Do electrons have structure?

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Frederick Seelig

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Aug 4, 2001, 9:52:27 PM8/4/01
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Do electrons have structure? From what I have read, electrons are still
considered point particles at today's colliders' energy levels. But
wouldn't it be reasonable to suppose that electrons were composite
particles, too? If hadrons consist of fractional charge thingies, oughtn't
one to suppose that leptons do too?

Doesn't the Standard Model consider leptons to be elementary particles? Is
there a simple to understand reason for this? Do any extensions of the
Standard Model treat leptons as composite particles? At what energy levels
would electrons show internal structure?

--
Fred Seelig

J. J. Lodder

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Aug 5, 2001, 12:31:15 PM8/5/01
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Frederick Seelig <fse...@mitre.org> wrote:

> Do electrons have structure? From what I have read, electrons are still
> considered point particles at today's colliders' energy levels. But
> wouldn't it be reasonable to suppose that electrons were composite
> particles, too? If hadrons consist of fractional charge thingies, oughtn't
> one to suppose that leptons do too?

Nature doesn't listen to our reason. It not even ought to,
it being far cleverer than we are :-)
However, the idea of composite electrons has severe problems;
electrons would have to consist of components -very- much heavier
than themselves, the remainder being supplied by a very large negative
binding energy.
While possible in principle, it would be hard to invent an 'elegant'
theory with such nearly, but not quite perfect, cancallations.
It would not be 'reasonable' :-)

> Doesn't the Standard Model consider leptons to be elementary particles?

Yes.


> Is there a simple to understand reason for this?

Yes, because the standard model is phenomenological.
It doesn't care for unobserved properties.


>Do any extensions of the
> Standard Model treat leptons as composite particles? At what energy levels
> would electrons show internal structure?

Won't know 'till seen,

Jan

Phil Gardner

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Aug 5, 2001, 12:30:48 PM8/5/01
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Frederick Seelig <fse...@mitre.org> wrote in message news:<3B69516A...@mitre.org>...

Does the "evidence" that they are point particles include anything
more than the facts that:
(a) the cross-section for elastic scattering of electrons through
angles exceeding (say) pi/2 decreases without limit as the electron
energy is increased.
(b) in the last 70 years or more no-one has constructed a plausible
model of an extended electron with a mass-energy density that is
everywhere finite and continuous.

Phil Gardner <pej...@oznetcom.com.au>

Moataz Emam

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Aug 6, 2001, 1:42:41 PM8/6/01
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The only known reasonable theory of electronic structure is within the
context of String theory, where electrons are extended strings in
spacetime. That is only apparent on the level of the Planck energy, or
length scale, about 10^-33 cm. This, of course, is quite unattainable
with today's technology. Some late ideas put the Planck scale much more
closer than that, making it possible to reach it in near future
accelerators.

--
Moataz H. Emam

URL: http://continue.to/emam
The Department of Physics
1129, Lederle Graduate Research Tower C,
University of Massachusetts, Amherst, 01003
e-mail : em...@physics.umass.edu
Tel. : (413) 545 0559
============================================

"I do not like it, and I am sorry I ever had anything to do with it."
Erwin Schrödinger, speaking of quantum
mechanics

"Those who are not shocked when they first come across quantum mechanics
cannot possibly have understood it."
Niels Henrik David Bohr

Matthew Nobes

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Aug 6, 2001, 1:43:20 PM8/6/01
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On Sun, 5 Aug 2001, Frederick Seelig wrote:

> Do electrons have structure?

To the best of our ablility to test, no.

> From what I have read, electrons are still considered point
> particles at today's colliders' energy levels. But wouldn't
> it be reasonable to suppose that electrons were composite
> particles, too? If hadrons consist of fractional charge
> thingies, oughtn't one to suppose that leptons do too?

People have tried to make theories like this. Look up
``preons''.

If you like string theory, then electrons (and other leptons) are
some sort of excited modes of tiny little strings. I not sure if
string theorists count this as substructure or not.

> Doesn't the Standard Model consider leptons to be elementary
> particles?

Yes.

> Is there a simple to understand reason for this?

No, in the SM that's just the way things are.

> Do any extensions of the Standard Model treat leptons as
> composite particles?

String theory is probably the best known contender.

> At what energy levels would electrons show internal
> structure?

That depends on the theory. THe current experimental bound is
pretty high (~5 TeV IIRC). Of course there could be more subtle
effects. For example, muon substructure could possibly explain
the recent magentic moment anomoly measurment.

--
"Neutral kaons are even more crazy than silly putty"
-G. 't Hooft
Matthew Nobes, c/o Physics Dept. Simon Fraser University, 8888 University
Drive Burnaby, B.C., Canada, http://www.sfu.ca/~manobes


Ralph E. Frost

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Aug 6, 2001, 3:47:02 PM8/6/01
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Matthew Nobes <man...@sfu.ca> wrote in message
news:Pine.GSO.4.30.010804...@fraser.sfu.ca...

> On Sun, 5 Aug 2001, Frederick Seelig wrote:
>
> > Do electrons have structure?
>
> To the best of our ablility to test, no.

I think this needs a bit of clarification. Consider that in our local
region/energy density, ALL imagery is created, conveyed and communicated in
terms of whole electron units. We can't leak one third of an electron out
of some membrane and then have that scuttle across into a detector.

In fact, when we power up the fields and equipment used to look for electron
substructure, folks do so in units of whole electrons. When we develop
photographs. we do so in terms of whole electrons.

So, since the result of ALL electron substructure queries will ultimately be
reduced down to ambient energy levels, is it POSSIBLE to have a resolved
image of a fractional electron?

I don't think so.


>
> > From what I have read, electrons are still considered point
> > particles at today's colliders' energy levels. But wouldn't
> > it be reasonable to suppose that electrons were composite
> > particles, too? If hadrons consist of fractional charge
> > thingies, oughtn't one to suppose that leptons do too?
>
> People have tried to make theories like this. Look up
> ``preons''.
>
> If you like string theory, then electrons (and other leptons) are
> some sort of excited modes of tiny little strings. I not sure if
> string theorists count this as substructure or not.
>
> > Doesn't the Standard Model consider leptons to be elementary
> > particles?
>
> Yes.
>
> > Is there a simple to understand reason for this?
>
> No, in the SM that's just the way things are.
>
> > Do any extensions of the Standard Model treat leptons as
> > composite particles?
>
> String theory is probably the best known contender.
>
> > At what energy levels would electrons show internal
> > structure?
>
> That depends on the theory. THe current experimental bound is
> pretty high (~5 TeV IIRC). Of course there could be more subtle
> effects. For example, muon substructure could possibly explain
> the recent magentic moment anomoly measurment.
>

Again, I don't think it matters WHAT energy level the equipment runs at.
The result will still be brought down and funnelled through local machinery.
This machinery only computes in units of whole electrons. Powering a
bigger system to higher energies is still not going to influence the ambient
outcome.


Er, don't some HEP test smash electrons into pieces?


J. J. Lodder

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Aug 6, 2001, 7:06:47 PM8/6/01
to
Phil Gardner <pej...@oznetcom.com.au> wrote:

> Frederick Seelig <fse...@mitre.org> wