, and Gross's
summary talk was pretty kewl. One can show that string theory is not a
system of dictators: even David Gross, Edward Witten, Brian Greene and
others that belong to the "predictive camp" do not have the privilige to
spank Lenny Susskind, for example. ;-) (Unfortunately, I would say.)
> which seems to mean "our colleagues in camp 1 aren't doing science
> anymore,
Yes, I think that assuming that we will never be able to predict one more
observable number from our theories implies that we are mostly giving up
further progress in science, and in my opinion it should mean that we
should not be paid for it. However I don't believe that we have reasons
to act so at this moment. Even if the number of very (meta)stable vacua is
10^1000, and I have doubts about this claim as well, there can still be a
single cosmology; a principle that tells us which vacua are preferred
statisticially or otherwise; until we learn everything, we will always
have ways, at least in principle, to learn more about our Universe.
I emphasize that some people in the field, for example Lenny Susskind,
would strongly disagree. The story was covered in the New York Times, 2nd
of September 2003 or so.
> but even though we don't have a glimmer of an idea about how to ever
> get a prediction out of string theory, we have far too much invested
> in it to admit defeat".
String theory has given a lot of predictions and possible scenarios for
physics beyond the Standard Model, and now we are - while working - also
waiting for the experimentalists to be able to observe at least one new
phenomenon that goes beyond the Standard Model and General Relativity.
It's just not happening, and there are well-known technical limitations
that cause this limited progress. Theorists are simply ahead a lot, and
without the experiments they can never be sure which answer is relevant
for our Universe.
> By the way, in case one has heard that string theory
> is an incredibly sophisticated mathematical formalism
> that requires years of mathematical training to follow,
> this new preprint has few equations and high school math
Because it is not really a paper of state-of-the-art string theory. Its
simplicity - and its independence of the calculational framework of string
theory - may make it important, but it is more likely that it makes it
less relevant. If you need to see some papers that you have no chance to
follow, I can give you a few hundreds of references. ;-)
Best wishes
Lubos
______________________________________________________________________________
E-mail: lu...@matfyz.cz fax: +1-617/496-0110 Web: http://lumo.matfyz.cz/
phone: work: +1-617/496-8199 home: +1-617/868-4487
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Superstring/M-theory is the language in which God wrote the world.
Peter Woit
>Indeed, 'vacuum selection' is a problem of string theory, but this is
>not new: At least at the perturbative level and semiclassically, there
>is plethora of of supersymmetric ground states with a variety of low
>energy excitation spectra.
The problem is not just at the perturbative or semi-classical level.
To quote the second sentence of the Banks et. al. preprint
"Early hopes that some of the highly supersymmetric moduli
spaces of solutions would prove inconsistent once non-perturbative
physics was understood, were dashed in a definitive manner by
the second superstring revolution."
>>and go on to note that in recent years
>>new such solutions have appeared ("flux compactifications").
>>Some estimate that there are at least 10^1000 families of
>>such solutions.
>You are misreading this bit: These flux compactifications are a step
>forward: You can have p-form gauge fields in these theories and you
>can have magnetic and electric fluxes on the cycles of your
>compactification manifold. But these fluxes obey some Dirac type
>quantization condition (like electric charges in the presence of
>magnetic monopoles) meaning that certain integrals of the field
>strength over certain submanifolds have to be integers.
>
>This means that the size of those submanifolds can not be any real,
>continuous number anymore but it has to be an integer (in the
>appropriate units). Thus the infinity of choices has been replaced by
>a discrete (and possibly finite, although large number) of choices.
I'm certainly not an expert on flux compactifications, so correct
if I'm wrong, but it seems to that the situation is as follows:
Without flux compactifications you have a possibly infinite (no
one knows whether the number of Calabi-Yaus is finite or
not) set of families of solutions, of varying and often large
dimension. String theorists like to believe that once
they figure out how to break supersymmetry then the
degeneracy in each family will be lifted, but they don't have
a viable supersymmetry breaking mechanism for doing this.
Adding in fluxes adds a discrete set of choices for each
family of Calabi-Yaus. The argument is that some of these
fluxes will give a potential for some of the moduli, possibly
for all of them. However, many choices of fluxes will
at best break part of the degeneracy, no? So now you just
have created new families of solutions, besides the ones with
no fluxes.
>>So the more fundamental a theory gets, the more parameters are shifted
>>from being parameters of the theory to being parameters of the initial
>>condition or solution or state (these terms are interchangeable
>>here).
I don't understand this argument at all. Before QED, in classical EM,
electrons could have any magnetic moment at all and one could have
argued that the magnetic moment of the electron was determined by the
initial state of the universe. With a more fundamental theory (QED),
you can calculate this magnetic moment to high accuracy. More
fundamental theories explain more and allow you to calculate and
predict things which are undetermined in less fundamental theories.
One's theory is supposed to include the information of what it can and
can't predict. The fact that string theory doesn't do this comes from
the fact that it isn't a theory, it is just a set of (increasingly
implausible) hopes that a theory of a certain kind exists.
>>By the way, in case one has heard that string theory
>>is an incredibly sophisticated mathematical formalism
>>that requires years of mathematical training to follow,
>>this new preprint has few equations and high school math
>>is all that is needed to understand most of them, except
>>perhaps for an ODE or two that you might need a first-year
>>calculus class to understand. As with the rest of this
>>kind of literature, while the math is straightforward, the
>>physics is obscure beyond belief.
>So, what's the problem? The papers of Banks and company recently have
>been discussions at some meta-level that indeed do not contain much
>maths (and some people might argue that the contend of substance is
>similar) but, just as an example, the papers where the flux
>compactifications are worked out (and you are supposed to have looked
>at them to appreciate Banks et al's comments) contain some more
>formulas. Again, you are confusing 'there are' and 'all': There are
>papers in string theory that contain some deep and profound maths but
>there are as well papers that are easily accessible as far as the
>math is concerned.
The standard reaction of most people I talk to in the physics
community is that at this point string theory is "not physics, but
math". I'm just pointing out that much of what is going on in string
theory these days is not math.
Serenus Zeitblom
> Let us admit that most string theorists think that most of his
> recent proposals are a bit weird.
Maybe. But actually this *particular* paper is concerned with
debunking an infinitely *weirder* paper by El Susskind. So this paper
is relatively mainstream. :)
> Even if he disagreed, it is just a matter of fact that there are many
> other, usually more specific directions that string theorists study
> nowadays, and they are also likely to get more citations.
Well, the idea that number of citations tells you very much about
a paper is ...ummm... a bit weird. I have seen lots of truly
excellent papers that have garnered few
http://arxiv.org/cits/hep-th/0304220 or *no*
http://arxiv.org/abs/hep-th/0308088 citations at all.
In fact, it seems to be getting really hard to get citations
these days: even the most determined efforts to launch a new
bandwagon, http://arxiv.org/abs/hep-th/0208048, seem to fall flat
these days. So I would not attach much importance to these data
if I were you.
> even David Gross, Edward Witten, Brian Greene and others that belong
> to the "predictive camp" do not have the privilige to spank Lenny
> Susskind, for example. ;-) (Unfortunately, I would say.)
"When I hear of "Poincare Recurrences", I reach for my gun."
> String theory has given a lot of predictions and possible scenarios for
> physics beyond the Standard Model, and now we are - while working - also
> waiting for the experimentalists to be able to observe at least one new
> phenomenon that goes beyond the Standard Model and General Relativity.
> It's just not happening,
I think that this is the correct answer to Peter Woit. *Any* theory
that he would have us work on would have to go without data ---
because there is [almost---see below] none.....
According to his website, Woit recommends that "Leading String
Theorists" should declare defeat and encourage work on other things
[such as?...] Of course, what Woit is recommending here is a way of
doing science that has never been tried before --- namely, he wants
"Leading String Theorists" to steer the subject in a particular
direction [in this case, off a cliff...]. Forgetting about the
practicality of this, I wonder whether he has a valid point in
general. I mean, should "celebrities" feel that their celebrity
imposes on them an obligation to work on particular problems, rather
than merely following their immediate interests like the rest of us?
For example, everyone knows that the most important observational
datum we have now is the dark energy. Should the celebrities feel
obliged to work on getting string theory to say something concrete
about this situation--- or should they feel free to re-arrange the
deck chairs on the Titanic, http://arxiv.org/abs/hep-th/0307195 ?? :)
Lubos Motl
Dear Robert,
it might be more constructive if we try to discuss these questions with
one another - and not just with the colleagues who want to prove string
theory irrelevant. ;-) I think that I agree with most of your post,
Robert.
> Indeed, 'vacuum selection' is a problem of string theory, but this is
> not new: At least at the perturbative level and semiclassically, there
> is plethora of of supersymmetric ground states with a variety of low
> energy excitation spectra. One is for example 10 or eleven dimensional
> flat Minkowski space, one that is definitely not realized in our
> universe.
I totally agree. It's almost guaranteed that we will have to admit the
existence of some vacua - at least the supersymmetric ones (in 4+
dimensions) - that do not describe our Universe. There has always been a
large number of string "theories" in the 1980s, and we learned in the
1990s that they are really different vacuum states of the same theory, and
moreover many of them are identified (or at least connected) by dualities.
The new thing are these super-astronomical ensembles of the possible
vacua. In my opinion, this whole business was started by Bousso and
Polchinski in their attempt to explain the smallness of the cosmological
constant; a single number - a big failure of physics - whose explanation
still might be very different. Various other people - including me with
Tom Banks and Mike Dine - constructed alternative procedures to generate
large ensembles of stringy vacua. The most interesting recent one was KKLT
(Kachru et al.) that got very close to the proof that their vacua should,
in fact, exist.
There might be still many loopholes. They assume that the Calabi-Yau
spaces with certain properties exist. Allan Adams explained me that it is
not guaranteed because no one has ever found a particular Calabi-Yau space
with the required properties. Another loophole might be some extra
channels for their decay. A newer paper from Santa Barbara found a faster
decay channel of the KKLT vacua, and I can imagine that a more detailed
analysis will reveal that these vacua are unstable. They have so many
fellow vacua to decay to and so many ways to do so and so many other
possible relations are unknown that I simply don't believe too much that
there can be a large number of isolated and similar almost-stable vacua.
Even if they are highly stable (metastable, more precisely), my prejudice
is that these are not the right vacua that should be studied as the
real-world candidates, simply because they are too artificial, and
unnaturally complicated (i.e. fine-tuned, in some sense), and they should
be disfavored by the quantum cosmological considerations that will be
hopefully found one day. There are simpler, more fundamental vacua -
Horava-Witten theory on a simple enough Calabi-Yau 3-fold, for example -
and I believe that the ultimate understanding of string theory will
explain why should we look at these "simplest" vacua in order to explain
all remaining puzzles about the fundamental physics of the real world!
What do you think about it?
> of topological (ie often discrete) choices you must make, these
> manifolds have continuous parameters (like volumes and sizes of their
> 'geometrical features'). So there is an infinite number of
> possibilities and only one can be realized in our world!
These continuous parameters are not real parameters in string theory; they
are rather scalar fields. And if their potential is exactly flat, they can
take different values at different points in the Universe (which would
most likely involve a time-evolution of the fine-structure constant, a
highly unlikely possibility), and they also imply the existence of extra
long-range forces. It seems pretty clear experimentally that these effects
don't exist, and therefore there exist no exact moduli in the real world.
After supersymmetry is broken, I think it is reasonable to expect, even on
theoretical grounds, a potential generated for all the scalar fields,
which removes all the moduli and implies that there are no arbitrary
continuous dimensionless parameters in string theory!
We don't quite know what is the structure of the full "configuration" or
"moduli" space or whatever generalizes it in string theory, and we can't
compute the potentials well and exactly enough, especially not after the
supersymmetry breaking. But once we are able to do that, I think that we
will see clearly that the dimensionless parameters just can't survive and
the predictive power of string theory should be unlimited.
> Currently, there is a lot of work going on on N=1 theories. There is
> some new understanding at least of holomorphic quantities like the
> super-potential which allows you to understand the vacuum structure
> (question everybody: What exactly (I mean physically) can we learn
> from W without knowing the Kaehler potential?).
I am pretty skeptical about that, too. It is just a subset of quantities
that is probably insufficient to answer a particular physical question,
such as what is the muon mass. Nevertheless, my amateurish guess is that
these matrix model methods might be useful to gain some insights about the
SUSY breaking.
> So the more fundamental a theory gets, the more parameters are shifted
> from being parameters of the theory to being parameters of the initial
> condition or solution or state (these terms are interchangeable
> here).
Well, we probably don't want to explain the precise radii of the planets'
orbits anymore. Kepler did it using Platonic polyhedra, and it took some
time before he realized what are the really physical numbers that deserve
an explanation. Nevertheless there are other numbers that should be
explained - the elementary particle mass matrices and the couplings of the
forces. Do you agree that they seem to be too constant to be immediately
labeled as historical coincidences? Maybe they can't be explained, but I
just don't believe that we should ever give up.
> > 1. Those who argue that string theory basically
> > can't be used to ever predict anything, but since you
> > thus can't show it is wrong, you should believe
> > it anyway, invoking some deus ex machina that determines
> > everything that isn't explained by the standard model.
>
> No, that's not true. As I argued above, this camp finds it likely that
> you cannot predict everything. There might well be a lot of structure
> or patterns to predict that are not yet predicted by standard model
> physics for example.
I more or less agree with Peter Woit's description of the two camps. This
camp #1 is essentially saying that there can't be any new reliable
quantitative predictions that go beyond the Standard Model - and they
often argue that it is enough that we understand the black hole
information puzzles etc. I totally disagree with their viewpoint. No new
explained quantitative data essentially means no solid progress in
physics, and even the explanation of the black hole puzzles must be
labeled as unphysical if we can't really show that our theory matches the
real world! We would really transform physics into philosophy (or a
religion) if the whole mathematical machinery were useless for actual
predictions - no quantitative results could be trusted - and only some
general philosophical conclusions should be extracted. This is not what I
call physics. Sorry guys.
I also agree with his description of our camp #2. String theory has the
power to explain all known phenomena - gauge fields, Higgs mechanism,
families of particles, gravity, black hole thermodynamics etc. - which
includes zillions of numbers. Its rigidity has been proved in many
situations. The whole ideology about the limited predictive power of
string theory results from attempts to solve the cosmological constant
problem - a problem related to a single number that we really don't
understand well - and I just find it unreasonable to discard all the
promises of a mathematically charming and powerful theory because of a
single stupid number whose understanding in 2100 might be explained in one
footnote of a high school physics textbook.
Aaron Bergman
In article <bkqdb4$md8$1...@newsmaster.cc.columbia.edu>,
Peter Woit <wo...@cpw.math.columbia.edu> wrote:
> However, many choices of fluxes will
> at best break part of the degeneracy, no? So now you just
> have created new families of solutions, besides the ones with
> no fluxes.
The whole point of flux compactifications is that they remove all the
moduli. You now have isolated vacua. You might even have a whole lot of
isolated vacua. This is in addition to vacua that do have moduli.
[...]
>
> The standard reaction of most people I talk to in the physics
> community is that at this point string theory is "not physics, but
> math". I'm just pointing out that much of what is going on in string
> theory these days is not math.
That should make you happy. They're talking about physics, not math. The
mathematics in string theory (and the mathematical tendencies of its
practitioners) has always run the spectrum from extraordinarily mathy to
no more math than you need to do ordinary perturbative QFT.
Aaron
Lubos Motl
Hi Serenus!
> Maybe. But actually this *particular* paper is concerned with
> debunking an infinitely *weirder* paper by El Susskind. So this paper
> is relatively mainstream. :)
Yes, I think it is a fair description! ;-) Tom Banks's interests were
always broad and he always knew why various wrong things were wrong. And
if he is saying something that does not sound too convincing to others, we
might still be doing some error!
> excellent papers that have garnered few
> http://arxiv.org/cits/hep-th/0304220 or *no*
> http://arxiv.org/abs/hep-th/0308088 citations at all.
Good advertisement! ;-) They can still become hot. May I add some papers
that should get 50 times more citations than they have? :-)
http://arxiv.org/abs/hep-th/0304080
http://arxiv.org/abs/hep-th/0306051
> In fact, it seems to be getting really hard to get citations
> these days: even the most determined efforts to launch a new
> bandwagon, http://arxiv.org/abs/hep-th/0208048, seem to fall flat
> these days.
Well, if you call 157 citations "flat", you're free to do it. I personally
think it is an appropriate number, and according to SPIRES this number
falls into the "famous" category.
> So I would not attach much importance to these data if I were you.
I don't attach too much importance, but nevertheless I belong to the
family of the people who think that citations are a better, albeit
imperfect, criterion for estimating the importance of research in science
than the recommendation letters and the number of publications in
journals.
> > even David Gross, Edward Witten, Brian Greene and others that belong
> > to the "predictive camp" do not have the privilige to spank Lenny
> > Susskind, for example. ;-) (Unfortunately, I would say.)
>
> "When I hear of "Poincare Recurrences", I reach for my gun."
That's great. But don't forget that exp(S) years ago, another Serenus was
forced to reach for his gun, too. Why are you repeating yourself so often? :-)
> I think that this is the correct answer to Peter Woit. *Any* theory
> that he would have us work on would have to go without data ---
> because there is [almost---see below] none.....
Thanks. I almost thought that I would be a lonely guy who made this
observation. ;-)
> According to his website, Woit recommends that "Leading String
> Theorists" should declare defeat and encourage work on other things
Well, a good strategy to improve the direction of the research. This
strategy has been successfully used by the Catholic Church in the Middle
Ages. Maybe it will work for us this time, too. String theory may be
identified as a society directed by a few dictators - who are themselves
directed by someone even higher in the hierarchy who keeps calculating the
best path to the truth from his office in Columbia University.
> imposes on them an obligation to work on particular problems, rather
> than merely following their immediate interests like the rest of us?
That's a very deep philosophical question. Do the leaders carry any
responsibility for a balanced development of the field? Although I prefer
freedom (especially the academic freedom) - even freedom for celebrities -
it is hard to hide that in some particular cases, it would feel much
better if the celebrities did not work on problems XY but rather AB, for
example. Let me add no details about the nature of XY. Nevertheless, in
the long run, it is obvious that tenured professors simply do work (and
can work) on the problems that they find relevant. And if a problem is
understudied, it is clear that eventually someone starts to work on it,
because it may be a relatively fast way to obtain very new results.
> about this situation--- or should they feel free to re-arrange the
> deck chairs on the Titanic, http://arxiv.org/abs/hep-th/0307195 ?? :)
That's an XY interesting paper, by the way! ;-)
Serenus Zeitblom
> that should get 50 times more citations than they have? :-)
>
> http://arxiv.org/abs/hep-th/0304080
> http://arxiv.org/abs/hep-th/0306051
Ummm...50 times zero? :)
> > In fact, it seems to be getting really hard to get citations
> > these days: even the most determined efforts to launch a new
> > bandwagon, http://arxiv.org/abs/hep-th/0208048, seem to fall flat
> > these days.
>
> Well, if you call 157 citations "flat", you're free to do it. I personally
> think it is an appropriate number, and according to SPIRES this number
> falls into the "famous" category.
But this paper will still be forgotten very soon.
Think how many citations the BMN paper had after a year! *THAT* was
expert bandwagon construction. Since then, the bandwagon factory
seems to have shut down....really, things are very slow these days!
What is going on? Where is Ed Witten when we need him? What is
Harvard doing?!
We should be grateful to Tom Banks and even --- yes! --- to El
Susskind for at least writing about things like deSitter space,
supersymmetry breaking and what not ---
things that we all want to know about!
> >
> > "When I hear of "Poincare Recurrences", I reach for my gun."
>
> That's great. But don't forget that exp(S) years ago, another Serenus was
> forced to reach for his gun, too. Why are you repeating yourself so often?
:-)
Adrian *told* me that people would laugh at
me for a hundred years for putting on that
stupid uniform....
>
> > imposes on them an obligation to work on particular problems, rather
> > than merely following their immediate interests like the rest of us?
>
> That's a very deep philosophical question. Do the leaders carry any
> responsibility for a balanced development of the field? Although I prefer
> freedom (especially the academic freedom) - even freedom for celebrities -
> it is hard to hide that in some particular cases, it would feel much
> better if the celebrities did not work on problems XY but rather AB, for
> example. Let me add no details about the nature of XY. Nevertheless, in
> the long run, it is obvious that tenured professors simply do work (and
> can work) on the problems that they find relevant. And if a problem is
> understudied, it is clear that eventually someone starts to work on it,
> because it may be a relatively fast way to obtain very new results.
Well, to be frank I would rather see the celebrities putting out
relatively lame papers about important things than brilliant
technical papers about totally boring things --- which seems to
be what is happening right now. It might be embarrassing for
them --- but there is a price to pay for being allowed to do
great work sometimes, as my old friend Leverkuhn used to say.....
http://arxiv.org/abs/hep-th/0307195 ?? :)
>
> That's an XY interesting paper, by the way! ;-)
XY definitely, but *interesting*??!!
Lubos Motl
> > http://arxiv.org/abs/hep-th/0304080
> > http://arxiv.org/abs/hep-th/0306051
> Ummm...50 times zero? :)
Nope! 50 times eight or one, respectively. ;-)
> But this paper will still be forgotten very soon.
> Think how many citations the BMN paper had after a year!
It is very possible - and many of us think so - that Cumrun Vafa is a
bigger genius than any of the other (even renowned) physicists including
those mentioned in your last line, and Robbert Dijkgraaf is the Spinoza
prize (Dutch Nobel prize) winner for 2003. Their papers from the last year
or so are rather interesting; they have led to a new subfield, but it did
not offer as many applications and ramifications as the BMN paper (2002,
long time ago, right?) and one can imagine that it is a less conceptual
progress than the BMN paper. Does the DV business really offer more than a
technical tool to calculate a small subset of (holomorphic) quantities in
a limited class of field theories? Although the authors are certainly
string theorists, do their results really imply something new for string
theory, or is it another example how string theory gives us ideas to
understand field theories?
> *THAT* was expert bandwagon construction. Since then, the bandwagon
> factory seems to have shut down....really, things are very slow these
> days!
This is a conspiratory interpretation of reality. ;-) If one neglects the
statistical fluctuations and human stupidity, it should still be believed
that the papers get as many citations as they deserve, and we just saw a
smaller number of super papers in 2003 than in other years. So far.
> What is going on?
We simply live in an era which does not offer too many breakthroughs
in theoretical physics. It might get better; it might get worse. If
the periodicity of the system is ten years, you should expect a new
revolution next year. ;-) Well, I would be happy if this sinusoidal
extrapolation were correct!
> Where is Ed Witten when we need him?
I disagree with this sort of cheap attack. Edward Witten has done more
good things for theoretical physics than 25 other physicists
altogether. And he has a very realistic understanding of reality in
physics - which nowadays means to be a bit sceptical. I personally
appreciate that Edward Witten does not jump at c=1 old matrix models,
for example. The old matrix models became a very fashionable topic
again, although there are no radically new insights, and therefore the
people study essentially the same problems like 10 years ago - a
period of the biggest frustration in string theory, I would say.
Edward Witten never forgot about the real goals of string theory - which
is NOT to describe unphysical toy models in 2D that are equivalent to free
fermions :-). He recently spent quite a lot of time with phenomenology
(proton decay in various scenarios, for example).
You know, perhaps he and others discovered all the things that could
have been discovered too quickly, and therefore we are thirsty for
truly exciting results in 2003. Maybe we were just too smart ;-) and
it is unhealthy to be too much ahead of the experimentalists.
> What is Harvard doing?!
Cumrun Vafa co-initiated the semi-bandwagon that even you mentioned. Many
people, led by Andy Strominger, are working on 2D string theory / old
matrix models which is another semi-hot topic. I could explain you why the
things that I am doing are important, but let me be more modest here. ;-)
OK, I think that we should understand many open questions about Matrix
theory and other pillars of the late 2nd revolution, for example open the
files at
http://arxiv.org/abs/hep-th/0309238
Maybe we need someone from outside the well-known places to make a really
qualitative progress.
> We should be grateful to Tom Banks and even --- yes! --- to El
> Susskind for at least writing about things like deSitter space,
> supersymmetry breaking and what not --- things that we all want to
> know about!
Let me simplify a bit: I personally don't. I don't believe that there is
something truly deep and "big" to know about de Sitter in string theory.
De Sitter space always has some degree of unpredictability, because of the
thermal radiation coming from the horizon, and I find it likely that there
is no exact mathematical formalism that describes quantum gravity in
finite space. The "stringiness" or "correctness" of a theory only appears
in the limit where the number of degrees of freedom goes to infinity, and
de Sitter can probably only be thought of as an excitation of something
infinite (flat space or AdS). In fact, this is what TB & LS more or less
also believe today, I think.
The whole interest in quantum cosmology in string theory was
premature, it seems. In my opinion, no solid results - results that
could be at least remotely compared to string dualities - have been
found, and the whole subfield was/is mainly motivated by a single
observed number - the cosmological constant. In this sense, the
attempt to make a contact with experiments whatever it costs was
counter-productive. The people in the 19th century should not have
tried to explain the expansion of the Universe with their Newton's
laws (they should have waited for GR) - and in the same sense, we
should not try to apply our limited formalism of current string theory
to the supercosmological problems. We just don't know how to do it
properly.
String theory can predict physics of billions of numbers in particle
physics in agreement (at least qualitative agreement) with the real
world, and these are the questions that we do understand. The
cosmological constant is just one number, whose origin we don't quite
understand, and I find it ridiculous to derive thousands of
outrageous, anthropic and other results about the things that we're
supposed to know very well (for example about the ability to calculate
particle masses) from a single number that we moreover don't
understand.
There is some more important business to do which is not directly related
to cosmology, and once we understand e.g. the origin of E_k U-duality in
M-theory and many other things, we might gain tools to study other issues
including the cosmological questions. But I don't believe that a single
number - Lambda - can be a good guide that helps us to find the whole
structure of M-theory.
> Well, to be frank I would rather see the celebrities putting out
> relatively lame papers about important things than brilliant
> technical papers about totally boring things --- which seems to
> be what is happening right now.
If I understand what you really intend ;-), I agree with you in this
general remark! Most of the leaders are very technically oriented and
they have high technical standards, and they would not be happy to do
what you suggest - and perhaps, they would not even be good at it.
> XY definitely, but *interesting*??!!
XY-interesting is something very different from interesting, and this
adjective has two parts: the information and the polite phrase. I think
you got the point.
Best wishes
Lubos
______________________________________________________________________________
E-mail: lu...@matfyz.cz fax: +1-617/496-0110 Web: http://lumo.matfyz.cz/
phone: work: +1-617/496-8199 home: +1-617/868-4487
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Superstring/M-theory is the language in which God wrote the world.
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From: Jason <pri...@excite.com>
Newsgroups: sci.physics.research
Subject: Superspace is a noncommutative space?
Date: Thu, 02 Oct 2003 12:47:16 -0400
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Status: OR
I was trying to explain superspace as a noncommutative space. I can
define a seminormed * algebra as follows: Let lambda_m be the Grassmann
unital *-algebra generated by m anticommuting elements. Then, if M is
an n-dimensional manifold, we can have a lambda_m vector bundle over M
and the *-algebra would be the sections of this bundle going to zero at
infinity. Unfortunately, if we define the seminorm as the supremum of
the "body" of the section, we only get a seminorm, so this doesn't
qualify as a C* algebra, but do noncommutative spaces necessarily have
to be C* algebras? Supersymmetry would then be a Hopf algebra acting
upon this superspace.
Serenus Zeitblom
>Though failing to cite himself, it was secretly Serenus Zeitblom who
>wrote:
> > Where is Ed Witten when we need him?
> I disagree with this sort of cheap attack. Edward Witten has done
> more good things for theoretical physics than 25 other physicists
> altogether.
Sorry, you misunderstood my intention completely. I really meant that
we *do* need Ed Witten right now! Apart from his direct physics
achievements, EW has revolutionized the way physics papers are
written----his 1998 paper on AdSCFT was among the best-written and
clearest physics papers of all time, and really made it clear why we
should care about Maldacena's paper. The situation now seems very
murky and unclear, and we need EW to tell us what the hell we should
all be working on! So it was far from a "cheap attack" on EW, just the
reverse!
> And he has a very realistic understanding of reality in physics -
> which nowadays means to be a bit sceptical. I personally appreciate
> that Edward Witten does not jump at c=1 old matrix models, for
> example.
I agree 100%, and this is what I was thinking of when I said we need
him!
> OK, I think that we should understand many open questions about Matrix
> theory and other pillars of the late 2nd revolution, for example open the
> files at
>
> http://arxiv.org/abs/hep-th/0309238
OK, I'll bite. :)
*Why* is it important to understand those things?
> I don't believe that there is something truly deep and "big" to know
> about de Sitter in string theory.
What an awful thing to say! I sure hope you are wrong!
> De Sitter space always has some degree of unpredictability, because
> of the thermal radiation coming from the horizon, and I find it
> likely that there is no exact mathematical formalism that describes
> quantum gravity in finite space. The "stringiness" or "correctness"
> of a theory only appears in the limit where the number of degrees of
> freedom goes to infinity, and de Sitter can probably only be thought
> of as an excitation of something infinite (flat space or AdS). In
> fact, this is what TB & LS more or less also believe today, I think.
Well, as you have said yourself, there is a vast difference between
understanding something in principle and understanding it in detail.
Trying to get deSitter out of string theory may be too hard to do in
detail ---- certainly, attempts to get exactly the right value for
the Constant are way too ambitious, and that has led into all this
crackpot anthropic stuff.
BUT on the other hand, it is just not good enough to wave our hands
and say that deS will "somehow" come out of AdS as an excitation. How
the heck is that supposed to happen when they are so utterly
different? I think it is still very much worthwhile to make sure that
we REALLY understand as much as we can about quantum gravity in deS,
even if we only have a programmatic idea about how to link it with
string theory. I mean, what if Juan M tries again and really proves a
no-go theorem saying that you just cannot get a positive Constant out
of string theory? Then the game is over! At least we need people to
reassure us that this is not necessarily true...and who knows,
somebody might find something about deS that will come in handy later.
> The whole interest in quantum cosmology in string theory was
> premature, it seems. In my opinion, no solid results - results that
> could be at least remotely compared to string dualities - have been
> found, and the whole subfield was/is mainly motivated by a single
> observed number - the cosmological constant.
You keep talking about the Constant as ONE number, as if that
proved that it is not worth worrying about. Have you heard the
famous Italian story about the father who, upon hearing that
his 15-year-old daughter was pregnant, consoled himself with
the thought that, well, she is only a TINY BIT pregnant at
the moment...... :) ? This one lousy little number --- and it's
so tiny, isn't it? ---- could easily ruin everything if
we don't watch out....[It's ok if it ruins the Universe, but
it would be worse if it ruins string theory. :)]
> In this sense, the attempt to make a contact with experiments
> whatever it costs was counter-productive. The people in the 19th
> century should not have tried to explain the expansion of the
> Universe with their Newton's laws (they should have waited for GR) -
> and in the same sense, we should not try to apply our limited
> formalism of current string theory to the supercosmological
> problems. We just don't know how to do it properly.
Well, I never thought I would see *you* being a pessimist about what
string theory can do! I am more of an optimist---I believe that we
would be in better shape re the Constant if more incredibly smart
people like CV [I'm not being sarcastic] and RD worked on dS instead
of the stuff they are doing now.
> String theory can predict physics of billions of numbers in particle
> physics in agreement (at least qualitative agreement) with the real
> world, and these are the questions that we do understand. The
> cosmological constant is just one number, whose origin we don't
> quite understand, and I find it ridiculous to derive thousands of
> outrageous, anthropic and other results about the things that we're
> supposed to know very well (for example about the ability to
> calculate particle masses) from a single number that we moreover
> don't understand.
Sorry, it may not be fair, it may not be reasonable, but that is not
how the real world works. The world does not care how many numbers we
get right ---- they have all swallowed Popper, and they are looking
out for what string theory gets *wrong*. [That's how this thread
started!] You can get a billion numbers right, but if nobody comes up
with some sort of believable *programme* for getting the Cosmological
Constant right --- at least for getting the *sign* right --- then
string theory will get into more and more serious trouble. It may not
be fair, but that's how it is.
> There is some more important business to do which is not directly
> related to cosmology, and once we understand e.g. the origin of E_k
> U-duality in M-theory and many other things, we might gain tools to
> study other issues including the cosmological questions. But I don't
> believe that a single number - Lambda - can be a good guide that
> helps us to find the whole structure of M-theory.
I see what you are saying, and I do think that *some* people should
work that way, but I would like to see more people, especially the
elite, telling us how M-theory *in principle* can cough up a positive
Constant. [I don't care about the precise value, as long as it is not
negative!!] Please don't tell the astrophysicists at Harvard that the
CC is just one number ---for most of them, the discovery of the CC is
*the* biggest news of their entire lives! People could easily get the
Nobel for this--- and they think that all string theorists are working
on this problem. And that is understandable.
> > Well, to be frank I would rather see the celebrities putting out
> > relatively lame papers about important things than brilliant
> > technical papers about totally boring things --- which seems to be
> > what is happening right now.
> If I understand what you really intend ;-), I agree with you in this
> general remark! Most of the leaders are very technically oriented and
> they have high technical standards, and they would not be happy to do
> what you suggest - and perhaps, they would not even be good at it.
They may not be good at it, but they should do it anyway!
Peter Woit
>>>In fact, it seems to be getting really hard to get citations
>>>these days: even the most determined efforts to launch a new
>>>bandwagon, http://arxiv.org/abs/hep-th/0208048, seem to fall flat
>>>these days.
>>>
>>>
>>Well, if you call 157 citations "flat", you're free to do it. I personally
>>think it is an appropriate number, and according to SPIRES this number
>>falls into the "famous" category.
>>
>>
>
>But this paper will still be forgotten very soon.
>Think how many citations the BMN paper had after a year! *THAT* was
>expert bandwagon construction. Since then, the bandwagon factory
>seems to have shut down....really, things are very slow these days!
>What is going on? Where is Ed Witten when we need him? What is
>Harvard doing?!
>We should be grateful to Tom Banks and even --- yes! --- to El
>Susskind for at least writing about things like deSitter space,
>supersymmetry breaking and what not ---
>things that we all want to know about!
>
>
This is either a clever parody, or an excellent example of the
mentality that has driven particle theory a long ways down a
blind alley. Too many leading string theorists continue to deal
with the failure of string theory by promoting ever more
pointless research programs, or doing bizarre things like
teaching string theory to undergraduates
http://www.mit.edu/afs/athena/course/8/8.251/www
It's going to be very difficult for them to admit failure,
but they really need to get on with it if hep-th is ever
again to have things worth reading.
John Baez
may enjoy - or be infuriated by - Carlo Rovelli's new paper:
A Dialog on Quantum Gravity
http://www.arXiv.org/abs/hep-th/0310077
Abstract: The debate between loop quantum gravity and string theory is
sometimes lively, and it is hard to present an impartial view on the
issue. Leaving any attempt to impartiality aside, I report here,
instead, a conversation on this issue, overheard in the cafeteria of
a Major American University.
Let me quote just a bit from the end, where the student Sal wraps
up her argument:
Prof, maybe I was a bit carried away by the polemical verve, so
let me be clear. I think that string theory is a wonderful theory. I
have a tremendous admiration for the people that have been able to
build it. Still, a theory can be awesome, and physically wrong. The
history of science is full of beautiful ideas that turned out to be
wrong. The awe for the math should not blind us. In spite of the
tremendous mental power of the people working in it, in spite of the
string revolutions and the excitement and the hype, years go by and
the theory isn't delivering physics. All the key problems remain wide
open. The connection with reality becomes more and more remote. All
physical predictions derived from the theory have been contradicted by
the experiments. I don't think that the old claim that string theory
is such a successful quantum theory of gravity holds anymore. Today,
if too many theoreticians do strings, there is the very concrete risk
that all this tremendous mental power, the intelligence of a generation,
is wasted following a beautiful but empty fantasy. There are alternatives,
and these must be taken seriously.
Aaron Bergman
wrote:
> People interested in this notion of "the string theory crackup"
> may enjoy - or be infuriated by - Carlo Rovelli's new paper:
I was waiting for someone to post about this.
My personal opinion is that each 'side' of this has people who don't
make the greatest advocates for their position. Rovelli is practically
the archetype for this.
If his goal is to get string theorists to listen to his position, he
does a pretty bad job of it.
Aaron
Jeffery
Carlo Rovelli obviously made this whole thing up, and was trying to
write a physics paper in the form of a dialog in a manner similar to
Galileo. I will admit that when I first came to this newsgroup, I
thought that John Baez was doing the same thing in the "Wizard and Oz"
threads. I thought that he was writing both the Wizard and Oz posts.
Later, I realized that Oz really was a different person.
Throughout Carlo Rovelli's paper he goes on and on about how there is
no experimental evidence for string theory, but there is no
experimental evidence for loop quantum gravity either. He spends the
entire time saying what he thinks is wrong with string theory but does
not say why he thinks loop quantum gravity is better. His criticisms
of string theory could just as easily apply to loop quantum gravity as
well. What is the experimental evidence for loop quantum gravity? How
could loop quantum gravity be falsified by experient? Does loop
quantum gravity explain the three generations, the small cosmological
constant, or symmetry breaking?
Now let's say someone was leveling these criticisms against not just
string theory but all post-Standard Model physics, such grand
unification, supersymmetry, string theory, M-theory, loop quantum
gravity, brane worlds, etc. Would it be justified to criticize these
theories on the grounds that there is no experimental evidence for
them? I would say no it's not. There is overwhelming experimental
evidence for the Standard Model but there are aspects of the Standard
Model that are unexplained. What should we do about that? Just ignore
it? Instead of ignoring it, we have tried to come up with theories
that explain unexplained aspects of the Standard Model. We don't have
direct experimental evidence for these theories because they exist at
higher energies than we can currently reach in our particle
accelerators. However, we have indirect evidence for them in that they
explain unexplained aspects of the Standard Model for which there's
enormous evidence. There is no experimental evidence against these
theories. They explain what they were intended to explain and are
consistent with all experimental data at the time they were invented,
which is all you could ask of any theory. Even if they turn out not to
be true, so what? We've never had a view of the Universe that was
actually true, and we never will. For instance, Newtonian mechanics is
not true. You don't criticize Newton for that since it was consistent
with all experimental data at the time it was invented. Today, string
theory is consistent with all experimental data.
Jeffery Winkler
Kevin A. Scaldeferri
Aaron Bergman <aber...@physics.utexas.edu> wrote:
>
>If his goal is to get string theorists to listen to his position, he
>does a pretty bad job of it.
>
Of course, it's entirely possible that string theorists aren't
actually the ones he's trying to convince....
--
======================================================================
Kevin Scaldeferri Calif. Institute of Technology
The INTJ's Prayer:
Lord keep me open to others' ideas, WRONG though they may be.
Urs Schreiber
news:<c7fd6c7a.03100...@posting.google.com>...
> Lubos Motl <mo...@feynman.harvard.edu> wrote:
[...]
> > OK, I think that we should understand many open questions about Matrix
> > theory and other pillars of the late 2nd revolution, for example open the
> > files at
> >
> > http://arxiv.org/abs/hep-th/0309238
>
> OK, I'll bite. :)
> *Why* is it important to understand those things?
I could imagine one or two reasons (after all, this is attacking the
"what is ST?" question, isn't it), but I would also rather hear Lubos
Motl talking about it.
Meanwhile I am privately approaching matrix theory from a maybe
unusual perspective, toying around with NSR-like worldsheet
formulations of discrete strings - and I'd like to know if anyone
would find this interesting or else tell me why I am misguided.
So we can derive matrix strings from D-brane mechanics, but can we
handle discretized strings as such? Can we construct string bit models
like those by Charles Thorne but supersymmetric and covariant? For
illustration of what I have in mind consider the following half-baked
ideas:
On a discretized NSR string we expect that every bit of the string
satisfies a Dirac equation with generalized mass term given by the
distance to its neighbouring bits:
y^m(@_m + i X'_m) |psi> = 0 .
Here y^m are the fermions, @ the partial derivative and X' some
discrete analogue of the sigma-derivative of the embedding
coordinates. This is just the supercurrent T_F written in functional
form (@ + iX' ~ alpha) and evaluated at a each single string-bit.
The simplest system of this sort would be a 2-bit system with one bit
fixed on some brane and the other one free to move. Denoting the
coordinate of the fixed string bit by X_b the equation of motion for
the remaining string bit should presumeably be
y^m(@_m + i(X - X_b)_m) |psi> = 0 .
Here X' is just the straight length of the 2-bit string. This might
even make sense since we know (e.g. Polchinski II, p.211) that in the
present context we can restrict to ground state open strings.
Now let the brane be in fact a D(-1)-brane with coordinates X_b = 0,
then the remaining string bit satisfies
D|psi> = y^m(@_m + iX) |psi> = 0 .
In a sense this is the most fundamental dynamics that one obtains from
a naive discretization of the NSR string. Remarkably, it formally
describes a Dirac particle coupled to a U(1) gauge field
A^m = X^m ,
which is locally pure gauge.
This has two interesting consequences:
1) The free string bit propagates in time and hence describes now a
D0 brane.
2) The identification A = X makes contact with the identification of
coordinates with gauge connections as known from the matrix model,
even though we did not presume any gauge fields on branes beforehand.
We just quantized a 2-bit NSR string with one end fixed in spacetime.
I am inclined to think that this is interesting, but I am prepared to
be disagreed with.
One obvious flaw of the above is that the resulting D0 brane is
massless. So let's try to improve on our description and consider a
2-bit string now glued to a classical massless D0-brane moving along
X^+. Using a parametrization on the string such that sigma is
perpendicular to X^+ yields for the free end of the 2-bit string the
equation
y^- @_- + y^+(@_+ + i X^-) + transversal terms .
The gauge background is now nontrivial and in fact describes a
homogeneous "electric" field along X^1. For comparison with the matrix
model let's boost to a high momentum frame along one of the
transversal directions. Then in addition a magnetic field appears and
our D0-brane, which locally still moves light like, has to perform
"ExB drift". Since this implies that its mean motion will no longer be
lightlike this could be interpreted as the D0 brane becoming massive,
because when taking the continuum limit the radius of gyration around
the "magnetic"field lines vanishes and only the time like drift motion
remains visible.
So it seems that by quantizing the simplest discretized NSR string one
arrives at the quantum mechanics of a single massive D0 brane whose
coordinates must be interpeted as a U(1) gauge connection, just as in
the matrix model.
One could now wave one's hands still a little more and replace U(1)
with U(N) by turning all the y^m, @_m and X^m in the above equations
into NxN matrices and taking traces. Then the Hamiltonian associated
with D^2 looks somewhat similar to the matrix model Hamiltonian. Maybe
it is its NSR analogue. Maybe not.
Robert C. Helling
> People interested in this notion of "the string theory crackup"
> may enjoy - or be infuriated by - Carlo Rovelli's new paper:
>
> A Dialog on Quantum Gravity
> http://www.arXiv.org/abs/hep-th/0310077
Wonderful! After two sessions of "Great Wakering" (see Douglas Adams'
Meaning Of Liff) I finally managed to get thru with this. I must say I
am very disappointed: We heard all that before. But the form of a
dialogue let me expect some discussion that is at least somewhat
balanced. But the (mainstream, pro string) professor is dumb, naive
and not really making his point. It might be that real world
discussions between loop people and string people often suffer from
the latter's ignorance but such a text should not make its point by
characterizing string theorists simply as stupid and thereby
disqualifying string theory. I was hoping for something better from
Rovelli.
Robert
--
..oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oOo.oO
Jason
> Now let's say someone was leveling these criticisms against not just
> string theory but all post-Standard Model physics, such grand
> unification, supersymmetry, string theory, M-theory, loop quantum
> gravity, brane worlds, etc. Would it be justified to criticize these
> theories on the grounds that there is no experimental evidence for
> them? I would say no it's not. There is overwhelming experimental
> evidence for the Standard Model but there are aspects of the Standard
> Model that are unexplained. What should we do about that? Just ignore
> it? Instead of ignoring it, we have tried to come up with theories
> that explain unexplained aspects of the Standard Model. We don't have
> direct experimental evidence for these theories because they exist at
> higher energies than we can currently reach in our particle
> accelerators. However, we have indirect evidence for them in that they
> explain unexplained aspects of the Standard Model for which there's
> enormous evidence. There is no experimental evidence against these
> theories. They explain what they were intended to explain and are
> consistent with all experimental data at the time they were invented,
> which is all you could ask of any theory. Even if they turn out not to
> be true, so what? We've never had a view of the Universe that was
> actually true, and we never will. For instance, Newtonian mechanics is
> not true. You don't criticize Newton for that since it was consistent
> with all experimental data at the time it was invented. Today, string
> theory is consistent with all experimental data.
Apparently, there's a theorem out there which says something to the
effect that the only renormalizable theories involving scalars, fermions
and vector fields in 3+1 dimensions are restricted to certain particular
forms and all the allowable terms do appear in the Standard Model.
(However, I've never actually seen any such theorem much less seen any
proof of it, so I'm not too sure if there's really such a theorem which
claims what many physicists claim it really claims.) If this were really
true, then, turning it around, it says that given almost any model at
high energy scales, when you run the renormalization group to low energy
scales, all the irrelevant terms become insignificant and the model
starts to look something like the standard model. Of course, things like
the gauge group, exact particle content, etc. aren't determined, but
you'd expect something like the Standard Model to come up. But this
means almost any family of models, if it covers a large enough parameter
space and/or combinatorial possibilites, would allow us, after fudging
the choice of gauge groups, particle content, etc. at high energy
scales, to get the Standard Model as the low energy limit naturally!
Haelfix
Quantum Gravity that has experimentally been confirmed with enough
precision that we should really be concerned about it.
It is also the simplest from a physical perspective. It describes the
state of absolute nothingness, if our theories can't even get that
right after many years of the smartest people on the planet working on
it, one should perhaps pause and rethink which direction we work our
theories from.
In the end, are we really that confident that GR's field theories are
sound, much less Lorentz Invariance? Experiment surely is conclusive
on small scales, but what about large scales? Carrol from UC lists it
as a grey area on his webpage, indeed attempts are being made in the
astrophysics community to slightly alter the functional form of GR.
Though the constraints on it are quite limiting, there is still some
wiggling room.
The various fields to solve the problem postulated by Quintesscence
and inflation are also way out of reach of particle and Stringy
physics. Indeed, the forms of those inflaton fields probably don't
even make sense in such contexts. Its amazing to me that two ripe
fields of physics progress even though one probably falsifies the
other, at least to an extent.
SUSY and SUGRA are still hopelessly out of reach of experiment, at
least for the next few years until the LHC etc goes live. I still
have not seen any good reasoning for supersoft symetry breaking
operators in SUSY, they are put in ad hoc such that they fit the
equations in a pleasing way. Nevermind the 110+ free parameters in
the Lagrangian, that could probably fit just about any Universe.
Hell, the pure mathematician is still stuck struggling to make sense
out of QFT. We haven't really escaped that demon yet.
The point is, people have outpaced themselves way too much. I have to
think that this is one of the most bizarre epochs of Physics in
history. I can't help but think that theorists won't get it right
until they get reconcile themselves with pure and axiomatic
mathematicians. And that experiment will probably make fools of us
all.
end spiel
Michael Petri
"Aaron Bergman" <aber...@physics.utexas.edu> wrote in
news:abergman-8E9B1E.13165510102003@localhost...
> In article <bm6s68$llf$1...@glue.ucr.edu>, ba...@galaxy.ucr.edu (John Baez)
> wrote:
> > People interested in this notion of "the string theory crackup"
> > may enjoy - or be infuriated by - Carlo Rovelli's new paper:
> I was waiting for someone to post about this.
>
> If his goal is to get string theorists to listen to his position, he
> does a pretty bad job of it.
I don't think that was Rovelli's intention. I cannot read minds, but
my guess is that he was rather trying to make LQG look attractive for
the yet undecided people, i.e. the "smart but stubborn" grad-student
Sal and the likes of him/her.
Actually, I found Rovelli's paper quite humorous. But I'm not the
right person to judge this. I neither understand string theory nor
loop quantum gravity on a truly fundamental level, so my shallow
understanding still allows me to delude myself, that *both* approaches
have achieved remarkable progress and insights (and both have quite
some homework to do) ;-)
> My personal opinion is that each 'side' of this has people who don't
> make the greatest advocates for their position. Rovelli is practically
> the archetype for this.
I don't know if Rovelli really has such a devastating effect on
others. I only know some of his writing. I always liked his papers,
because he does a good job in explaining the physical concepts and
consequences of LQG at a level, that I can understand (or at least
makes me believe I understand). So taking the emotional stuff aside,
at least for me the *content* of his papers has been quite a good
advocate of LQG, so far.
Mike
Lubos Motl
realistic. Although Prof. Simp makes some good points, he obviously does
not know all the developments in string theory during the last years too
well. There are various points that indicate that Simp can't be a real
string theory professor, but merely a Rovelli's fictious person. However,
Simp knows much more than Sal.
It was not hard to guess that the discriminated side had to be a stubborn
female grad student (Sal), while the opressor had to be a male string
theorist. Rovelli has obviously learned some basic facts about the ways in
which ideologies spread. Sal's comments about string theory and loop
quantum gravity are realistic, too. Most of the people in loop quantum
gravity generate these empty claims - much like Sal's statements - most of
the time.
Fortunately Rovelli made his story up, otherwise I would be upset that a
person like Sal with such very superficial and biased opinions about
science could have become a grad student at MAU (Major American
University).
In my opinion, even Rovelli's story makes it quite clear that the results
that have been obtained by loop quantum gravity are essentially equal to
zero; they just can't be compared to the results of string theory. Sal is
not convinced by the semi-realistic models with all necessary matter and
all forces, naturally implied and unified by string theory, and she wants
string theory to explain all the parameters of the Standard Model with
perfect accuracy today. Otherwise she does not want to waste her time even
by studying string theory. On the other hand she finds it perfectly
acceptable that it is essentially impossible to include the realistic
matter contents into loop quantum gravity (even as an artificial construct
put by hand) and she is not worried by the fact that loop quantum gravity
seems to predict that the smooth space is impossible.
Rovelli's paper just highlights the fact that string theory - as of today
- has no real competitors. I guess that Rovelli wanted to show just the
opposite - but it is just plain impossible, despite Rovelli's pedagogical
skills that are exceptional, no doubts about that. Rovelli is constrained
by reality, and reality is that loop quantum gravity does not work.
Arun Gupta
More specifically, is the following statement experimentally falsifiable:
"SUSY has relevance for that realm of physical phenonmena currently explained
using the Standard Model."
backdoorstudent
jeffery...@mail.com (Jeffery) wrote in message news:<575262ce.03101...@posting.google.com>...
>
> Throughout Carlo Rovelli's paper he goes on and on about how there is
> no experimental evidence for string theory, but there is no
> experimental evidence for loop quantum gravity either. He spends the
> entire time saying what he thinks is wrong with string theory but does
> not say why he thinks loop quantum gravity is better. His criticisms
> of string theory could just as easily apply to loop quantum gravity as
> well. What is the experimental evidence for loop quantum gravity? How
> could loop quantum gravity be falsified by experient? Does loop
> quantum gravity explain the three generations, the small cosmological
> constant, or symmetry breaking?
In reading the dialog, "Sal" a.k.a. Rovelli did not give me the
impression that Loops are any better than strings or seem to argue for
loops instead of strings. She/he is simply claiming that we should
study alternatives. Many of us find this argument refreshing because
we're growing tired string theorists claiming that there is no other
way. I even think the media is starting to sicken of people
pontificating about a "theory of everything." Well, maybe not all of
the media: http://www.pbs.org/wgbh/nova/elegant/
Peter Woit
Aaron Bergman wrote:
>>People interested in this notion of "the string theory crackup"
>>may enjoy - or be infuriated by - Carlo Rovelli's new paper:
>>
>>
>.
>
>My personal opinion is that each 'side' of this has people who don't
>make the greatest advocates for their position. Rovelli is practically
>the archetype for this.
>
>
It will be interesting to see if any experienced string
theorists are willing to specifically point out mistakes
made by Rovelli or flaws in the argument he is
implicitly making. Based on my experience, the
attempted counter-argument is more likely to be
1. "There's nothing new to see here folks, move
along now, keep moving"
2. "This is unbalanced, he should also be
explaining how glorious and successful
string theory is."
3. "He's being really mean to us, implying
that we are engaged in a misguided research
program. This is highly uncollegial."
4. "Rovelli is ignorant and too stupid
to really understand string theory. "
Can anyone point to a major mistake or flaw
in Rovelli's article?
Kevin A. Scaldeferri
In article <575262ce.03101...@posting.google.com>,
Jeffery <jeffery...@mail.com> wrote:
> For instance, Newtonian mechanics is
>not true. You don't criticize Newton for that since it was consistent
>with all experimental data at the time it was invented. Today, string
>theory is consistent with all experimental data.
I think this is completely a false analogy.
Newtonian mechanics was directly and simply consistent with the
experimental data of the day.
String theory can only claim this under extreme contortions. String
theory predicts 10 (or 11, or whatever) dimensions, which is not
observed. String theory predicts supersymmetry, which is not
observed. String theory predicts proton decay, which is not observed.
As far as I know, no one knows how to get string theory not to predict
that all observed particles are massless, except in a very handwaving
manner.
There is also an issue of the cosmological constant, which again
can only be dealt with in a handwavy manner, as far as I know.
There is, of course, the possibility of progress on these line and the
possibility of new experimental data, but to claim that string theory
is consistent with all experiments to date is a rather misleading
statement.
Thomas Larsson
Aaron Bergman <aber...@physics.utexas.edu> wrote in message news:<abergman-8E9B1E.13165510102003@localhost>...
>
> If his goal is to get string theorists to listen to his position, he
> does a pretty bad job of it.
>
I have the impression that Rovelli has tried to communicate with string
theorists for a decade, e.g. by repeatedly explaining why background
independence was Einstein's main conceptual breakthrough. If string
theorists haven't reacted in such a long time, he may have concluded
that they don't want a dialogue.
Aaron Bergman
In article <30731f05.03101...@posting.google.com>,
macgu...@yahoo.com (Arun Gupta) wrote:
One of the main driving phenomenological justifications for SUSY is that
it can explain why the Higgs mass is so small. If we don't see SUSY at
the weak scale, it loses a lot of its reason for being. It can probably
be pushed an order of magnitude or two higher, but if it's not seen at
LHC, a lot of people will be unhappy.
Aaron
Kevin A. Scaldeferri
Arun Gupta <macgu...@yahoo.com> wrote:
On the contrary, that statement seems manifestly false
(anti-tautologous?). If the phenomena are succesfully explained by
the Standard Model as-is, how can SUSY be relevant?
alejandro.rivero
> Throughout Carlo Rovelli's paper he goes on and on about how there is
> no experimental evidence for string theory, but there is no
> experimental evidence for loop quantum gravity either. He spends the
> entire time saying what he thinks is wrong with string theory but does
> not say why he thinks loop quantum gravity is better.
Perhaps it should be noted that Rovelli is not an expert on Loop
Quantum Gravity but an expert on Gravity, plainly. He has tried, as
far as I can remember, a good bunch of mathematical techniques
on the subject, LQG being just one of them.
Jerzy Karczmarczuk
Lubos Motl wrote:
> I found Rovelli's paper http://arXiv.org/abs/hep-th/0310077 amusing and
> realistic. Although Prof. Simp makes some good points, he obviously does
> not know all the developments in string theory during the last years too
> well. There are various points that indicate that Simp can't be a real
> string theory professor, but merely a Rovelli's fictious person. However,
> Simp knows much more than Sal.
...
Yes, all very nice. Just one (-)... I don't like too much when people
take the old, venerable history, and stretch it to their needs,
knowing that most people who learnt it are retired... //This is a
partial answer to Peter Woit's quest for flaws in Rovelli's
article. But I don't think that this is a major flaw, just an
oversimplisticized <<licentia poetica>>...)//
I mean concretely the Veneziano model.
Sal And does the Veneziano formula describe correctly the cross sections
that we observe?
Simp No, it does not. The physical high energy behavior of the strong
interactions cross sections is not the one predicted by this formula.
It looks like if Veneziano proposed something, which was then killed
by the observation. This is false. At that time we knew quite a lot
about Regge trajectories, and also by the growing sigma, which could
not be explained by some small number of dominant Regge poles, and
Veneziano knew that. This was a rather simplistic, *explicitly*
non-unitary model, and mot people suspected that is was the first term
of a - possible - future unitary dual theory...
Sal Therefore a good physicist concludes that Veneziano formula was a nice
theoretical idea, but not one that Nature likes, and abandons it, to
study something else . . . [...]
Simp In fact, the Veneziano formula was abandoned for strong interactions,
but so much was found out of it, it was understood it could emerge from
a string theory and it was realized that it could be used in a much
better way.
Good physicists at that time were aware of all the deficiencies of the
*model* and using the verb *abandon* is a clear abuse.
Sal Wait a minute, this story gives a historical account of the birth of
an idea. But it has nothing to do with providing reasons for believing
that this idea is physically correct. If anything, it shows that it was
a wrong idea to start with. The Veneziano amplitude and the
Dolen-Horn-Schmid duality in strong interactions were motivated by the
observation of the resonances with high spin, the rough proportionality
between mass and spin. . .
Don't let your students read that. This observation *alone* led to
Regge poles. The duality stuff is the *interplay* between three
speculobservfacts:
* The scattering amplitude is dominated by resonances at low energies.
(s-channel poles)
* It is dominated by the Regge poles at high energies (t-channel singularities)
* Otherwise it is a nice analytical beast, so the dispersion relations and
other maths works well, and one produce the "sum rules" which say that the
average over the resonances is EQUAL to the average over the Regge poles.
So, people searched for analytic 'implementations' of such a
behaviour. Veneziano found the Beta function, other produced 5- 6-
a,d N-particle amplitudes which factorized appropriately. They were
generalized hypergeometrics, extremely elegant and easy to
manipulate, but non-unitary, and there was no doubt that this was a
phenomenologic approximation. Sal and Simp are in the state of sin.
Sal continues -
If I understand correctly, all this is today understood on the basis of
QCD. Even the apparent stringy behavior can be understood from QCD;
tubes of flux of the color lines of force behave like small strings in
some approximation. Therefore it is reasonable that some sort of string
theory gives an approximate account of the phenomenology. The correct
physical conclusion is that string theory is an approximate description
at some scale, not a fundamental theory.
If I understood correctly, the relation between strings (rudimentary
as they were) and the dual amplitudes was *OBVIOUS* 30 years ago. OK,
Sal hasn't been born yet... But perhaps Simp should have helped him.
Thank you.
Jerzy Karczmarczuk
Caen, France
Peter Woit
The minimal supersymmetric extension of the standard
model (MSSM) is not falsifiable, except if the
standard model itself is falsified. Deviations from
the standard model predictions can be made as
small as you want by choosing appropriate values
for the 105 extra parameters it contains.
Some supersymmetric GUTs can be falsified,
for instance if experimentalists manage to
dramatically push up the bound on the proton
lifetime. This is how the simplest
non-supersymmetric SU(5) GUT was falsified.
There is a list of reasons that people give
as advantages of supersymmetric models,
and some reasons in this list can be, if not
falsified, at least made less convincing. One
of the main arguments for supersymmetry has
been that it explains why the electroweak
breaking scale is so much smaller than the
GUT scale. If no supersymmetric particles
are found after going up to mass scales
much higher than the electroweak scale, this
will count against this particular argument
for supersymmetry. The other main
argument has been from the running
of coupling constants in the MSSM, the
"three lines meet in a point" argument. My
understanding is that at two-loops the
prediction of one coupling constant from the
other two is off by 10%, which is within
the uncertainties of the calculation (you
have 105 parameters to play with...). So
this is a fuzzy enough prediction that,
while it could have been falsified by
very different numbers, better data can't
falsify it now.
Arnold Neumaier
>
> In my opinion, even Rovelli's story makes it quite clear that the results
> that have been obtained by loop quantum gravity are essentially equal to
> Rovelli's paper just highlights the fact that string theory - as of today
> - has no real competitors.
Apparently we have two competitors in a race measured in terms of
infinitesimal gains, debating about their relative merits.
The results on both sides are essentially equal to zero,
as regards experimentally verifiable predictions that are not
already consequences of the (simpler and predictive) standard model
and classical relativity.
Neither loop quantum gravity nor string theory have so far contributed
anything to understanding quantum gravity, at least not to my understanding.
Instead they added a lot of fancy formalism and difficulties that far
outweigh their explanatory value. At the present state of the art,
one wastes much more time learning to understand these theories than what
one gains in understanding quantum gravity (namely next to nothing).
I felt disappointed by every survey paper I read.
If an explanation is not much more compact than the union of those phenomena
it explains which are not already explained in simpler terms, it does
not deserve to be called an explanation.
Postulating fancy concepts like strings, extra dimensions, or spin foams
just to 'explain' a few items that currently fall from heaven is worse
than postulating a panoply of Gods to explain the forces of Nature.
Contrast the plethora of real phenomena explained and predicted quantitatively
by thermodynamics, by statistical mechanics, by quantum mechanics, or by
general relativity with the paucity of real phenomena explained and
predicted quantitatively by either loop quantum gravity or string theory.
Seen from this perspective, loop quantum gravity and string theory
look more like two elephants who gave birth to a flea.
If theoretical physics were a commercial enterprise, loop quantum gravity
and string theory programs would both have been stopped since long and
deferred to a time (far in the future) when the conflict between theory
and experiment in quantum gravity becomes so severe that it gives clear
hints of the direction theory should take.
But in a science based on peer reviews, a field is able to perpetuate itself
once it has attracted a minimal number of researchers if it beats the drums
to melodies of great dreams.
Thus fields like loop quantum gravity and string theory may serve to make
a living, to create beautiful myths for readers of the Scientific American,
or to give their researchers the reputation of working on something
very difficult and advanced (passing milestones in a direction
not leading to the goal may require tremendous amounts of skill),
but they hardly serve the quest for understanding nature.
Arnold Neumaier
Jeffery
macgu...@yahoo.com (Arun Gupta) wrote in message news:<30731f05.03101...@posting.google.com>...
I don't think it can be totally disproven but if they don't detect
supersymmetric particles at the Large Hadronic Collider that will be a
huge strike against it. It will remove the main reason for which it
was invented in the first place. Fortunately, it will be several years
before we have to worry about it.
Jeffery Winkler
alejandro.rivero
Perhaps strings theoretists believe that their theory is background
independent, of sort of.
Serenus Zeitblom
> But in a science based on peer reviews, a field is able to perpetuate itself
> once it has attracted a minimal number of researchers if it beats the drums
> to melodies of great dreams.
>
> Thus fields like loop quantum gravity and string theory may serve to make
> a living, to create beautiful myths for readers of the Scientific American,
> or to give their researchers the reputation of working on something
> very difficult and advanced (passing milestones in a direction
> not leading to the goal may require tremendous amounts of skill),
> but they hardly serve the quest for understanding nature.
In the context of a physics discussion group, this is nothing but
abuse. In a non-moderated group you could fling obscenities at
string theorists and loop theorists, and it would amount to the
same thing. Tell me, in the discussion groups on numerical
analysis and optimisation --- if they exist --- do you have
string theorists irrupting and telling you all that your field
is so stupefyingly boring that research on it is a total waste
of time, and that no honest researcher would accept public funds
to do research on anything that simple and dull? I didn't think so.
So why not accord us the same courtesy? If you have concrete
criticisms of these theories, let's hear them. Otherwise,
``It is not good to have zeal without knowledge,
nor to be hasty and miss the way.''
(Proverbs 19:2)
Thomas Larsson
> I don't think it can be totally disproven but if they don't detect
> supersymmetric particles at the Large Hadronic Collider that will be a
> huge strike against it. It will remove the main reason for which it
> was invented in the first place. Fortunately, it will be several years
> before we have to worry about it.
People seem to think that SUSY is in trouble if no light Higgs boson
(< 130GeV) is detected at Tevatron II. So far, I haven't heard any rumours,
but maybe somebody else has.
Lubos Motl
> > Thomas Larsson:
> > I have the impression that Rovelli has tried to communicate with string
> > theorists for a decade, e.g. by repeatedly explaining why background
> > independence was Einstein's main conceptual breakthrough. If string
> > theorists haven't reacted in such a long time, he may have concluded
> > that they don't want a dialogue.
> Alejandro Rivero:
> Perhaps strings theoretists believe that their theory is background
> independent, of sort of.
Lubos Motl:
This point has been discussed on SPR many times. But let me say a couple
of basic points again, perhaps in a different way:
1. Physics of string theory at low energies fully agrees with
generalizations of general relativity, and therefore it also fully
respects its general covariance. (Using the language of quantum field
theory, the general covariance is necessary to decouple negative-norm
states of the metric field which we know to automatically happen in string
theory.) In fact, it is true not only at low energies. One can explicitly
show (see chapters 3 of Green Schwarz Witten and of Polchinski) that an
infinitesimal change of the background in perturbative string theory is
equivalent to a condensation of a state that existed in the previous
background. This guarantees that "theories" living on different
backgrounds are actually the same theory. Effective field theory analyses
of non-perturbative string theory indicate that this version of background
independence holds non-perturbatively.
2. String theory is more than just a theory of gravity. It implies the
existence of a large amount of other physical phenomena, including those
well-known objects such as Yang-Mills gauge fields, fermions, Higgs
mechanism etc., but also higher-energy states and their corresponding
higher-spin symmetries etc. In this sense, string theory extends the
notion of general covariance into a much broader and more powerful
symmetry whose precise character is not fully known. However, gauge
symmetries may be also viewed as redundacies of the description (see works
by Seiberg - and Seiberg and Witten - both in string theory and quantum
field theory) and therefore there is no unique answer to the question
"what is the gauge symmetry of (a background of) string theory?".
3. Spacetime does not seem to be fundamental, but rather a derived notion
in string theory. There is a lot of evidence for this claim. For example,
at very short distances the usual notions of spacetime geometry cease to
be meaningful, and the whole infinite tower of excited string states
starts to be as important as the graviton. Also, string theory allows
miraculous equivalences between theories defined on different backgrounds
- T-duality, mirror symmetry, M-theory on K3 / heterotic strings on T^3
duality and many others. In these dualities, the momentum in some
direction - for example - may be mapped into the electric charge (or the
winding number) in the dual description. This all suggests that general
covariance and geometry are just shadows of a much grander theoretical
structure - a structure that we can calculate a lot about, but whose most
fundamental principles are still mostly unknown to us. Geometry only
admits the usual description for large manifolds. At short distances, it
becomes fuzzy and it mixes with other physical effects. Also, it has been
also showed that topology of space can change in string theory, and the
evolution is totally smooth.
4. String theorists are not quite satisfied with their formalisms to
calculate string-theoretical quantities because they're not *manifestly*
background independent, and in many cases (Matrix theory, AdS/CFT) they
make it almost impossible to calculate physics on other backgrounds. Many
people in the field hope that we will find a better formalism that is
manifestly background-independent. This is one of the dreams of string
theory. At the same moment, I must say that the assumption is that it
won't change the results of physical calculations that have been done so
far. It will only change the formalism. Such a background-independent
formalism is not just a dream that would satisfy our sense of beauty: the
realistic backgrounds (heterotic strings on Calabi-Yau three-folds is the
classic example) are relatively complicated, more complicated than the
backgrounds that we understand well, and it is reasonable to assume that
if we are able to calculate such backgrounds, we will have the tools to
calculate *all* backgrounds. A background-independent formalism is also
supposed to answer at least some questions about the vacuum selection
problem.
5. I am among the people who would be happy if some tools of loop quantum
gravity (but almost certainly not the *whole* loop quantum gravity) were
applicable to string theory. This would be a very exciting - because a
very easy - way to make progress. However, let me also admit that I find
it unlikely because the nature of geometry in the full theory
(string/M-theory) seems to be much deeper, while the caricature of the
world as pictured by loop quantum gravity seems to be full of naive
prejudices that seem to contradict the lessons on spacetime that string
theory has taught us - e.g. space can have extra dimensions; geometry is
an approximate notion; geometry can transform into other physics;
topology can change; there must be things beyond geometry.
6. Yes, most string theorists don't want to spend too much of their time
with colleagues like Carlo Rovelli because they don't see the point of
such discussions. In this sense, I belong to the minority, but
unfortunately Carlo Rovelli has never answered my e-mails.
Best wishes
Lubos
Gerard Westendorp
[..]
> Contrast the plethora of real phenomena explained and predicted
> quantitatively by thermodynamics, by statistical mechanics, by
> quantum mechanics, or by general relativity with the paucity of real
> phenomena explained and predicted quantitatively by either loop
> quantum gravity or string theory.
Perhaps we should take into account that we are pushing the limits of
the unknown further and further away from the world of everyday life.
Classical thermodynamics and mechanics helped us to make engines.
These could be made in a garage.
Quantum mechanics helped us to make transistors. Perhaps someone
has succeeded in making one in a garage, but we tend to think more
in terms of huge buildings full of people in plastic suits.
Special relativity helped us to make nuclear bombs. Luckily they
cannot be made in a garage.
But post 1930 particle physics has not had a big social impact yet.
It looks like our ancestors ate all the low energy fruit, so
we have to go increasingly high in energy to discover new
things. But our life takes place mostly below a few eV per
particle, so all the TeV stuff is just too exotic for our
homes, kitchens and gardens.
Now, we are even getting bored of TeVs. It we should not
expect that any theory concerned with such energies
will have applications, or even be testable.
Luckily for physicists, as humans get richer, usefulness
becomes less and less important. Most of the value of quantum
gravity is probably the fact that it is fun.
Gerard
Arun Gupta
ke...@sue.its.caltech.edu (Kevin A. Scaldeferri) wrote in message
> Arun Gupta <macgu...@yahoo.com> wrote:
> >Is SUSY experimentally falsifiable?
> >
> >More specifically, is the following statement experimentally falsifiable:
> >
> >"SUSY has relevance for that realm of physical phenonmena currently
> >explained using the Standard Model."
> On the contrary, that statement seems manifestly false
> (anti-tautologous?). If the phenomena are succesfully explained by
> the Standard Model as-is, how can SUSY be relevant?
The quoted sentence was simply an attempt to delineate a particular
realm of experimental phenomenon (stuff in the Standard Model range of
energies).
Anyway, I do have a further set of questions :
1. True or false? - whatever is found at LHC will be compatible with
string theory. There are no predictions from string theory at all.
2. Seeing that String Theory has no concrete predictions for what will
be seen at LHC, perhaps we can say :
String theory is not a Theory of Everything. It is a Theory of
Anything. (That is, given any likely experimental phenomena that we
will find in the next few decades, String Theory is compatible with
it.)
Do you agree?
3. In trying to understand String Theory as a Theory of Anything :
Suppose quantum field theory had a true theorem - namely, that the
only mathematically consistent QFTs in 4 dimensions are gauge
theories. Suppose this holds, we would not say that "QFT predicts the
Standard Model".
Of course, the different (low energy) gauge group and fermion choices
are different QFTs, unlike in String theory, where these are different
vacuum states.
Nevertheless, seemingly, the search for gauge groups/string vacua that
work for the given universe, are amazing alike - yes or no? There are
no dynamical principles involved. It is a sophisticated form of
curve-fitting. Right now neither can tell us why the universe
exhibits particular symmetries.
Can we make a (limited) analogy:
QFT:Standard Model :: String Theory:(theory of everything)?
4. One limitation to the analogy, is of course, that String Theory
naturally incorporates a massless spin-2 particle, the graviton, while
conventional wisdom is that QFT cannot do so consistently. Is this
the danger that loop gravity presents to String Theory, that it might
demonstrate the existence of a consistent QFT theory of gravity?
(The second limitation to the analogy is that String Theory has many
more constraints than QFT does, from the requirement of mathematical
consistency.)
A. Garrett Lisi
I'd like to inject into the physics meme pool that perhaps:
"String theory is the luminiferous aether theory of the twenty-first
century."
I am not particularly well versed in the history of science, but the
analogy seems striking -- worth mentioning and discussing. When we
repeat history, it's always with new characters.
Rovelli's paper was a fun read. And I'm amused he posted it in
hep-th. I expect the resulting silence from the string theory
community will be deafening.
-Garrett
Tim S
on 14/10/03 8:01 am, Lubos Motl at mo...@feynman.harvard.edu wrote:
> I found Rovelli's paper http://arXiv.org/abs/hep-th/0310077 amusing and
> realistic. Although Prof. Simp makes some good points, he obviously does
> not know all the developments in string theory during the last years too
> well. There are various points that indicate that Simp can't be a real
> string theory professor, but merely a Rovelli's fictious person. However,
> Simp knows much more than Sal.
>
> It was not hard to guess that the discriminated side had to be a stubborn
> female grad student (Sal), while the opressor had to be a male string
> theorist.
I think these names are short for Simplicio and Salviati, 2 of the people in
Galileo's _Dialogo sopra i due massimi sistemi del mondo_ (_Dialogue on the
two chief world systems_). Simplicio is the Aristotelian and Saliviati is
his opponent.
Tim
Aaron Bergman
wrote:
IIRC, SUSY does favor a light Higgs. However, last I heard
Tevatron's been having issues with beam intensity, so it's not
clear what they could see.
Aaron
--
Aaron Bergman
<http://www.princeton.edu/~abergman/>
Aaron Bergman
"Michael Petri" <petr...@freenet.de> wrote:
> I don't think that was Rovelli's intention. I cannot read minds, but
> my guess is that he was rather trying to make LQG look attractive for
> the yet undecided people, i.e. the "smart but stubborn" grad-student
> Sal and the likes of him/her.
Well, I think that would have turned me off when I was looking for stuff
to go into. You can look in the archives of this group to see my
personal development.
The real elephant in the room here is jobs. Jobs aren't great in string
theory right now. LQG is even worse, unless I misunderstand the
situation.
[...]
> > My personal opinion is that each 'side' of this has people who don't
> > make the greatest advocates for their position. Rovelli is practically
> > the archetype for this.
>
> I don't know if Rovelli really has such a devastating effect on
> others.
About a decade ago, he gave a somewhat infamous talk at Princeton or the
Institute. That it didn't convince anyone is probably the best way to
summarize what I've heard of this talk.
Rovelli has always, for me, been one of those who keeps going on how
string theorists don't understand the lessons of GR, how background
independence is god's holy writ and any theory lacking it explicitly is
anathema, how string theorists are really just field theorists overly
infatuated with a finite perturbation expansion, that LQG is rejected
because string theorists are insular and prejudiced, and so on and forth.
Needless to say, it's all a load of crap. I can tell you that going to
graduate school, I talked about all these things with a number of string
theorists and they were hardly a bete noire. Frankly, I think people
were probably thinking about these things even back in the 80s at the
height of superstring triumphalism.
Now, I understand the triumphalism annoyed people. I've also heard that
a lot of string theorists have said some not very nice things to LQG
people, especially back then. I wasn't there, so I couldn't tell you. It
wouldn't surprise me, though, as string theory has a lot of very strong
personalities who won't hesitate to tell you in an rather unrelenting
manner just how wrong you are. Why do many string theorists ignore LQG
now? Because, at some point, it was examined and found wanting by some
very smart people. And I'm sure they said so. In so many words.
So, maybe some people got treated badly. Well, lots of people in string
theory get treated badly, too, often by the same people. The result,
apparently, is that there is, at least for some, a deeply unproductive
(and pretty much unrequited) rivalry as exemplified by things like the
aforementioned paper.
It's not helpful. It's about as tiresome as the same critiques that get
repeated as nauseum in this and other fora. You want people to study
LQG, go write papers about how great LQG. If you build it, they will
come. Really. In the meantime, can't we skip the tendentious Galilean
dialogues?
Aaron
Peter Woit
Aaron Bergman wrote:
>.... back in the 80s at the
>height of superstring triumphalism.
>
>Now, I understand the triumphalism annoyed people.
>
String theory triumphalism unfortunately isn't
a phenomenon only to be discussed in the past tense.
A couple years ago David Gross entitled his talk
at an AAAS meeting "The Power and the Glory of
String Theory". The cover story of this month's
Scientific American (available on www.sciam.com)
begins by describing the state of string theory as
follows:
"But in the mid-1990s the theory started to
click together conceptually. It made some
testable, if qualified, predictions"
String theory has a huge and continuing problem
with being overhyped by many of its practicioners.
Under the circumstances, string theorists should
consider doing something about this before
criticizing other physicist's attempts to put
forward a more balanced view of the current
situation.
Jeffery
ke...@its.caltech.edu (Kevin A. Scaldeferri) wrote in message news:<bmar9a$ad3$1...@clyde.its.caltech.edu>...
> In article <575262ce.03101...@posting.google.com>,
> Jeffery <jeffery...@mail.com> wrote:
> > For instance, Newtonian mechanics is
> >not true. You don't criticize Newton for that since it was consistent
> >with all experimental data at the time it was invented. Today, string
> >theory is consistent with all experimental data.
>
> I think this is completely a false analogy.
>
> Newtonian mechanics was directly and simply consistent with the
> experimental data of the day.
>
> String theory can only claim this under extreme contortions. String
> theory predicts 10 (or 11, or whatever) dimensions, which is not
> observed.
We don't observe them because they are compacified on very small
distance scales, on order of the planck scale, or alternatively
because of the brane world scenario, that fermions and vector bosons
are confined to the 3brane that is our universe.
String theory predicts supersymmetry, which is not
> observed.
Because supersymmetry breaks around 1 TeV, so the masses of the
supersymmetric particles should around that mass, so that's why you
don't observe. If we don't observe supersymmetric particles at the
Large Hadronic Collider, then that will be an inconsistency with the
data.
String theory predicts proton decay, which is not observed.
Because if you combine grand unification with supersymmetry it
predicts a proton lifetime of 10^32 years, which is too rare for us to
separate from the background.
Also, the simple naive form of string theory predicts rapid decay but
we can take care of the problem using an H group.
>
> As far as I know, no one knows how to get string theory not to predict
> that all observed particles are massless, except in a very handwaving
> manner.
That is not true. I could show you the particle spectrum of the E_8 x
E_8 superstring compacified on a Calabi-Yau manifold. The fermions do
have mass.
>
> There is also an issue of the cosmological constant, which again
> can only be dealt with in a handwavy manner, as far as I know.
Superstring theory contains scalar fields that by rolling down their
potentials could give inflation, and then the current acceleration of
the expansion.
>
>
> There is, of course, the possibility of progress on these line and the
> possibility of new experimental data, but to claim that string theory
> is consistent with all experiments to date is a rather misleading
> statement.
It is consistent. String theory does not predict that we should
observe 10 uncompactified dimensions, or supersymmetric particles at
every day energies, or rapid proton decay, for all the reasons I cited
above. For instance, string theory does not predict 10 dimensions that
would be observable by us, because the extra ones are compactified.
So, in what way is string theory inconsistent with experimental data?
Jeffery Winkler
Kevin A. Scaldeferri
This is good, a discussion of physics, not rhetoric. But, I'm going
to continue as the skeptic here... perhaps we can have an improved
version of Rovelli's dialogue.
In article <575262ce.03101...@posting.google.com>,
Jeffery <jeffery...@mail.com> wrote:
>
>ke...@its.caltech.edu (Kevin A. Scaldeferri) wrote in message news:<bmar9a$ad3$1...@clyde.its.caltech.edu>...
>> Newtonian mechanics was directly and simply consistent with the
>> experimental data of the day.
>>
>> String theory can only claim this under extreme contortions. String
>> theory predicts 10 (or 11, or whatever) dimensions, which is not
>> observed.
>
>We don't observe them because they are compacified on very small
>distance scales, on order of the planck scale, or alternatively
>because of the brane world scenario, that fermions and vector bosons
>are confined to the 3brane that is our universe.
Clearly, since you present multiple choices, this is only a
possibility of string theory, not a prediction. At least for now,
there is no firm way of saying that one of these phenomena which
(sort-of vaguely) resemble the real world must occur, yes? This is a
contortion introduced to correct for a bad prediction of the theory.
>
> String theory predicts supersymmetry, which is not
>> observed.
>
>Because supersymmetry breaks around 1 TeV
Is it a prediction of string theory that SUSY breaks at that scale?
Or is that just a number picked to be tantalizingly outside of current
experimental reach.
>If we don't observe supersymmetric particles at the
>Large Hadronic Collider, then that will be an inconsistency with the
>data.
Do you know a string theorist who will go on record saying they will
abandon ST if we don't see SUSY at the LHC?
>>String theory predicts proton decay, which is not observed.
>
>Because if you combine grand unification with supersymmetry it
>predicts a proton lifetime of 10^32 years, which is too rare for us to
>separate from the background.
>
>Also, the simple naive form of string theory predicts rapid decay but
>we can take care of the problem using an H group.
So, to turn this around, the naive prediction of string theory is
experimentally ruled out. So, you must as another twist
(magnificently mathematically elegant, I'm sure) to patch things up.
>> As far as I know, no one knows how to get string theory not to predict
>> that all observed particles are massless, except in a very handwaving
>> manner.
>
>That is not true. I could show you the particle spectrum of the E_8 x
>E_8 superstring compacified on a Calabi-Yau manifold. The fermions do
>have mass.
Okay, what are the masses? Let me guess, in conflict with experiment?
>>
>> There is also an issue of the cosmological constant, which again
>> can only be dealt with in a handwavy manner, as far as I know.
>
>Superstring theory contains scalar fields that by rolling down their
>potentials could give inflation, and then the current acceleration of
>the expansion.
Sounds handwavy to me. Is there an actual model you had in mind which
is consistent with observed cosmology?
>>
>> There is, of course, the possibility of progress on these line and the
>> possibility of new experimental data, but to claim that string theory
>> is consistent with all experiments to date is a rather misleading
>> statement.
>
>It is consistent. String theory does not predict that we should
>observe 10 uncompactified dimensions, or supersymmetric particles at
>every day energies, or rapid proton decay, for all the reasons I cited
>above.
But, also, it doesn't predict that we shouldn't observe any or all of
these, right?
> For instance, string theory does not predict 10 dimensions that
>would be observable by us, because the extra ones are compactified.
>So, in what way is string theory inconsistent with experimental data?
It is (so far as I can see) inconsistent with experimental data in the
same way as all the other beyond-the-SM theories whose naive
predictions are incorrect and which require various contortions to
squeeze out something which isn't contradicted.
Michael Petri
news:abergman-4555CB.23573512102003@localhost...
> The real elephant in the room here is jobs. Jobs aren't great in
> string theory right now. LQG is even worse, unless I misunderstand
> the situation.
I agree. The last sentence of Rovelli's "Galilean Dialogue" points
into this direction. His paper closes with: "By the way, Sal is still
looking for a job..."
> So, maybe some people got treated badly. Well, lots of people in string
> theory get treated badly, too, often by the same people. The result,
> apparently, is that there is, at least for some, a deeply unproductive
> (and pretty much unrequited) rivalry as exemplified by things like the
> aforementioned paper.
This seems to be the human nature. Scientists are not very much different
from other people or from other social animals: Always alert on defending
*their* territory, building leadership and barking away any potential
threats.
Is it necessary? I don't think so: If you're convinced about what
you're doing, just write it up - preferentially in simple terms, so
that others might understand or point out the flaws. If you're work is
good, it doesn't need promotion other than its raw content. Making a
big fuzz about the gloriousness of your approach or even dropping
derogatory remarks about the mediocre alternatives, is rather a sign
of weakness. My definition for a respectable scientist (or a
respectable person in general) is this: State your opinion, admit your
mistakes and go to work, that's it. And if you happen to have a big
ego, don't take it too seriously. ;-)
> You want people to study LQG, go write papers about how great
> LQG. If you build it, they will come.
Exactly! This strategy always works. Maybe not in the short-term, but in the
long run: always!
Best regards
Mike
Lubos Motl
> "String theory is the luminiferous aether theory of the twenty-first
> century."
Well, I am not sure whether this sentence was written as a serious topic
for new discussions, or as a not-too-entertaining joke, but at any rate,
string theory is just the opposite of the theory of aether. String theory
respects all major symmetry principles, and it shows that all physical
objects are dynamical, they affect each other, and they have a common
origin. String theory is the simplest possible and the most natural
consistent extension of the successful theories in physics, including
gauge (quantum field) theories and general relativity.
Loop quantum gravity is much, much more similar to aether theory - it
tries to construct the spacetime from explicit constituents, much like
aether is made out of some sort of molecules. Consequently, loop quantum
gravity most likely breaks Lorentz invariance, much like aether does.
Aether has the advantage that it allows the spacetime to be smooth at long
distances, which is unlikely to be the case of loop quantum gravity.
> Rovelli's paper was a fun read. And I'm amused he posted it in
> hep-th. I expect the resulting silence from the string theory
> community will be deafening.
Be sure that string theorists discuss Rovelli's paper. We just returned
from a dinner with Gary Gibbons, and Rovelli's paper was one of our
topics. Well, we are certainly not going to write many papers based on the
ideas from Rovelli's paper - because there are not too many new scientific
ideas in that paper :-), and writing papers in Rovelli's format would sort
of mean to accept his standards (which would mean to fall into much lower
scientific categories). Some string theorists find the paper entertaining,
many other string theorists are shocked by the visibly painful situation
of loop quantum gravity today, as demonstrated by many things including
Rovelli's paper. A renowned physicist who attended the dinner tonight
pointed out the alarming low productivity of loop quantum gravity within
science. While the amount of money that flow to loop quantum gravity today
- and the number of people in this field - is about 10 per cent of the
people in string theory, its production is closer to 1 per cent of the
production in string theory.
The same physicist - whose identity won't be revealed here (and I won't
even say whether he is a string theorist or not), but whose number of
citations is much above 10,000 - analyzed some claims of loop quantum
gravity, for example its "background independence". Loop quantum gravity
does not even seem to respect Lorentz invariance (even this elementary
question is open in LQG) - it works in a non-relativistic form of a
Hamiltonian formalism. General covariance in LQG is, of course, even more
controversial because it is not clear whether LQG is compatible with large
smooth manifolds whose existence is necessary for general covariance to be
meaningful. LQG also requires a pre-existing manifold for its definition -
for example, LQG does not allow topology change.
Kevin A. Scaldeferri
>ke...@sue.its.caltech.edu (Kevin A. Scaldeferri) wrote in message
>> Arun Gupta <macgu...@yahoo.com> wrote:
>> >Is SUSY experimentally falsifiable?
>> >
>> >More specifically, is the following statement experimentally falsifiable:
>> >
>> >"SUSY has relevance for that realm of physical phenonmena currently
>> >explained using the Standard Model."
>> On the contrary, that statement seems manifestly false
>> (anti-tautologous?). If the phenomena are succesfully explained by
>> the Standard Model as-is, how can SUSY be relevant?
>The quoted sentence was simply an attempt to delineate a particular
>realm of experimental phenomenon (stuff in the Standard Model range of
>energies).
Yes, I sort of figured. I was just trying to push you to a more
precise statement which could actually be answered.
>Anyway, I do have a further set of questions :
>
>1. True or false? - whatever is found at LHC will be compatible with
>string theory.
I have no doubt.
> There are no predictions from string theory at all.
I would not go that far. It's not clear to me that there are robust
predictions below the Planck scale, though. This may well be a flaw
of any QG theory, though.
Anyways, I'm going to decline to answer the rest of your questions
since I've done it too many times before.
Jaime T Melo
> Lubos Motl wrote:
> > In my opinion, even Rovelli's story makes it quite clear that the
> > results that have been obtained by loop quantum gravity are
> > essentially equal to zero; they just can't be compared to the
> > results of string theory. [...] Rovelli's paper just highlights
> > the fact that string theory - as of today - has no real
> > competitors.
I hadn't ever read a so clear opinion on the battle for conquering the
primacy over QG related theories. Regarding the analogy to a
commercial enterprise - Yes, both have been a successful one, selling
books, lectures etc. etc. worldwide.
Regards,
Jaime
anbar
Lubos Motl <mo...@feynman.harvard.edu> wrote in message
news:<Pine.LNX.4.31.031013...@feynman.harvard.edu>...
> I found Rovelli's paper http://arXiv.org/abs/hep-th/0310077 amusing and
> realistic. Although Prof. Simp makes some good points, he obviously does
> not know all the developments in string theory during the last years too
> well. There are various points that indicate that Simp can't be a real
> string theory professor, but merely a Rovelli's fictious person.
This impression is generated by the fact the Rovelli omitted the role
*I* played in the discussion... let me introduce myself: S.A. Gredo.
I work as a bartender at the MAU cafeteria.
I tried to make some points of my own (besides serving some wine to
Prof. Simp... he shouldn't drink, you know, it slows his arguing
capabilities) but I was essentially ignored, and since the discussion
is now public I wish to resume my views as well.
Sal is a a little too enthusiastic about the loop approach (although
I'd now call it the state sum approach), which is plagued by as much
problems and has delivered as much physics as string theory.
The major problem is of course the lack of a definite model for the
dynamics, which is not quite satisfactory for a physical theory... it
seems indeed that all "hamiltonian constraints" put forward so far
fail to give general relativity in some classical limit.
Even discreteness of area and volume, which Sal mentioned as results
of LQG, would be truly sound results if our world were euclidean and
the local isometry group were compact... given that we live in a
lorentzian world, there is no guarantee that the correct dynamics will
preserve the corresponding operators as hermitean.
On the string theory account, Sal failed to mention an historical
analogue which IMO could be appropriate: the Ptolemaean theory of
what we now call the solar system.
The fundamentals of the theory are strikingly simple: all the planets
move on large spheres centered on the Earth. However, to account for
the observed motions we have to attach little spheres on the large
spheres and even tiny spheres on the little spheres: of course, by
introducing a sufficient number of epicycles we can reproduce any
planetary motion. Ptolemaean theory, despite being "true", is not a
physical theory because it is not predictive and it is not
falsifiable.
String theory, as it is formulated (or non-formulated... we don't even
know the "true" degrees of freedom) right now, has the same virtues
and suffers from *exactly* the same diseases. This is a bare fact,
which should raise on those working in the field at least a little
doubt about being in possess of the Book of Nature.
> Rovelli's paper just highlights the fact that string theory - as of
> today - has no real competitors.
And this is too bad, because string theory - as of today - is not much
of a competitor as a physical theory, according to criteria other than
the huge amount of *mathematics* that has been - and is waiting to be
- extracted from it.
best
S.A.
Aaron Bergman
In article <bmqhsn$22m$1...@inky.its.caltech.edu>,
ke...@inky.its.caltech.edu (Kevin A. Scaldeferri) wrote:
> In article <575262ce.03101...@posting.google.com>,
> Jeffery <jeffery...@mail.com> wrote:
> >
> >ke...@its.caltech.edu (Kevin A. Scaldeferri) wrote in message
> >news:<bmar9a$ad3$1...@clyde.its.caltech.edu>...
>
> > String theory predicts supersymmetry, which is not
> >> observed.
> >
> >Because supersymmetry breaks around 1 TeV
>
> Is it a prediction of string theory that SUSY breaks at that scale?
> Or is that just a number picked to be tantalizingly outside of current
> experimental reach.
It's picked to solve the naturalness problem (but you knew that.)
[...]
> >> As far as I know, no one knows how to get string theory not to predict
> >> that all observed particles are massless, except in a very handwaving
> >> manner.
> >
> >That is not true. I could show you the particle spectrum of the E_8 x
> >E_8 superstring compacified on a Calabi-Yau manifold. The fermions do
> >have mass.
>
> Okay, what are the masses? Let me guess, in conflict with experiment?
We don't want them to have masses. The fermions (except possibly the
neutrino) are all massless in the standard model. You do get massless
fermions and Yukawa couplings.
As for what they are, beats me.
> >> There is also an issue of the cosmological constant, which again
> >> can only be dealt with in a handwavy manner, as far as I know.
> >
> >Superstring theory contains scalar fields that by rolling down their
> >potentials could give inflation, and then the current acceleration of
> >the expansion.
>
> Sounds handwavy to me.
Getting inflation out of string theory is hard. He's making it too easy.
There's a conference at UCSB right now about all of this:
<http://online.kitp.ucsb.edu/online/strings03/>
[...]
> > For instance, string theory does not predict 10 dimensions that
> >would be observable by us, because the extra ones are compactified.
> >So, in what way is string theory inconsistent with experimental data?
>
> It is (so far as I can see) inconsistent with experimental data in the
> same way as all the other beyond-the-SM theories whose naive
> predictions are incorrect and which require various contortions to
> squeeze out something which isn't contradicted.
Contortions is a bit stronger than I'd use, but, yeah. The naive
predictions of string theory (you're simply not going to figure out how
to detabilize flat 10 or 11D, 32 supersymmetries as best I can tell) are
wrong. But, we're not naive and it doesn't mean that string theory is
wrong.
Aaron
John Baez
Aaron Bergman <aber...@physics.utexas.edu> wrote:
>In article <bm6s68$llf$1...@glue.ucr.edu>, ba...@galaxy.ucr.edu (John Baez)
>wrote:
>> People interested in this notion of "the string theory crackup"
>> may enjoy - or be infuriated by - Carlo Rovelli's new paper:
>If his goal is to get string theorists to listen to his position, he
>does a pretty bad job of it.
Yeah, he's having way too much fun needling them to make it easy
for them to listen sympathetically. Maybe he's trying to influence
students who are still making up their minds what to do. Or maybe
he just feels like saying publicly what he's been saying privately
for years! I don't know. Maybe I'll ask him sometime.
As for whether it helps to increase the polarization between
stringies and loopies like this, I don't know. Ashtekar is
taking a very different line, running that workshop with Nicolai
at the Albert Einstein Institute to get string theorists and
loop quantum gravity people to talk constructively with each
other.
John Baez
Tim S <T...@timsilverman.demon.co.uk> wrote, regarding Rovelli's
characters "Simp" and "Sal":
>I think these names are short for Simplicio and Salviati, 2 of the people in
>Galileo's _Dialogo sopra i due massimi sistemi del mondo_ (_Dialogue on the
>two chief world systems_). Simplicio is the Aristotelian and Saliviati is
>his opponent.
Right - and I believe Rovelli quotes this dialog at one point in
his own story, both in translation and in the original Italian.
John Baez
>Carlo Rovelli obviously made this whole thing up, and was trying to
>write a physics paper in the form of a dialog in a manner similar to
>Galileo. I will admit that when I first came to this newsgroup, I
>thought that John Baez was doing the same thing in the "Wizard and Oz"
>threads. I thought that he was writing both the Wizard and Oz posts.
Heh.
>Later, I realized that Oz really was a different person.
Right. I've even met him, and he didn't look at all like me.
>Throughout Carlo Rovelli's paper he goes on and on about how there is
>no experimental evidence for string theory, but there is no
>experimental evidence for loop quantum gravity either.
True.
>He spends the
>entire time saying what he thinks is wrong with string theory but does
>not say why he thinks loop quantum gravity is better.
That's not true. He actually spends a lot of time saying
why he thinks loop quantum gravity is better. You may not
agree with it, but he certainly does say it.
Lubos Motl
Hi Jaime,
> I hadn't ever read a so clear opinion on the battle for conquering the
> primacy over QG related theories. Regarding the analogy to a
> commercial enterprise - Yes, both have been a successful one, selling
> books, lectures etc. etc. worldwide.
I think that this is a good context to announce that PBS will be
broadcasting a three-hour-long show "The Elegant Universe" on October 28th
and November 4th - about Einstein's life and dreams and especially string
theory. A lot of computer-assisted animations will make it a show worth
watching. The full program will be available on their web:
http://www.pbs.org/wgbh/nova/elegant/
There are several new popular scientific books that will be published in
2004. Cambridge University Press will print a textbook of string theory
for undergrads - which looks really good - and a book about loop quantum
gravity. Also, a very informative and entertaining book about extra
dimensions will be released. In February, a very successful a talented
physicist (and writer) will publish his or her new excellent book, and I
suppose it will be a bombastic news for all the readers. Stay tuned. ;-)
Best
Alfred Einstead
Aaron Bergman <aber...@physics.utexas.edu> wrote:
> > People interested in this notion of "the string theory crackup"
> > may enjoy - or be infuriated by - Carlo Rovelli's new paper:
> My personal opinion is that each 'side' of this has people who don't
> make the greatest advocates for their position. Rovelli is practically
> the archetype for this.
Actually, I'm still waiting for both Motl and Baez from an earlier
incarnation of this discussion to actually notice the key point
in the article "Strings, Loops and Quantized Gravity." (which
followed up on the thread "Loop Quantum Gravity vs. M-Theory")
which they both completely missed reading in the process of replying
to the inessential preliminaries in it.
The key parts are reproduced below.
Subject: Strings, Loops and Quantized Gravity
From: whop...@csd.uwm.edu (Alfred Einstead)
Date: 19 Jun 2001 14:48:10 -0700
>Perhaps another perspective will offer some insights.
>
>It's a little known fact that a cone is flat. It only looks curved,
>but the curvature is entirely concentrated at one point -- the vertex.
>
>The vertex can be characterized, precisely, by [...] its deficit angle.
>
>In effect, the curvature of that surface measures the density of
>vertices on the sheet.
>
>So, a long time ago, I got this really neat idea: let's create
>a particle-based theory of gravity by pretending that all of
>space-time curvature actually resides at points and that
>these vertices are actually the particles of gravity.
>
>Of course, as you know, this isn't possible [...] because the key items
>of interest are NOT points, but subspaces of (n-2) dimensions. Thus, in 3-D,
>one has to conceive of the curvature as being concentrated on ...
... strings.
> If you parallel transport a x-y-z coordinate frame in a circle [...]
> then the frame will come back with a fixed rotation ONLY IF the loop
> links one of these singular lines or circles. And, like before,
> one finds that associated with each singularity is a characteristic
> value which indicates by how much a general frame will be transformed
> for each cycle it takes around one of these singularities...
... in an otherwise flat background spacetime.
>A "particle-based" theory of gravity would then actually be a
>string theory for these curvature singularities. And the
>manifold, itself, would be thought of as being nothing more
>than a flat space populated by a dense thicket of these
>singularities. But the underlying space is purely secondary.
>The manifold really reduces to NOTHING BUT a these singularities
>and the "underlying space" is really just us projecting a geometry
>on them by pretending they reside on a singular locally-flat
>manifold.
... which means you both have a background and at the same time
you don't.
To carry out the corresponding generalization, strings (and
more generally branes) would be endowed with singular
curvature in the space they're contained in and (indeed), the
space itself would be flat apart from these singularities.
For instance, for N-2 branes, the curvature singularities
would be characterized by ... loop invariants, generalizing
the notion of deficit angle.
Thomas Larsson
Aaron Bergman <aber...@princeton.edu> wrote in message news:<slrnbou2da....@cardinal1.Stanford.EDU>...
> IIRC, SUSY does favor a light Higgs. However, last I heard
> Tevatron's been having issues with beam intensity, so it's not
> clear what they could see.
>
The Tevatron has been taking data for half a year, and the quality
of Run II data now exceeds Run I. Some exciting new results have
already been presented:
"At the Lepton-Photon conference at Fermilab in August, the D0
collaboration announced the most stringent limit so far on the
scale of large extra dimensions, based on 205 pb^-1 of data from
Tevatron Runs I and II combined. The new limit on the fundamental
Planck scale is 1.37 TeV (in the GRW convention). "
(Cern Courier 43:8, October 2003)
Maybe this result already rules out extra dimensions, large enough
to explain the electro-weak scale at ~0.8 TeV. After all, 1.37 > 0.8.
Starblade Darksquall
> even say whether he is a string theorist or not), but whose number of
> citations is much above 10,000 - analyzed some claims of loop quantum
> gravity, for example its "background independence". Loop quantum gravity
> does not even seem to respect Lorentz invariance (even this elementary
> question is open in LQG) - it works in a non-relativistic form of a
> Hamiltonian formalism. General covariance in LQG is, of course, even more
> controversial because it is not clear whether LQG is compatible with large
> smooth manifolds whose existence is necessary for general covariance to be
> meaningful. LQG also requires a pre-existing manifold for its definition -
> for example, LQG does not allow topology change.
>
> Best wishes
> Lubos
>
Those are some weaknesses of LQG... but I think not all canonical
quantum gravity type theories suffer from the same problem. Think of
causal set theory. It works fine, as long as there are no timelike
loops. However, if we base a canonical quantum gravity theory on the
path light would trace out, then we could easily restore Lorentz
Symmetry.
The thing I don't like about perturbative quantum gravity is, simply
that it's perturbative. That is, it assumes that gravity has a
'graviton' that's just like the photon, weak gauge bosons, and the
gluons. This may be what the problem is. Whereas the photon, weak
gauge bosons, and the gluons occupy space, the graviton IS space, so
the 'classical' (IE non quantum gravity) wave equations may not even
apply to it, or not in the same way.
BTW, I heard there was a website devoted to quantum gravity... an
official website, kind of like a website to a school. It's the one I
also heard John Baez is going to. Is this true? Or did I just hear
wrong?
(...Starblade Riven Darksquall...)
A. Garrett Lisi
> On Thu, 16 Oct 2003, A. Garrett Lisi wrote:
>
> > "String theory is the luminiferous aether theory of the twenty-first
> > century."
>
> Well, I am not sure whether this sentence was written as a serious topic
> for new discussions, or as a not-too-entertaining joke,
Considering that the majority of theorists are dedicating their lives to
research in string theory, it is indeed a very serious topic. Or maybe
a very dark joke.
> but at any rate,
> string theory is just the opposite of the theory of aether. String theory
> respects all major symmetry principles, and it shows that all physical
> objects are dynamical, they affect each other, and they have a common
> origin. String theory is the simplest possible and the most natural
> consistent extension of the successful theories in physics, including
> gauge (quantum field) theories and general relativity.
Aether theory, past and present, is rich in symmetry. Aether theory
showed that all physical objects, and how they effect each other, had
a common origin as dynamical motions of the aether. Aether theory
was the simplest possible, most beautiful, and most physically natural
description of electromagnetism -- the most successful theory in
physics. And aether theory was extended to describe all physics, with
particles described as vortices in the aether.
ALL these things were said and believed of aether theory, the dominant
theory of its age. How could waves propagate without a transmission
medium, any more than strings could vibrate without a spacetime
background?
I do not wish to defend aether theory. It was wrong -- it demanded a
larger framework of complicated mathematics and handwaving
arguments than was justified by the phenomena it was able to explain.
A much simpler description of electromagnetism eventually emerged
that was without the superfluous theoretical baggage and still fit
experiment.
> Loop quantum gravity is much, much more similar to aether theory - it
> tries to construct the spacetime from explicit constituents, much like
> aether is made out of some sort of molecules. Consequently, loop quantum
> gravity most likely breaks Lorentz invariance, much like aether does.
> Aether has the advantage that it allows the spacetime to be smooth at long
> distances, which is unlikely to be the case of loop quantum gravity.
Loop quantum gravity is in a nascent stage of exploration -- it shows
potential but is just getting started and has problems, much as string
theory did in the early nineties. However, I don't wish to defend it
either as it is, as you say, in a similar state of being quite possibly
wrong.
But defending the present competition is not my point. The point of my
argument, and of my initial observation, is that string theorists are hugely
overconfident in the physical truth of their theory given the complete lack
of experimental support. The whole physics community, in the 19th
century and now, can go down dead ends. These dead ends can be wildly
successful in gaining popular support, seemingly independent of their real
promise, purely because of the political and human nature of conducting
theoretical research. I think history may be repeating itself, and the
competing theory that will eventually supplant string theory just hasn't
been found yet. But we should be looking for it. That is my point.
> Be sure that string theorists discuss Rovelli's paper. We just returned
> from a dinner with Gary Gibbons, and Rovelli's paper was one of our
> topics. Well, we are certainly not going to write many papers based on the
> ideas from Rovelli's paper - because there are not too many new scientific
> ideas in that paper :-), and writing papers in Rovelli's format would sort
> of mean to accept his standards (which would mean to fall into much lower
> scientific categories).
The fact that Rovelli's paper contained nothing new is the point of my
observation -- history is repeating itself. ;)
But, his paper was primarily to point out that the emperor has no clothes.
For that he is to be applauded -- or attacked, depending on your political
camp.
> Some string theorists find the paper entertaining,
> many other string theorists are shocked by the visibly painful situation
> of loop quantum gravity today, as demonstrated by many things including
> Rovelli's paper. A renowned physicist who attended the dinner tonight
> pointed out the alarming low productivity of loop quantum gravity within
> science. While the amount of money that flow to loop quantum gravity today
> - and the number of people in this field - is about 10 per cent of the
> people in string theory, its production is closer to 1 per cent of the
> production in string theory.
Yes, 1% of nothing is still nothing.
Nevertheless, LQG is just getting started, whereas strings have been
around for quite a while now.
> The same physicist - whose identity won't be revealed here (and I won't
> even say whether he is a string theorist or not), but whose number of
> citations is much above 10,000 - analyzed some claims of loop quantum
> gravity, for example its "background independence". Loop quantum gravity
> does not even seem to respect Lorentz invariance (even this elementary
> question is open in LQG) - it works in a non-relativistic form of a
> Hamiltonian formalism. General covariance in LQG is, of course, even more
> controversial because it is not clear whether LQG is compatible with large
> smooth manifolds whose existence is necessary for general covariance to be
> meaningful. LQG also requires a pre-existing manifold for its definition -
> for example, LQG does not allow topology change.
I am not the best suited person to counter these points raised against LQG.
But these problems you state seem no different than those of Regge
calculus, which is theoretically useful. Also, from my limited understanding,
I don't think LQG requires a pre-existing manifold or disallows topology
change.
However, Rovelli's paper, and my analogy with aether theory, is really political
commentary. In particular, I observe that the physics community, through
human social and economic pressure, can influence theorists to proceed down
dead ends en mass. It is a herd mentality. Observe your very phrasing above,
in which you refer to a "renowned" physicist who's very name you shudder at
mentioning because of his greatness. These are the kinds of men who
influence an age. In the 19th century these men were Lorentz, Thomson
(Kelvin), Stokes, Planck, Cauchy, Poincare', ... -- all strong proponents and
researchers of aether theory.
> Best wishes
> Lubos
> Superstring/M-theory is the language in which God wrote the world.
Ah, so it comes down to religious faith then... I can't argue with that. (Yes, I
know it's your sig file, and not part of your argument, but it is telling.)
This can be a slow process. Many proponents of aether theory only
relinquished their faith in their theory at their deaths. Even today many
people do research in aether theory -- it is that beautiful and compelling
-- only now we call them crackpots, since with hindsight we know it was
a dead end.
But, in what direction to point young physicists... Running with the herd
is safe. It is well funded. It is taking the blue pill. It is easiest to search
under the light post, and to drill where the wood is soft. But really it
should be, and to an extent is, up to them.
-Garrett
alejandro.rivero
> Aaron Bergman <aber...@physics.utexas.edu> wrote:
> > > People interested in this notion of "the string theory crackup"
> > > may enjoy - or be infuriated by - Carlo Rovelli's new paper:
> > I was waiting for someone to post about this.
> > My personal opinion is that each 'side' of this has people who don't
> > make the greatest advocates for their position. Rovelli is practically
> > the archetype for this.
>
> Actually, I'm still waiting for both Motl and Baez from an earlier
> incarnation of this discussion to actually notice the key point
> in the article "Strings, Loops and Quantized Gravity." (which
> followed up on the thread "Loop Quantum Gravity vs. M-Theory")
> which they both completely missed reading in the process of replying
> to the inessential preliminaries in it.
Well, actually, I can not see how it relates to the issue of the
background, if we are speaking, as I supposse, of the underlying
10 or 26 dimensional space where the string is moving.
> >It's a little known fact that a cone is flat. It only looks curved,
> >but the curvature is entirely concentrated at one point -- the vertex.
Indeed this fact is well known. Now I wonder, on other hand, if the
so called "conical singularities", as used by Crane to define
elementary particles, share this property.
> >So, a long time ago, I got this really neat idea: let's create
> >a particle-based theory of gravity by pretending that all of
> >space-time curvature actually resides at points and that
> >these vertices are actually the particles of gravity.
> >
> >Of course, as you know, this isn't possible [...] because the key items
> >of interest are NOT points, but subspaces of (n-2) dimensions. Thus, in 3-D,
I am not sure of how the argument translates to n dimensions. Remember
that the curvatures expressed with Riemann Tensor are not the
only ones you could generalize.
> >one has to conceive of the curvature as being concentrated on ...
>
> ... strings.
>
You mean, one-dimensional subvarieties. I guess that string theory
carries some aditional structure beyond worldsheets.
On other hand, if I understand correctly, spin-networks are seen as
line configurations that actually codify the curvature of the space,
thus your description seems easier to fit in the LQF world. Then, it
seems that you are suggesting to employ a spin network as if it were a
string.
Lubos Motl
> The Tevatron has been taking data for half a year, and the quality
> of Run II data now exceeds Run I. Some exciting new results have
> already been presented:
> ...
> Maybe this result already rules out extra dimensions, large enough
> to explain the electro-weak scale at ~0.8 TeV. After all, 1.37 > 0.8.
I am not sure which "electroweak scale at 0.8 TeV" model you really mean
(don't we know that the electroweak scale simply IS at 0.2 TeV or so?),
but at any rate, I don't find such new limits too exciting. I always
believed - and still believe - that the conventional models with the GUT
scale around 10^{16} GeV and the fundamental scale nearby are much more
likely and more natural than the models with large/warped extra
dimensions. It would be much more exciting if they found them! ;-)
______________________________________________________________________________
E-mail: lu...@matfyz.cz fax: +1-617/496-0110 Web: http://lumo.matfyz.cz/
phone: work: +1-617/496-8199 home: +1-617/868-4487
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Lubos Motl
On 18 Oct 2003, anbar wrote:
> Sal is a a little too enthusiastic about the loop approach (although
> I'd now call it the state sum approach), which is plagued by as much
> problems and has delivered as much physics as string theory.
Well, it depends on what you mean by physics. If you used the word
"physics" in the same sense as string physicists, LQG has delivered less
than one percent of physics that has been delivered by string theory. If
you mean physics beyond the Standard Model that has already been
experimentally tested, then not only string theory and loop quantum
gravity, but any other theory is logically in trouble. If SSC were not
cancelled, we would have a lot of data today. But because it was
terminated, everyone must add 10-15 years of waiting.
> The major problem is of course the lack of a definite model for the
> dynamics, which is not quite satisfactory for a physical theory...
> it seems indeed that all "hamiltonian constraints" put forward so far
> fail to give general relativity in some classical limit.
This, by the way, also means that loop quantum gravity, as formulated
today, is a non-relativistic theory. By this adjective I mean that it does
not respect the rules of special relativity, both in the Hilbert space as
well as the path integral formalism. Therefore the claims about
"background independence" are unrealistic, because even much smaller
physical requirements do not hold in loop quantum gravity.
In the Hilbert space formalism, the impossibility to incorporate Lorentz
invariance manifests itself as the problems to find a Hamiltonian
constraint. What about the path integral formalism?
It is not hard to see that if one inserts a generic "spinfoam" into a
Minkowski space, a preferred reference frame will be chosen - it is the
frame in which the "spinfoam" seems more or less isotropic with respect to
a *euclidean* metric inserted to the *Minkowski* spacetime. In order to
respect Lorentz invariance, arbitrarily strongly deformed simplices would
have to appear in the path integral with the same frequency as the
non-singular ones. This would probably be another reason that makes the
long-distance physics singular.
Note that this argument is quite general: all attempts to discretize
Minkowski spacetime should be either singular OR they should violate
Lorentz symmetry. All attempts to discretize gravity in dimension d>3 have
failed so far.
Of course, one might continue to hope that the path integral formalism may
be defined in the Euclidean spacetime.
The analyses of 10-j symbols also indicate that some singular
configurations *do* dominate the asymptotic 10-j symbols, and consequently
the whole path integral. This type of divergence is presumably a
discretized version of the multi-loop divergences that we know from
canonical quantization of GR.
Pure quantum gravity simply *is* plagued by divergences. New physics is
needed at very short distances to regulate these problems away. One of the
beauties of string theory is that it shows that in order to reconcile
gravity with quantum mechanics, other forms of matter - fermions, gauge
bosons, Higgses and so on - *must* exist as well.
> Even discreteness of area and volume, which Sal mentioned as results
> of LQG, would be truly sound results if our world were euclidean and
> the local isometry group were compact...
That's right. The formal equivalence between the Euclidean spacetime and
Minkowski spacetime holds in all quantum field theories that we use every
day: the amplitudes in these two signatures are related by analytical
continuation. It also holds in string theory - string theory is radically
conservative and it shares all these "good features" with quantum field
theories. Because loop quantum gravity does not have spacetime to start
with, it is not really allowed to analytically continue the amplitudes.
The problems with Minkowski spacetime in loop quantum gravity reflect the
fact that the analytical continuation is impossible once the background is
completely removed.
Let me say something about the area quantization. The quantization of the
angular momentum is a beautiful result of quantum mechanics, and the area
quantization is an attempt to prove the same thing for geometry.
Therefore, the very idea is nice but not new at all, and it was not too
difficult to redefine the variables in such a way that some quantities
will behave like the angular momentum. However this choice of variables is
just a proposal - and there exists a lot of evidence that it is a proposal
that does not work.
Although the area quantization is a nice result, it would be
experimentally untestable - even in principle. *All* of geometry in this
spacetime (even if this spacetime existed) would be quantized, and this
simply limits our ability to measure areas. One Planck length is really a
minimal error of the length measurements in loop quantum gravity, much
like it is the case in string theory. Therefore an exact formula for the
allowed areas - with a subplanckian precision - is unphysical.
An exact value of the area also violates the uncertainty principle because
the time-derivative of the area is roughly speaking a canonically dual
variable to the area itself. Consequently, if the area is well-defined,
the uncertainty of its time-derivative must be infinite. In this case it
means that a component of metric would be infinite. In reality, all these
quantities become finite. The values of the area become fuzzy, and a
precise formula - which claims to know it much more precisely than with
Planckian precision - has no physical meaning.
> String theory, as it is formulated (or non-formulated... we don't even
> know the "true" degrees of freedom) right now, has the same virtues
> and suffers from *exactly* the same diseases.
No, this statement is absolutely wrong. String theory is *exactly* the
first and the only known theory that does *not* allow us to add any
corrections such as some new epicycles. We can study a *different* vacuum
of string theory, much like we are allowed to study ice, water, steam -
different phases of water. But once we say which solution we study, the
theory is completely unique and everything is determined, and in many
cases that we know well, it is even determined in practice.
In the 19th century, you might imagine to add almost arbitrary corrections
(and of course, new fields) to the electromagnetism. In quantum field
theory this freedom was reduced - we only wanted to add renormalizable
terms, roughly speaking, but we could have still added new fields etc. In
string theory you are not allowed to add any new fields or new
interactions, otherwise you make the theory inconsistent. All parameters
are dynamical, and eventually they become either time-dependent, or they
develop a potential whose minimum can be calculated.
Yes, we must still make a discrete choice, and our today's understanding
seems to say that the number of solutions is pretty large. But there are
no *continuous* dimensionless static constant/parameters that can be used
to deform string theory. This fact is totally essential for our feeling
that we are on the right track.
> This is a bare fact,
It is not a bare fact but rather a rudimentary misunderstanding of string
theory.
> which should raise on those working in the field at least a little
> doubt about being in possess of the Book of Nature.
Well, talented bartenders should be given more time to study string
theory, so that their contribution could be more relevant than these
sentences.
> And this is too bad, because string theory - as of today - is not much
> of a competitor as a physical theory,
Fortunately this sentence is meaningless, too. Competitor of "whom"?
Competitor of general relativity (or quantum field theory)? Well, string
theory is a much grander structure that includes all good ideas about GR
(and quantum field theory) - without them being put into spacetime. There
is no competition going on here. As long as we want to continue with
physics beyond the Standard Model, string theory is the only game in town,
the only well-established framework to continue.
Best wishes
Lubos
Lubos Motl
A. Garrett Lisi wrote:
> "String theory is the luminiferous aether theory of the twenty-first
> century." ...
>
> > Lubos Motl wrote:
> > Well, I am not sure whether this sentence was written as a serious topic
> > for new discussions, or as a not-too-entertaining joke,
>
> Considering that the majority of theorists are dedicating their lives to
> research in string theory, it is indeed a very serious topic. Or maybe
> a very dark joke.
Lubos Motl:
Theoretical high-energy physicists don't have too much new experimental
data, but there is still a whole generation (or two generations) of
theoretical physicists that don't want to give up. They must think about
very deep ideas, and a significant part of them believes that string
theory is the right way to go. String theory "feels" good; it is very
unique; it always turns out to be consistent even though the consistency
seems to follow from mathematical facts that look like miracles.
Surprisingly, it predicts the world in a qualitative agreement with the
observed one, without putting the specific field into the theory. String
theory has already taught us a lot of things about quantum field theories,
many fields of mathematics and other intellectual enterprises, and even if
these were the only achievements, they would be big ones. I am among those
who don't believe that this will be the only achievement.
It is not difficult to leave theoretical physics, and there are many
example of physicists that have left, and many will follow. I don't see a
problem here. People continue to study theoretical physics, and they are
ready to take the risk. The only alternatives that would be:
1. Cancel research in theoretical physics.
2. Ask laymen to decide what theoretical physicists should believe
and what sort of theories should they study.
In my opinion, both possibilities would return this important science to
the Middle Ages. It is very important that people are trying to study the
fundamental questions, even though it is difficult given the small amount
of new experimental data, and it is also very important that the
researchers enjoy intellectual freedom to work on the problems that they
find relevant, challenging, interesting, and important. An honest
physicist could simply *not* work on something that is wrong. I am just
convinced that I have collected an overwhelming scientific evidence that
loop quantum gravity, as well as any other approach that claims to be an
alternative of string theory to construct a quantum gravitational theory,
simply IS wrong. You can't expect *me* to seriously work on some piles of
ideas that pretend to be important. The only thing that you could do is to
fire all string theorists on political grounds, even though most of them
objectively had the best result among all physicists in their class etc.
Do you really want to do that? Do you want laymen and politicians decide
which physics is correct? There has not been a single example in history
of physics when physicists were wrong and a "large group of laymen" was
right.
> Aether theory, past and present, is rich in symmetry.
No, it's not. Aether theory breaks symmetries, especially the Lorentz
symmetry.
> Aether theory showed that all physical objects, and how they effect
> each other, had a common origin as dynamical motions of the aether.
No, it did not. The aether theory would prevent us from understanding the
common (relativistic) origin of the electric and magnetic fields, as well
as the electromagnetic and weak nuclear interactions. The only "virtue" of
aether theory was to reconcile the new electromagnetic phenomena with the
old mechanistic prejudices. String theory is not doing anything like that
- just the opposite is true.
> Aether theory was the simplest possible, most beautiful, and most
> physically natural description of electromagnetism -- the most
> successful theory in physics. And aether theory was extended to
> describe all physics, with particles described as vortices in the
> aether.
Aether theory is extremely ugly, the specific theories that had a chance
to reproduce electromagnetism were extremely contrived, and as the
development in the early 20th century showed, it was unnatural to expect
this explanation of electromagnetism.
> ALL these things were said and believed of aether theory, the dominant
> theory of its age. How could waves propagate without a transmission
> medium, any more than strings could vibrate without a spacetime
> background?
We are not saying anything like that nowadays. Maybe loop quantum gravity
people say that the space must be made of many "atoms of space", much like
the "atoms of aether". Ask them why they say so - but at any rate, it is
their specific idea how space at short distances is regulated by quantum
mechanics. It is not our idea.
> Loop quantum gravity is in a nascent stage of exploration -- it shows
> potential but is just getting started
It has been getting started for 20 years, and if I include similar
(failed) attempts to discretize gravity, it has been getting started for
45 years.
> But defending the present competition is not my point. The point of my
> argument, and of my initial observation, is that string theorists are hugely
> overconfident in the physical truth of their theory given the complete lack
> of experimental support.
You know, such a statement has absolutely no value unless it is supported
by an argument or a calculation. A comparison of string theory with aether
theory certainly does not count as an argument... You know, physicists
demand certain intellectual qualities from an argument that they treat
seriously.
> The whole physics community, in the 19th century and now, can go down
> dead ends.
Instead, you should tell us what Lorentz and Einstein said about the
aether, if you think that we are as unreasonable as Maxwell was. It would
be certainly more useful than your fuzzy and offending statements.
> theoretical research. I think history may be repeating itself, and the
> competing theory that will eventually supplant string theory just hasn't
> been found yet. But we should be looking for it. That is my point.
Try to look for it. I am doing it 50 per cent of my time. It just seems
obvious that such a "second" theory does not exist.
> But, his paper was primarily to point out that the emperor has no clothes.
Which emperor?
> Nevertheless, LQG is just getting started, whereas strings have been
> around for quite a while now.
Ashtekar invented his new variables in 1986, superstring theory started as
a big field in 1984 after Green and Schwarz discovered the anomaly
cancellation. The roots of string theory are in the late 1960s - the
Veneziano amplitude, Regge physics etc. - and the roots of loop quantum
gravity and discretization of space are in the early 1960s when people
started to discretize space in similar naive ways.
These two candidates are equally old.
> I am not the best suited person to counter these points raised against
> LQG. But these problems you state seem no different than those of
> Regge calculus, which is theoretically useful. Also, from my limited
> understanding, I don't think LQG requires a pre-existing manifold or
> disallows topology change.
Ask Lee Smolin or anyone and you will learn that LQG forbids topology
change.
> However, Rovelli's paper, and my analogy with aether theory, is really
> political commentary.
That's right. Therefore you should not expect that it will influence
*physics* in any way.
> Ah, so it comes down to religious faith then... I can't argue with
> that. (Yes, I know it's your sig file, and not part of your argument,
> but it is telling.)
This is however a very rational type of religion.
Best wishes
Lubos
______________________________________________________________________________
E-mail: lu...@matfyz.cz fax: +1-617/496-0110 Web: http://lumo.matfyz.cz/
phone: work: +1-617/496-8199 home: +1-617/868-4487
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Peter Woit
Lubos Motl wrote:
>String theory is *exactly* the
>first and the only known theory that does *not* allow us to add any
>corrections such as some new epicycles. We can study a *different* vacuum
>of string theory, much like we are allowed to study ice, water, steam -
>different phases of water. But once we say which solution we study, the
>theory is completely unique and everything is determined, and in many
>cases that we know well, it is even determined in practice.
>
>In the 19th century, you might imagine to add almost arbitrary corrections
>(and of course, new fields) to the electromagnetism. In quantum field
>theory this freedom was reduced - we only wanted to add renormalizable
>terms, roughly speaking, but we could have still added new fields etc. In
>string theory you are not allowed to add any new fields or new
>interactions, otherwise you make the theory inconsistent. All parameters
>are dynamical, and eventually they become either time-dependent, or they
>develop a potential whose minimum can be calculated.
>
>Yes, we must still make a discrete choice, and our today's understanding
>seems to say that the number of solutions is pretty large. But there are
>no *continuous* dimensionless static constant/parameters that can be used
>to deform string theory. This fact is totally essential for our feeling
>that we are on the right track.
>
>
>
String theorists continually repeat the claim that string theory
is a unique theory, with all parameters determined dynamically.
Then they tell us that they don't know what this unique theory
is, what the parameters are, or
what the dynamics is that determines the parameters.
Some of us find this extremely unconvincing, especially
when they are unwilling or unable to distinguish between
what is known to be true and what they would like to be
true.
When asked to justify this uniqueness claim, many
basically fall back on "Ed Witten says so", which is
quite a good argument, but not good enough. If you
try and get them to actually write down a consistent
theory, what they write down (for instance M-theory
on a flat 11-dim background) may or may not be
consistent, but clearly looks nothing like the real
world.
String theorists should stop going on about how
beautiful, wonderful and unique their theory is
until they actually have figured out what (and
whether) it is.
Jeffery
g...@lisi.com (A. Garrett Lisi) wrote in message
news:<ac79fe0e.0310...@posting.google.com>...
> Lubos Motl <mo...@feynman.harvard.edu> wrote:
> > On Thu, 16 Oct 2003, A. Garrett Lisi wrote:
> > > "String theory is the luminiferous aether theory of the twenty-first
> > > century."
> > string theory is just the opposite of the theory of aether. String
> > theory respects all major symmetry principles, and it shows that
> > all physical objects are dynamical, they affect each other, and
> > they have a common origin. String theory is the simplest possible
> > and the most natural consistent extension of the successful
> > theories in physics, including gauge (quantum field) theories and
> > general relativity.
> Aether theory, past and present, is rich in symmetry. Aether theory
> showed that all physical objects, and how they effect each other, had
> a common origin as dynamical motions of the aether. Aether theory
> was the simplest possible, most beautiful, and most physically natural
> description of electromagnetism -- the most successful theory in
> physics. And aether theory was extended to describe all physics, with
> particles described as vortices in the aether.
>
> ALL these things were said and believed of aether theory, the dominant
> theory of its age. How could waves propagate without a transmission
> medium, any more than strings could vibrate without a spacetime
> background?
>
> I do not wish to defend aether theory. It was wrong -- it demanded a
> larger framework of complicated mathematics and handwaving
> arguments than was justified by the phenomena it was able to explain.
> A much simpler description of electromagnetism eventually emerged
> that was without the superfluous theoretical baggage and still fit
> experiment.
The flaw in the analogy is that the aether theory contradicted the
Michelson-Morley Experiment, that the speed of light is the same no
matter what direction you measure it in, regardless of the motion of
the Earth, so it has the same value for all observers. Therefore the
aether theory contradicted experimental data. As I said to Kevin on
another thread, string theory is currently consistent with all
experimental data, since it exists at higher energies that we can
currently reach in our particle accelerators. The fact it has 10 or 11
dimensions is not a contradiction since the extra dimensions are
compacified. You judge the success of a theory based on how well it
explains what it is intended to explain, and it's consistency with
experimental data. The aether theory contradicted the Michelson-Morley
Experiment. String theory is currently consistent with all
experimental data.
Jeffery Winkler
George Wilkie
news:<bmteef$rka$1...@glue.ucr.edu>...
> >Throughout Carlo Rovelli's paper he goes on and on about how there is
> >no experimental evidence for string theory, but there is no
> >experimental evidence for loop quantum gravity either.
> True.
> >He spends the
> >entire time saying what he thinks is wrong with string theory but does
> >not say why he thinks loop quantum gravity is better.
> That's not true. He actually spends a lot of time saying
> why he thinks loop quantum gravity is better. You may not
> agree with it, but he certainly does say it.
John,
I'm thinking Rovelli's efforts were more to convince the reader that
there are other approaches to quantum gravity than string theory, not
necessarily that one is better than the other. In the absense of
experimental data, the best we have to go with on which is better is
pure intuition, and I think this is the message he was trying to get
across. That, and that his intution is better, hehe.
It certainly doesn't look like he'll convince anyone well established
in the field, no matter which side of the fence they're on, that
string theory is the crap he makes it out to be. Rather, I think he's
trying to die down the hype for incoming theorists saying "Sure,
string theory can unify all forces, but it also makes extraordinary
claims that take extraordinary evidence. Things aren't looking too hot
for strings right now, so why don't you help let loops catch up in
popularity?"
It did annoy me at how stupid Rovelli made the professor look though.
Having him say things like "I'm not a historian" in response the the
explosion of ideas in the early 20th century, or just flat out not
understanding GR except as a lagrangian could use a little more work.
Maybe that helped make it humorous and perhaps that was his goal, who
knows.
Urs Schreiber
> Note that this argument is quite general: all attempts to discretize
> Minkowski spacetime should be either singular OR they should violate
> Lorentz symmetry.
A _random_ discretization apparently can respect Lorentz
symmetry in the sense that the probability distribution is
invariant. This is for instance mentioned on pp.9 of Sorkin's
hep-th/0309009.
Lubos Motl
> Those are some weaknesses of LQG... but I think not all canonical
> quantum gravity type theories suffer from the same problem. Think of
> causal set theory. It works fine, ...
Well, it might work fine, but it is not a complete physical theory -
rather a formalism to keep track of some elementary features of spacetime.
> as long as there are no timelike loops. However, if we base a
> canonical quantum gravity theory on the path light would trace out,
> then we could easily restore Lorentz Symmetry.
Do you mean some sort of light cone gauge? ;-) Yes, I think that light
cone gauge is a great way to obtain a theory of quantum gravity in the
Hamiltonian formalism - in fact, string theory has achieved this already
decades ago, and today we have even a nonperturbative version of this
formalism for some simplest backgrounds (with 7-11 large dimensions)
called "Matrix theory".
> The thing I don't like about perturbative quantum gravity is, simply
> that it's perturbative. That is, it assumes that gravity has a
> 'graviton' that's just like the photon, weak gauge bosons, and the
> gluons.
But graviton almost certainly *is* like photon as far as this basic
question is concerned. If you believe general relativity, you must also
believe that gravitational waves exist. Maybe LIGO will detect them soon,
who knows. But according to quantum mechanics, waves with frequency "f"
simply CAN'T carry energy smaller than "E=hf" (except for zero). The
wavefunction of the "Universe" that contains a gravitational wave must be
a periodic function of time, with the period equal to the period of the
classical wave (or its divisor). This simply implies that the energy of
gravitational waves *is* carried in quanta with energy E=hf, just like for
photons, and these quanta are called gravitons.
In fact, we believe that we can study the combined effects of quantum
physics and general relativity much further - for example, we can
calculate the black hole evaporation following Hawking's recipe.
Whatever is your correct theory of quantum gravity, it must eventually be
able to calculate the scattering of gravitons with other particles and
themselves etc. And because gravity is so weak at long distances, it is
obvious that the perturbative calculation of these amplitudes must exist
for large separations. Gravity is in this context a very weak
perturbation, indeed.
What you "don't like" about perturbative gravity is a necessary
requirement for any theory to be a meaningful predictive quantum theory.
One can calculate the Taylor (or asymptotic) expansion of any reasonable
function describing the interactions caused by gravity, and whatever the
answer is, there *must* exist a perturbative approximative scheme
answering these questions. If a candidate for a theory of quantum gravity
is not able to calculate a perturbative S-matrix, it can't ever be
accepted as a serious candidate by particle physicists. If your favorite
theories will *never* be able to calculate something like that (because
you just want these questions to be hidden forever), your theories will
never be viewed as physical theories by particle physicists.
We are not saying that there must exist preferred backgrounds in the
fundamental formulation of the theory or anything like that. We are just
saying that your theory *must* allow one to calculate what really happens
on a given background, and a background similar to Minkowski (or de
Sitter) space exists as we know from our observations. And because the
gravitational force etc. looks morally the same as the electromagnetic
force (and between two electrons, it is even 10^42 times weaker), this
means that the correct theory *must* allow us to calculate the effect of
the gravitons in a perturbative fashion. Canonically quantized pure
general relativity can't do it - it is plagued by divergences - so it is a
wrong candidate for a quantum theory of gravity. String theory can do it,
so it is a correct candidate.
> BTW, I heard there was a website devoted to quantum gravity... an
> official website, kind of like a website to a school. It's the one I
> also heard John Baez is going to. Is this true? Or did I just hear
> wrong?
I've seen many "official" web sites of quantum gravity with a lot of
incorrect information, but let me write a link to a more reasonable
official quantum gravity web site instead:
http://www.superstringtheory.com/
Lubos Motl
> A _random_ discretization apparently can respect Lorentz
> symmetry in the sense that the probability distribution is
> invariant. This is for instance mentioned on pp.9 of Sorkin's
> hep-th/0309009.
Hi Urs, I guess that you meant gr-qc/0309009. Well, the reader should not
get discouraged by the change of the archive. Yes, it sounds convincing
that a Poisson-like random distribution of points does not pick any
preferred reference frame, as Sorkin writes on page 10.
Note that this is very different from spin foams in loop quantum gravity.
One can only obtain Lorentz-invariant ensembles of *points*, but once you
start to connect the "adjacent" points with "edges", you're in trouble
again. Even if you pick a Lorentz-invariant random distribution of points,
connecting them with edges will break Lorentz symmetry unless most edges
are infinitely long in the coordinate space. It is simply because Lorentz
invariance requires that all directions of the edges with respect to the
Minkowski space appear equally frequently, and "most" directions in
Minkowski space are directed along the light cones.
Best
Kevin A. Scaldeferri
Jeffery <jeffery...@mail.com> wrote:
>
>aether theory contradicted experimental data. As I said to Kevin on
>another thread, string theory is currently consistent with all
>experimental data, since it exists at higher energies that we can
>currently reach in our particle accelerators. The fact it has 10 or 11
>dimensions is not a contradiction since the extra dimensions are
>compacified. You judge the success of a theory based on how well it
>explains what it is intended to explain, and it's consistency with
>experimental data. The aether theory contradicted the Michelson-Morley
>Experiment. String theory is currently consistent with all
>experimental data.
And, as I said there, a more accurate statement is that string theory
is believed to be consistent with all experimental data. That is, it
is believed that there exists some stable vacuum in which the extra
dimensions are compactified, and SUSY is broken appropriately, and the
right gauge group is realized, etc.
Arun Gupta
> "physics" in the same sense as string physicists, LQG has delivered less
> than one percent of physics that has been delivered by string theory. If
> you mean physics beyond the Standard Model that has already been
> experimentally tested, then not only string theory and loop quantum
> gravity, but any other theory is logically in trouble. If SSC were not
> cancelled, we would have a lot of data today. But because it was
> terminated, everyone must add 10-15 years of waiting.
"Physics" in the sense of the string physicists appears to be a
different definition from that of the previous 200 years.
There appears to be no conceivable finite amount of experimental data
that can falsify string theory.
Kevin A. Scaldeferri
Aaron Bergman <aber...@physics.utexas.edu> wrote:
>
>In article <bmqhsn$22m$1...@inky.its.caltech.edu>,
> ke...@inky.its.caltech.edu (Kevin A. Scaldeferri) wrote:
>
>> In article <575262ce.03101...@posting.google.com>,
>> Jeffery <jeffery...@mail.com> wrote:
>> >
>> >ke...@its.caltech.edu (Kevin A. Scaldeferri) wrote in message
>> >news:<bmar9a$ad3$1...@clyde.its.caltech.edu>...
>>
>> > String theory predicts supersymmetry, which is not
>> >> observed.
>> >
>> >Because supersymmetry breaks around 1 TeV
>>
>> Is it a prediction of string theory that SUSY breaks at that scale?
>> Or is that just a number picked to be tantalizingly outside of current
>> experimental reach.
>
>It's picked to solve the naturalness problem (but you knew that.)
It solves _a_ naturalness problem. However, doesn't breaking SUSY
introduce a host of naturalness problems of its own?
Jeffery
http://www.superstringtheory.com/
Did you really mean to this site? All unmoderated physics forums
become a mecca for crackpots, and for some reason it's more for this
site than any other site I've ever found. It's Crackpot Heaven, and
it's an embarrassment. It's the mecca to all the crackpots in the
world.
Jeffery Winkler
Urs Schreiber
>
> On 20 Oct 2003, Urs Schreiber wrote:
>
> > A _random_ discretization apparently can respect Lorentz
> > symmetry in the sense that the probability distribution is
> > invariant. This is for instance mentioned on pp.9 of Sorkin's
> > hep-th/0309009.
> Note that this is very different from spin foams in loop quantum gravity.
> One can only obtain Lorentz-invariant ensembles of *points*, but once you
> start to connect the "adjacent" points with "edges", you're in trouble
> again. Even if you pick a Lorentz-invariant random distribution of points,
> connecting them with edges will break Lorentz symmetry unless most edges
> are infinitely long in the coordinate space. It is simply because Lorentz
> invariance requires that all directions of the edges with respect to the
> Minkowski space appear equally frequently, and "most" directions in
> Minkowski space are directed along the light cones.
Sounds plausible. On the other hand there might be
discretizations that circumvent this problem, I'd think.
For instance take a Poisson distributed random causal set a la
Sorkin and draw an edge between every pair of timelike
connectable points. Lorentz transforming the resulting graph
does not affect the probability distribution of the points.
But since the edge structure is determined by time-likeness,
which is invariant under Lorentz transformations, I would
expect that the probability distribution on the links is
likewise invariant.
As another example, it is possible to have "exact" (as opposed
to probabilistic) Lorentz symmetry on the lattice by
appropriately generalizing the function algebra on which it is
realized. For instance hep-th/0310013 demonstrates how that
works using "umbral calculus" on the lattice, which
generalizes the commutative algebra of coordinate functions x
on the lattice by the noncommutative algebra generated by "x
beta", where beta is some "shift operator".
On the other hand, it is not clear why and that we should
expect Lorentz symmetry to be a fundamental and exact
symmetry of a theory of quantum gravity. Of course it could
be.
Is it clear that the non-perturbative formulation of
string/M-theory will necessarily have local Lorentz invariance
on the fundamental level?
Doesn't the matrix model do away with spacetime as a manifold
and replace it with the set of eigenvalues of the matrices?
(At least that's the point of view emphasized by some authors,
such as in hep-th/0310019.) Such a discretization breaks
Lorentz invariance, too, doesn't it?
John Baez
>ke...@its.caltech.edu (Kevin A. Scaldeferri) wrote in message
>news:<bmar9a$ad3$1...@clyde.its.caltech.edu>...
>> String theory predicts supersymmetry, which is not
>> observed.
>Because supersymmetry breaks around 1 TeV, so the masses of the
>supersymmetric particles should around that mass, so that's why you
>don't observe.
You state that "supersymmetry breaks around 1 TeV" as if it
were a fact, or at least a consequence of superstring theory.
But, you can't really do a calculation that starts with string theory
and concludes that supersymmetry breaks around 1 TeV, unless you
cleverly sneak this fact into your assumptions somewhere. In fact,
superstring theory doesn't make *any* firm predictions concerning
supersymmetry breaking. It would be much easier to explain the
complete *absence* of supersymmetry breaking, if that were only the case.
The 1 TeV figure is just a nice figure that would make the existence
of supersymmetry compatible with:
1) current-day experiments
and
2) the desire for unification at energy scales close to where
extrapolated graphs of the running coupling constants of various
forces come close to crossing.
If we drop desire 2), I'm pretty sure there are superstring-inspired
models where supersymmetry breaks at energies much higher than 1 TeV,
that are still compatible with experiment.
>If we don't observe supersymmetric particles at the Large Hadronic
>Collider, then that will be an inconsistency with the data.
Really? It would be great if all superstring theorists would
sign a pledge to this effect, promising to renounce work on
supersymmetry if the LHC doesn't see supersymmetric particles.
But in fact, they may just drop desire 2).
(There may be other escape hatches I'm not familiar with, too.)
>String theory predicts proton decay, which is not observed.
>Because if you combine grand unification with supersymmetry it
>predicts a proton lifetime of 10^32 years, which is too rare for us to
>separate from the background.
Again this prediction is based on desires 1) and 2).
>> As far as I know, no one knows how to get string theory not to predict
>> that all observed particles are massless, except in a very handwaving
>> manner.
>That is not true. I could show you the particle spectrum of the E_8 x
>E_8 superstring compactified on a Calabi-Yau manifold. The fermions do
>have mass.
Show us.
John Baez
Starblade Darksquall <Starb...@Yahoo.com> wrote:
>Lubos Motl wrote:
>> Loop quantum gravity does not even seem to respect Lorentz invariance
>> (even this elementary question is open in LQG) - it works in a
>> non-relativistic form of a Hamiltonian formalism.
1) First of all, it's worth noting that the theory of general
relativity doesn't have the Lorentz group as a group of symmetries.
The Lorentz group only acts on certain special solutions, namely
those that are small perturbations of Minkowski spacetime.
2) So, if we start from a background-free formalism, where Minkowski
spacetime is not put in by hand, we should not expect to see
Lorentz invariance staring us in the face.
3) We should, however, find it - at least approximately - when we
construct solutions that correspond to small perturbations of
Minkowski spacetime.
(One can argue that we should find it *exactly*, but only on
the grounds that a beautiful exact symmetry of perturbative
classical gravity should remain an exact symmetry of perturbative
quantum gravity. This is an aesthetic argument, and thus not
watertight.)
By the way, none of this so far has anything particular to do
with loop quantum quantum gravity. Everything I'm saying is
just general stuff about any background-free theory that reduces
to general relativity in some limit.
As for loop quantum gravity, the problem is that while we do
have background-free formalisms, both canonical and path-integral,
we don't have a specific theory where we can construct solutions
that correspond to small perturbations of Minkowski spacetime.
So, right now we are at stage 2) and haven't gotten to stage 3).
Of course, this is terrible, since it means we're not sure we're
on the right track. But, that's life.
>> General covariance in LQG is, of course, even more
>> controversial because it is not clear whether LQG is compatible with large
>> smooth manifolds whose existence is necessary for general covariance to be
>> meaningful.
No, manifolds are not necessary for the concept of general covariance
to make sense.
>> LQG also requires a pre-existing manifold for its definition -
>> for example, LQG does not allow topology change.
That's not true. It's difficult to talk about topology change
in canonical quantization, but not so difficult in the path-integral
approach. This is one reason I helped invent a path-integral
approach to loop quantum gravity, which goes by the name of "spin foams".
Spin foam models include a sum over topologies.
>BTW, I heard there was a website devoted to quantum gravity... an
>official website, kind of like a website to a school. It's the one I
>also heard John Baez is going to. Is this true?
Going to school? Or going to a website? I'm confused.
I don't go to school any more - I just stay there and teach.
But I go to lots of websites. Here are a couple of cool websites
on quantum gravity - maybe you were thinking of one of these:
Center for Gravitational Physics and Geometry
http://cgpg.gravity.psu.edu/research/index.shtml#qg
Loop Quantum Gravity
http://relativity.livingreviews.org/Articles/lrr-1998-1/
Don't trust that superstringtheory.com website Lubos told you
about... it says on the first page that the universe is for
sale now. Would you trust a website that promised to sell
you the universe? :-)
Tony Smith
> People interested in this notion of "the string theory crackup"
> may enjoy - or be infuriated by - Carlo Rovelli's new paper:
> A Dialog on Quantum Gravity
> http://www.arXiv.org/abs/hep-th/0310077 ...
> Let me quote just a bit from the end, where the student Sal wraps
> up her argument: ...
> ... in spite of the
> string revolutions and the excitement and the hype, years go by and
> the theory isn't delivering physics. ...
> ... There are alternatives,
> and these must be taken seriously.
The only alternative to the subject of this thread, string theory,
that is discussed in hep-th/0310077 is loop quantum gravity.
I would like to point out another alternative, a theory locally based
on the geometry of the D4, D5, and E6 Lie algebras and related groups
and symmetric spaces, which is outlined in a paper at
http://www.innerx.net/personal/tsmith/TQ3mHFII1vNFadd97.pdf
I regret that I cannot give an arXiv reference to the full paper,
but I am blacklisted from posting papers on arXiv.
However, here are some of its results:
According to the model, geometry of the Hermitian Symmetric Spaces
D5 / D4xU(1) and E6 / D5xU(1) and related Shilov Boundaries, along
with combinatorial relations, allows the calculation of ratios of
tree-level particle masses:
Me-neutrino = Mmu-neutrino = Mtau-neutrino = 0 [small masses might
occur beyond tree level] ...
Me = 0.5110 MeV
Md = Mu = 312.8 MeV (constituent quark mass)
Mmu = 104.8 MeV
Ms = 625 MeV (constituent quark mass)
Mc = 2.09 GeV (constituent quark mass)
Mtau = 1.88 GeV
Mb = 5.63 GeV (constituent quark mass)
Mt = 130 GeV (constituent quark mass)
W+ mass = W- mass = 80.326 GeV
Z0 mass = 91.862 GeV
Higgs mass = 145.8 GeV
weak force - Higgs VEV = 252.5 GeV (assumed, since ratios are
calculated)
With respect to the T-quark mass, the paper discusses the possibility
that observed events around 173 GeV and 225 GeV could be due to
excited states in theT-quark - Higgs - Vacua system.
Gravity comes from a modification of the MacDowell-Mansouri mechanism.
Tony Smith
web site http://www.innerx.net/personal/tsmith/TShome.html
---------------------------------------------------------------
Lubos Motl
On Mon, 20 Oct 2003, Peter Woit wrote:
> String theorists continually repeat the claim that string theory
> is a unique theory, with all parameters determined dynamically.
Despite this repeated fact, it is just hard for many others to understand
why.
> Then they tell us that they don't know what this unique theory is,
We don't know what the full theory is and what fundamental principles will
it be derived from in the next century, but we know enough to say that the
theory exists - and we can calculate a great deal of things about it in
many contexts.
> what the parameters are,
For example in the vacua with moduli - e.g. those with 8 supercharges or
more - we can classify all the massless scalar fields, relate them to the
same moduli spaces in various dual descriptions, and precisely calculate
their influence on low energy physics as well as other physics. In other
contexts, we can calculate the potentials for the scalar fields. It just
happens that in the most realistic contexts we don't know the precise
rules to compute the potentials.
> or what the dynamics is that determines the parameters.
We are aware of the existence of many mechanisms that are relevant, but
the true fact is that we have not finished the theory yet. Your three
complaints may be summarized as one complaint only: string theorists have
not completed their research of string theory yet. Well, this is exactly
why we keep on working on it.
> Some of us find this extremely unconvincing, especially
> when they are unwilling or unable to distinguish between
> what is known to be true and what they would like to be
> true.
The uniqueness of string theory - especially the relations between all of
its "versions" - is known to be true for all the backgrounds that we count
as parts of string theory today, and there are no indications that this
uniqueness of the *theory* (as opposed to its *solutions*) should be
reduced. On the contrary, the continuing research started in the mid 90s
showed that string theory is more unique than was thought previously.
> When asked to justify this uniqueness claim, many
> basically fall back on "Ed Witten says so", which is
> quite a good argument, but not good enough.
I've never heard this "argument" - although I agree that even this would
be a better argument than many arguments against. String theory is unique
because every possible modification that you can imagine to make during
its construction (for example, try to modify anything in Polchinski's
books) either leads to an equivalent theory; or a different state in the
same theory (that is related by various transitions or dualities); or an
inconsistent (non-)theory. To show the uniqueness in full glory, one would
have to go through all aspects of string theory which would require
thousands of pages of text.
> If you try and get them to actually write down a consistent theory,
> what they write down (for instance M-theory on a flat 11-dim
> background) may or may not be consistent, but clearly looks nothing
> like the real world.
If one looks at these vacua *naively*, he sees no relations to the real
world. If one looks with a bit of physical instinct, he or she can see
that these vacua are closely related to the real world. The real world can
be obtained by choosing a related point in the configuration space, and
even without doing it, the qualitative physics just agrees with reality
much better than in any other theory - except those where the result was
put in.
> String theorists should stop going on about how beautiful, wonderful
> and unique their theory is until they actually have figured out what
> (and whether) it is.
I think, on the contrary, that they should emphasize these very important
points much more intensely simply because these points are essential for
the fact that people continue to study theoretical physics - and many
people simply do not understand why anyone works on proposals in
theoretical physics. The 3-hour-long show on PBS is another step to convey
these important concepts to the public.
http://www.pbs.org/wgbh/nova/elegant/
Serenus Zeitblom
OK, Lubos, I agree with most of what you
have said. But I would like to hear what you
have to say in response to this, from Steve
Weinberg:
Weinberg: A disappointing aspect of string theory is that
it has so far failed to shed any light at all on what is
probably the biggest outstanding problem in the physics of
what we can actually see in nature -- the failure to
understand the energy of empty space, the so-called
cosmological constant. If you try to calculate the energy
in empty space, taking into account only fluctuations in
fields of wavelengths where we understand the physics, you
get an incredibly large energy, much too large to possibly
fit what we know about the expansion of the universe.
There must be some complicated cancellations that make the
energy in empty space very small.
String theory provides not the slightest shred of insight
as to why the energy of empty space is as low as it
experimentally seems to be. And that's precisely the kind
of thing that one would think string theory would be able
to help with. I'd say that's the biggest disappointment so
far, that in the one area where you might expect some kind
of quantitative general idea to come out of string theory
that might actually be useful, it has failed to provide it.
[end quote]
I know you think that the CConstant is "just one number".
The difference is that we know, *in principle*, how to get
other numbers [like Yukawa couplings etcetcetc] out of
string theory, in a beautiful way. Linde &co *supposedly*
got a positive CC out of string theory, but in such a
grotesque way that it turned them all into anthropoids.
The CC is "just one number" that could screw up
everything. More people should work on it.
Maybe you need to explain to us why you don't agree with
me [and Steve Weinberg] :)
Thomas Larsson
Lubos Motl <mo...@feynman.harvard.edu> wrote in message news:<Pine.LNX.4.31.031020...@feynman.harvard.edu>...
> but at any rate, I don't find such new limits too exciting.
Nobody expects you to find experimental constraints exciting - you are a
string theorist!
> I always
> believed - and still believe - that the conventional models with the GUT
> scale around 10^{16} GeV and the fundamental scale nearby are much more
> likely and more natural than the models with large/warped extra
> dimensions.
The word "large" in the context of extra dimensions means around 1 TeV;
large enough to cause the electroweak scale. This is what can conceivably
be tested experimentally. Small extra dimensions, at the GUT or Planck
scales, belong to the realm of philosophy.
However, another point is that the Tevatron is taking serious data, and
that they should be able to rule out a light Higgs within a few years.
If so, we know for sure that supersymmetry, and thus superstrings, have
nothing to do with physics at the electroweak scale. I don't think that
anybody seriously expects accelerators ever to go more than one or two
orders of magnitude further - they would just be too big - so this would
complete the decoupling of string theory from physics.
Lubos Motl
On 21 Oct 2003, Jeffery wrote:
> http://www.superstringtheory.com/
> ...
> site than any other site I've ever found. It's Crackpot Heaven, and
> it's an embarrassment. It's the mecca to all the crackpots in the
> world.
Yes, I agree, and it is impossible to believe what the people write in the
forum. However, the web site is not *merely* a forum - a second: I think
that the person who asked was looking for something of this sort. Well,
you may also consider http://www.qgravity.org but there will also be some
issues about that web site. ;-)
Ralph E. Frost
"Kevin A. Scaldeferri" <ke...@sue.its.caltech.edu> wrote in message
news:bn2jjr$gpo$1...@sue.its.caltech.edu...
> In article <575262ce.03102...@posting.google.com>,
> Jeffery <jeffery...@mail.com> wrote:
> >
> >aether theory contradicted experimental data. As I said to Kevin on
> >another thread, string theory is currently consistent with all
> >experimental data, since it exists at higher energies that we can
> >currently reach in our particle accelerators. The fact it has 10 or 11
> >dimensions is not a contradiction since the extra dimensions are
> >compacified. You judge the success of a theory based on how well it
> >explains what it is intended to explain, and it's consistency with
> >experimental data. The aether theory contradicted the Michelson-Morley
> >Experiment. String theory is currently consistent with all
> >experimental data.
>
> And, as I said there, a more accurate statement is that string theory
> is believed to be consistent with all experimental data. That is, it
> is believed that there exists some stable vacuum in which the extra
> dimensions are compactified, and SUSY is broken appropriately, and the
> right gauge group is realized, etc.
But, is the difficulty, and the motivation for the big quibble in these
parts, that said hypothesized stable vacuum is not actually assessible or
verfiable from or within our physical reality?
fynn
Yes, the forums on that site may be called Crackpot's Heavens (why
shouldn't also crackpots have their heaven?).But note that apart from
the forums, which you want to avoid, the site has a lot of nice
introductory information on string theory and offers serious study
groups if one wants really to get into some subjects, and cannot
afford to take a suitable university course.In the past, there were 4
subsequent study groups in which an essential part of the two volumes
by Polchinski were worked through. I took those study groups, so I
know what I am talking about: I found them excellent.They were much
more useful to learn the subject systematically than e.g. the
discussions here.Now three new study groups are about to start on that
site. It is easy to find them (without having to stop by at the
forums) by following the links there to the school. One course is on
quantum theory, based on Sakurai's text, a second one on quantum field
theory, based on Peskin and Schr"oders's text, and a third on
Seiberg-Witten theory, based on the 1996 review by Peskin. It's really
fun, I can highly recommend those courses.Cheers, Fynn.
Peter Woit
Lubos Motl wrote:
>On Mon, 20 Oct 2003, Peter Woit wrote:
>
>
>
>>String theorists continually repeat the claim that string theory
>>is a unique theory, with all parameters determined dynamically.
>>
>>
>
>Despite this repeated fact, it is just hard for many others to understand
>why.
>
>
>
Continually repeating that this is a "fact" is not an argument.
Neither is telling people who challenge your "facts" that
you can't tell them specifically why a "fact" is true since
they are not smart enough to understand. In my
experience in life this is a last-ditch tactic adopted by people when
they don't have a leg to stand on.
>>what the parameters are,
>>
>>
>
>For example in the vacua with moduli - e.g. those with 8 supercharges or
>more - we can classify all the massless scalar fields, relate them to the
>same moduli spaces in various dual descriptions, and precisely calculate
>their influence on low energy physics as well as other physics. In other
>contexts, we can calculate the potentials for the scalar fields. It just
>happens that in the most realistic contexts we don't know the precise
>rules to compute the potentials.
>
>
>
In other words: when we can consistently calculate things they
come out wrong, and in other contexts we would like to believe that
even though our approximate calculations come out wrong there
is some kind of other more consistent calculation that will come
out right.
You're still not telling us what the parameters are. You don't
know the answer and what you do know is very discouraging. One
piece of the parameter space is supposed to be the space of all
Calabi-Yaus. There is a huge and possibly infinite set of discrete
classes of these, each class having a potentially large dimension.
This is presumably only one small part of parameter space.
>String theory is unique
>because every possible modification that you can imagine to make during
>its construction (for example, try to modify anything in Polchinski's
>books) either leads to an equivalent theory; or a different state in the
>same theory (that is related by various transitions or dualities); or an
>inconsistent (non-)theory. To show the uniqueness in full glory, one would
>have to go through all aspects of string theory which would require
>thousands of pages of text.
>
>
>
I'm willing to stipulate that no one knows how to deform anything
in Polchinski's books into something else no more inconsistent
than the starting point, except in a way that can somehow be
claimed to be a change in background. Even so, such a fact
has nothing to say about whether there are zero, one, many or
an infinite number of different theories from which you
could recover Polchinski's examples as special cases.
Some of the people doing S-matrix theory during the
sixties believed that, subject to a few simple constraints
including analyticity, there would be a unique consistent
S-matrix and it would describe the real world
(or at least the strong interactions). They
believed this because analytic functions have impressive
uniqueness properties and because they couldn't produce
even one non-trivial S-matrix of the kind they were
looking for. It is now clear that this was purely wishful
thinking (there are lots of consistent S-matrices), of
exactly the sort that string theorists are now engaged in.
By analogy with the S-matrix history, perhaps someday
someone will find consistent "M-theories", lots of them,
which are really just QFTs in a dual picture. These may
be useful for doing calculations in QCD, but have nothing
to do with unification or gravity. The AdS/CFT story is
pointing in that direction.
Lubos Motl
On Mon, 20 Oct 2003, Alfred Einstead wrote:
> Actually, I'm still waiting for both Motl and Baez from an earlier
> incarnation of this discussion to actually notice the key point ...
OK, I am an incarnation of Motl, and let me try to write something about
the key points by Al. Einste. I won't have terribly much to say, but let
me say already at the beginning that Einstead's proposal is almost as
good as loop quantum gravity, which is unfortunately not enough to create
a working theory of quantum gravity.
> >So, a long time ago, I got this really neat idea: let's create
> >a particle-based theory of gravity by pretending that all of
> >space-time curvature actually resides at points and that
> >these vertices are actually the particles of gravity.
Well, this is not terribly much different from loop quantum gravity. The
difference is that in loop quantum gravity, the whole area (or length and
volume) is concentrated in the edges (or ...) of a spin network. In your
case, you only want the curvature to be made of "delta functions"; the
curvature is morally the second derivative of the metric. I think that the
example of a single point is misleading, and already if you want to
combine two conical singularities within the same flat space, you may face
problems (especially in higher dimensions). Moreover, I think that the
problems of quantized general relativity suggest just the opposite cure:
we should try to smooth out the space at very short distances, and not to
concentrate the curvature to points! The regions of the configuration
space where two "curvature particles" approach one another would lead to
singularities and divergences, I guess. What sort of goal do you expect
from your general proposal, Alfred? Do you expect a working quantum theory
of gravity?
> >Of course, as you know, this isn't possible [...] because the key items
> >of interest are NOT points, but subspaces of (n-2) dimensions.
That's correct. The (D-2)-dimensional surfaces are also the same thing
that carries the entropy of the horizons in a D-dimensional spacetime
unless "n" meant the dimension of *space* only. At any rate, I think that
your reasoning is very advanced: you try to check your quantum gravity
idea by generalizing it to D spacetime dimensions. This is something that
a loop quantum gravity person would hardly do.
> Thus, in 3-D, one has to conceive of the curvature as being
> concentrated on ... strings.
Can you write a metric whose curvature equals what you need, namely the
combination of the delta functions? Note that in d>3, the Riemann tensor
of curvature has many components, and it might be difficult to construct a
general value of the Riemann tensor out of your delta-function sources.
The Riemann tensor in 4 dimensions has 20 components, for example.
> > If you parallel transport a x-y-z coordinate frame in a circle [...]
> > then the frame will come back with a fixed rotation ONLY IF the loop
> > links one of these singular lines or circles.
Right, it's like loop quantum gravity with an extra derivative in its
fundamental equations. If you write an equation that relates it to the
real metric, and it is conceivable to imagine that you would succeed, you
will obtain as advanced a proposal for a quantum theory of gravity as loop
quantum gravity. You should then obtain as much money as the LQG people
are getting. In LQG, you measure areas by summing over the spin network's
edges that intersect that area. In YQG (your quantum gravity), you measure
the angle resulting from a parallel transport by the linking number. Very
good.
> >A "particle-based" theory of gravity would then actually be a
> >string theory for these curvature singularities.
Well, note that although (cosmic) strings are usually thought of as
carrying a deficit angle in 4 (spacetime) dimensions, in 10 or 26
dimensions you would need 7-branes and 23-branes instead of strings to
generate what you want - so your proposal is not likely to be equivalent
to string theory.
> ... which means you both have a background and at the same time
> you don't.
I disagree. Your theory has a background. If you remove all the curvature
quanta, you obtain a Minkowski space. It is a background without curvature
- curvature must be added through your quanta. Loop quantum gravity has
"less" derivatives, but it is also a sort of expansions around a
background - but it is a g_{mn}=0 background. The metric, distances and
areas are zero until you insert a "spin network". The advantage of YQG
over LQG is that your YQG can describe the Minkowski background with
Lorentz symmetry while LQG probably cannot.
Best wishes
Lubos
Lubos Motl
Dear Serenus,
I've already read this text by Weinberg. Yes, I agree that the missing
explanation for the value of the cosmological constant seems as an obvious
failure. My guess is that Weinberg would agree that the cosmological
constant is one number. ;-) I would even add that string theory has not
given us any impressive results about the early cosmology - an obvious
place where GR and QM must be considered simultaneously. It is an obvious
context where string theory should display its muscles, but in practice it
has not showed almost anything. I don't want to be cruel, but most works
in string cosmology so far remind me of loop quantum gravity. While string
theory contains both QM as well as GR, so far it only seems very powerful
in backgrounds with some large spatial dimensions, not so much in extreme
cosmology.
> I know you think that the CConstant is "just one number".
> The difference is that we know, *in principle*, how to get ....
That's right. But I would actually say that we do *not* know how to
reliably calculate the size of the cosmological constant after
supersymmetry breaking. Before supersymmetry breaking the cosmological
constant can be zero, and after supersymmetry breaking we are using the
estimates based on low-energy effective field theory that might be
misleading. A proper resummation of all perturbative and nonperturbative
effects might lead to an exponentially small value of Lambda.
If it does not, the cosmological constant could still have an anthropic
explanation along the lines of Weinberg's argument. Yes, all these things
would be disappointing, but one can imagine that this is how the Universe
works. Let's not give up, but let's be open-minded.
> The CC is "just one number" that could screw up
> everything. More people should work on it.
I just don't think so. Your proposal is not a new idea. People have been
intensely working on the question of Lambda in string theory - and they
have emphasized its importance - from 1998 (at least), and unlike other
questions, this question has not been given any convincing answers. Is it
an accident? I don't think so. In my viewpoint, it is just a wrong
question for present and a wrong direction of research. People should be
working on the things that *do* work and where they can make more progress
and extend the success, and it is just not the case of the cosmological
constant puzzle.
We obviously do not know many important things even about the highly
supersymmetric backgrounds - for example, we have no deep geometric
understanding of the E_{k(k)}(Z) U-duality groups of M-theory on tori, and
we can't define dynamics even of those backgrounds in a covariant
nonperturbative fashion (and for M-theory on T^k for k>5 we have *no* full
nonperturbative definition at all). I still believe that we have much more
chance to make progress in this direction because we essentially know the
answers to most important *physical* questions in these backgrounds, and
the remaining task is to improve the *formalism*. Once we do it, we might
be able to generalize the new formalism to other backgrounds which we
don't understand well enough today.
The correct calculation of the cosmological constant may emerge naturally
after many other steps are understood. For example, we might find that a
mechanism remotely similar to the idea of Anthony Zee
http://arxiv.org/abs/hep-th/0309032
might follow from string/M-theory. I recommend you to read this paper -
unless you have done so - because Zee shows very nicely how crazy the long
lifetime of the proton would look like to a theorist that is using some
effective description at low energies.
Lubos Motl
On Tue, 21 Oct 2003, Thomas Larsson wrote:
> Nobody expects you to find experimental constraints exciting - you are a
> string theorist!
I don't expect *anyone* in particle physics to find new limits on the size
of extra dimensions too exciting simply because the general feeling is
that these dimensions are probably much smaller (and some cynics say that
they don't exist at all).
> The word "large" in the context of extra dimensions means around 1 TeV;
> large enough to cause the electroweak scale. This is what can conceivably
> be tested experimentally. Small extra dimensions, at the GUT or Planck
> scales, belong to the realm of philosophy.
That's not true. Extra dimensions always have dramatic consequences even
for low-energy physics. For example, heterotic strings on Calabi-Yau
manifolds of minuscule (Planckian) size predict N generations of quarks
and leptons where N is |chi/2| where chi is the Euler character of the
Calabi-Yau space - a topological invariant. Many properties of low-energy
physics are determined by the size and shape of the extra dimensions, and
if we mastered string theory better, we could probably calculate even the
particle masses from an analysis of string/M-theory in the extra
dimensions.
> However, another point is that the Tevatron is taking serious data, and
> that they should be able to rule out a light Higgs within a few years.
What mass of Higgs do you dream about to help you in your plans to damage
string theory? ;-) Higgs can't be too heavy even according to other,
simpler theories.
> If so, we know for sure that supersymmetry, and thus superstrings, have
> nothing to do with physics at the electroweak scale. I don't think that
> anybody seriously expects accelerators ever to go more than one or two
> orders of magnitude further - they would just be too big - so this would
> complete the decoupling of string theory from physics.
This is not true either. Physicists in the 21st century are not as
primitive as you imagine them, and they can prove many theories in less
direct, but equally convincing ways. Above, I roughly described the
relation between the shape of the extra dimensions and the observed
low-energy physics.
Best
DickT
ba...@galaxy.ucr.edu (John Baez) wrote in message news:<bn45vi$n74$1...@glue.ucr.edu>...
>
(much good stuff snipped)
>
> Don't trust that superstringtheory.com website Lubos told you
> about... it says on the first page that the universe is for
> sale now. Would you trust a website that promised to sell
> you the universe? :-)
A site where a bunch of us amateurs are discussing LQG is
physicsforums.com. It's nearly all about the canocicle quantization
program so far, but if somebody wanted to start a thread or two on
spin foams I'm sure they'd be welcome. The site is strongly moderated
and what I like to call personal theories are gently transferred to a
special location. No guarantees that everyone there has a Ph. D., but
the level of discourse is surprisingly high.
Kevin A. Scaldeferri
In article <vpadakd...@corp.supernews.com>,
Ralph E. Frost <ra...@REMOVEfarmerdealer.com> wrote:
>
>
>"Kevin A. Scaldeferri" <ke...@sue.its.caltech.edu> wrote in message
>news:bn2jjr$gpo$1...@sue.its.caltech.edu...
>>
>> is believed to be consistent with all experimental data. That is, it
>> is believed that there exists some stable vacuum in which the extra
>> dimensions are compactified, and SUSY is broken appropriately, and the
>> right gauge group is realized, etc.
>
>But, is the difficulty, and the motivation for the big quibble in these
>parts, that said hypothesized stable vacuum is not actually assessible or
>verfiable from or within our physical reality?
>
No, not at all. The hypothesized stable vacuum is precisely our
physical reality.
Kevin A. Scaldeferri
>
>> [some stuff critical of string theory and LQG]
In article <c7fd6c7a.03101...@posting.google.com>,
Serenus Zeitblom <serenusze...@yahoo.com> wrote:
> [some stuff critical of Arnold Neumaier for being critical of
> ST and LQG]
First, I'm going to put on my moderator hat for a moment and ask
in an official-sounding voice for everyone to take it down a notch.
That out of the way...
>In the context of a physics discussion group, this is nothing but
>abuse.
This seems to perpetuate the idea that particle theory is the true
physics and the rest is mere details, which is both rather
condescending to the rest of physics and has been debunked by a
variety of others in this thread.
Also, it seems to perpetuate the common misconception that SPR is a
particle theory discussion group, which is also not true. Admittedly
the discussion is often dominated by these topics. Sometimes I think
this is because everyone else is too busy actually doing something
useful :-)
>If you have concrete
>criticisms of these theories, let's hear them.
I believe that the criticism was that some tens of thousands of
man-years have gone into these pursuits with no falsifiable
predictions produced.
Arnold Neumaier
> > once it has attracted a minimal number of researchers if it beats the drums
> > to melodies of great dreams.
> >
> > Thus fields like loop quantum gravity and string theory may serve to make
> > a living, to create beautiful myths for readers of the Scientific American,
> > or to give their researchers the reputation of working on something
> > very difficult and advanced (passing milestones in a direction
> > not leading to the goal may require tremendous amounts of skill),
> > but they hardly serve the quest for understanding nature.
>
> In the context of a physics discussion group, this is nothing but
> abuse.
I do not think so. I think my statements are true.
Though they may be subjective, they are defendable and do not
contain abusing language. Saying that a subject hardly serves
the quest for understanding nature can hardly qualify as abuse.
> Tell me, in the discussion groups on numerical
> analysis and optimisation
There are also areas in numerical analysis and optimisation
where the same analysis (peer review supported groups doing
mainly irrelevant work) applies. But there are no threads like
'The optimization theory crackup', so there is little point
posting such comments. I wouldn't have seen any reason for my statement without
having read all that was going on in this
thread before my posting.
> do you have
> string theorists irrupting and telling you all that your field
> is so stupefyingly boring that research on it is a total waste
> of time, and that no honest researcher would accept public funds
> to do research on anything that simple and dull?
First, I didn't say that no honest researcher should do research
on these topics, neither did I claim that the field is boring.
Contributing to difficult things is rarely boring.
There are many good motives to work on something,
and I respect people working on these things. Also, I sometimes
work on problems that I know are fairly irrelevant, because I like
them or because there is another special reason for it.
But it is a different matter to assess the value of a field as
a whole, and I made my assessment without any personal attacks or
rude words, giving reasonble reasons for my statements.
Second, I am not only a mathematician but also a physicist,
though I don't have a formal degree. I know all the material
a physics PhD is expected to know, and much much more, have
published 9 papers on different areas of computational physics
including quantum mechanics, have a working knowledge of quantum
field theory at least at the level of what is in typical
textbooks.
I knew about exceptional Lie groups and the Leech lattice
long before these made their appearance in physics,
and know a significant amount about string theory
and loop quantum gravity - not enough to do research there myself
but enough to understand it, and at a level that I could do
research with half a year of further preparation (if I'd find
it worthwhile). In particular, I try to keep up to date on news
that would indicate that getting deeper into the matter would
become important. So I know quite well what I am talking about.
If numerical analysis would contain a prominent research group
working on techniques that were claimed to be the future of the field (and even
the most important work in the field) but
these techniques never yielded significant algorithms that could
be applied to real life problems, and someone would mention that
in sci.math.num-analysis or sci.op-research, and let people know
that the claims of importance are exaggerated, in a style like my
post to s.p.r., I'd heartily agree and write a corresponding
reply.
> If you have concrete criticisms of these theories, let's
> hear them.
I gave concrete criticism: Both string theory and loop quantum gravity explain
very little beyond what the standard model
explains, but needs a much more elaborate theory.
If an explanation is not much more compact than the union of
those phenomena it explains which are not already explained
in simpler terms, it does not deserve to be called an explanation.
I can be more specific in mentioning things that are lacking
(independent of experimental verification) and would be needed
to convince me that something important is going on:
Both theories have not been related quantitatively to the
established theories. A convincing classical limit
(to general relativity), nonrelativistic limit (to a
multiparticle Schroedinger equation with Newtonian interaction),
and low energy limit (at currently accessible energies no new
particles apart from the graviton) has not been demonstrated.
Moreover, one would want something more, namely at least to be
able to calculate the leading corrections to these limits
(whether or not their magnitude is observable).
If you can show me signficant progress along these lines
(maybe I have missed some important contributions), I'd be happy
to study them and revise my assessment in the light of such work.
But as long as there is no such work, I think my statements
are fully compatible with the state of the art in theoretical
physics.
Arnold Neumaier
Michael Petri
news:3F8C08A2...@univie.ac.at...
> Neither loop quantum gravity nor string theory have so far contributed
> anything to understanding quantum gravity, at least not to my
understanding.
You are being a litte harsh here. I don't consider my understanding to be
good either, but there are some things that are truly worth mentioning.
1) LQG has succeeded with very little ballast and by steadfastly holding on
to one very important physical principle, diffeomorphism-invariance, to
predict area- and volume quantization. The formula for area and volume are
actually manageable (whereas the theory itself is difficult!). And who knows
what will follow from the assumption, that space-time is discrete on a
fundamental level? We already found the black hole entropy formula, there is
some interesting research going on with respect to singularities, ...
2) String theory is a formidable lesson about mathematical beauty and
probably deserves to be studied just for its own sake, even if there weren't
several physically beautiful results and predictions which might soon be
testable (I wish I would understand more to appreciate string theory's true
range). It certainly suffers from it's heavy mathematical machinery, which
requires years of training, just to get started. But wasn't the situation
for GR similar at the beginning of the last century? I myself can't say much
about string theory's actual achievements, but a lot of people here in this
group are doing an excellent job in this respect. [and more often than not
patentiently answering the - often not too smart - questions of anybody who
signals that he is willing to learn a bit more]
> At the present state of the art,
> one wastes much more time learning to understand these theories than what
> one gains in understanding quantum gravity (namely next to nothing).
> I felt disappointed by every survey paper I read.
The waste of time somewhat depends on the perspective. I am just a week-end
scientist (and not for all week-ends, mind you), therefore - yes - for me it
*would* be a waste of time to dig deeply into one of those fields. It would
be like Achilles trying to catch up with the turtle: Whenever I made a tiny
bit of progress, the turtle would also have moved on (the only difference to
the original paradox being, that the turtle sits in a Ferrari). But even if
your time is limited, you can eventually understand some results, as long as
you accept, that by reading a paper you don't have to understand everything
on the way.
But if you are doing theoretical physics on a professional level the "waste
of time" might not seem such a bad thing after all. Even if it took five to
ten years to get up working, just think how boring life would have been, had
you wasted these precious years by writing papers with the LPU (least
publishable unit) about the newest high precision measurement on the
vibrational frequency of say H2O2, improving the result of your sole
competitor X (who measured f=1.123456789) by roughly 0,1 digits (you're most
recent measurement came out f=1.1234567891). The sad fact is, that many
professionals are doing just this kind of thing.
> Seen from this perspective, loop quantum gravity and string theory
> look more like two elephants who gave birth to a flea.
[some text deleted]
> But in a science based on peer reviews, a field is able to perpetuate
itself
> once it has attracted a minimal number of researchers if it beats the
drums
> to melodies of great dreams.
I actually liked this piece of poetry, although I don't agree with with song
that you are singing ;-)
Mike
Peter Woit
>
>People seem to think that SUSY is in trouble if no light Higgs boson
>(< 130GeV) is detected at Tevatron II. So far, I haven't heard any rumours,
>but maybe somebody else has.
>
>
>
Unfortunately Tevatron II is having serious problems getting
anywhere near design luminosity. To start pushing up the
Higgs mass limit they need about 2fb^{-1} of
integrated luminosity, see e.g.
http://fnth37.fnal.gov/higgs/higgs.html
They've only got about .3 fb^{-1} right now and under
their latest plan it won't be until 2006-7 that they get
enough to maybe increase the Higgs mass limit.
Thomas Larsson
> > once it has attracted a minimal number of researchers if it beats the drums
> > to melodies of great dreams.
> >
> > a living, to create beautiful myths for readers of the Scientific American,
> > or to give their researchers the reputation of working on something
> > very difficult and advanced (passing milestones in a direction
> > not leading to the goal may require tremendous amounts of skill),
> > but they hardly serve the quest for understanding nature.
>
> In the context of a physics discussion group, this is nothing but
> string theorists and loop theorists, and it would amount to the
> same thing. Tell me, in the discussion groups on numerical
> analysis and optimisation --- if they exist --- do you have
> is so stupefyingly boring that research on it is a total waste
> of time, and that no honest researcher would accept public funds
> to do research on anything that simple and dull?
In a recent review of quantum gravity, one could read
"The aim of the present paper is to discuss in some detail established
results on the field. In some strong sense, the review could be finished
at once, because there are none."
- E. Alvarez, http://www.arxiv.org/abs/gr-qc/0307090 , p 2.
I don't know much about numerical analysis, but I would be very surprised
if one can find a review paper where a similar statement is made about
that field.
However, there is one solid result, better known to the readers of this
newsgroup than to the physics community in general. The correct symmetry
principles underlying general relativity (diffeomorphism invariance) and
quantum theory (projective, lowest-energy representations) have been
unified. One may of course have different opinions about the relevance of
the unification of the symmetry principles to the unification of the
theories themselves, but that the multi-dimensional Virasoro algebra
successfully unifies the correct symmetry principles of GR and QM can not
be reasonably denied.
Lubos Motl
> The real elephant in the room here is jobs. Jobs aren't great in string
> theory right now. LQG is even worse, unless I misunderstand the
> situation.
That's a very pragmatic approach, but in my viewpoint, the jobs are mainly
a tool of the society to regulate the number of people and resources that
are dedicated to theoretical high energy physics, for example. I guess
that you did not mean that the situation in the job market decides about
your answers to questions about physics!
> About a decade ago, he gave a somewhat infamous talk at Princeton or the
> Institute. That it didn't convince anyone is probably the best way to
> summarize what I've heard of this talk.
According to many people I've heard, he is a very good, intense and
emotional teacher. My (former) advisor explained me that one could not
even have played a card game with him.
> Rovelli has always, for me, been one of those who keeps going on how
> string theorists don't understand the lessons of GR, how background
> independence is god's holy writ
It seems almost clear that any reasonable enough criterion would reveal
that string theorists understand GR better than average relativists - and
they are also using its tools in a larger number of papers per person and
year. Also, loop quantum gravity is so "much" background independent that
even Lorentz invariance itself is likely to be violated in LQG.
> It wouldn't surprise me, though, as string theory has a lot of very
> strong personalities who won't hesitate to tell you in an rather
> unrelenting manner just how wrong you are.
Would you agree that this is particularly true for the leading figures -
that the percentage of straightforward personalities is higher among the
elite? ;-) I am sure that David Gross, Lenny Susskind, and even Cumrun
Vafa are not the only examples.
> Why do many string theorists ignore LQG now?
It is probably true that many younger string theorists have not heard much
about LQG - and some of them are pretty tolerant about LQG (the reason is
usually because they have not heard too much about it). However I know for
sure that most leading figures know about LQG and they're not working on
it simply because they have good reasons to think that it (LQG) is almost
certainly wrong.
Let me say one important thing at the end: I find the current situation -
in which the quantum gravity community is split into subcommunities that
do not have to talk to one another - unacceptable. There is ONE physical
world only whose laws should be studied and none in theoretical physics
should be "allowed" to ignore - and to know nothing about - the key ideas
that have been studied in the field. This means that the physicists who
count themselves as "string theorists" should know about LQG (well, there
is not too much stuff that has been found here, so it is a finite task)
and have an opinion about that - and indeed, I think that many of us
(string theorists) understand LQG and its possible role in the scheme of
things better than most LQG practitioners. On the other hand, I just find
it completely impossible for a professional quantum gravity theorist to
know nothing specific and technical about string theory; sorry, some
layperson's general emotional statements are not counted as knowledge.
For example, there should be ONE quantum gravity / high energy theory
archive on the web. According to a quantum gravity theorist, the proposals
of other people can either be correct, wrong, or partially correct (and
often the status is not known), but they are certainly not "independent"
because these people are not working in a "different field". Loop quantum
gravity, in particular, has become an umbrella for various people who
don't need to learn or produce almost anything and who can always justify
it by saying that "they are working on alternatives that are
discriminated". That's just wrong.
Best wishes
Michael Petri
news:575262ce.03101...@posting.google.com...
> I could show you the particle spectrum of the E_8 x
> E_8 superstring compacified on a Calabi-Yau manifold. The fermions do
> have mass.
I always wanted to look into this. Could you point out one or two
references (preferentially not too technical)?
Thanks, Mike
Borcis
> For instance, string theory does not predict 10 dimensions that
> would be observable by us, because the extra ones are compactified.
Shouldn't that read : "String theory predicts 6 dimensions to be
compactified (unless it finds a distinct but equivalent way to deal with them)
*because* we only observe 4 (and it would otherwise predict 10 dimensions to
be observable)".
> So, in what way is string theory inconsistent with experimental data?
Well, one might say that its way to reach consistency with experimental data
is furiously reminiscent of the epicycles of geocentric systems. To me, the
one really astonishing experimental feature of string theory is the
faithfulness to it of its adepts, given the difficulty of providing it(s
standard interpretation) with other sorts of experimental support. I read this
to mean that its extraordinary mathematics relate to the physicists' minds
like the mass of the Earth does to the body of astronomers : a fascinating
clue to my eyes.
That the exceptional beauty of the math is so convincing by itself that a lot
of smart people hold fast to it despite the clear experimental difficulties, I
see as reason enough to start fishing for *useful* interpretations of the
math, as describing something different than initially intended.
Regards, Boris Borcic
--
"Hope achieves the square root of the impossible"
backdoorstudent
American about the future of string theory:
http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=000073A5-C100-1F80-B57583414B7F0103
He was also on David Letterman's show the other night.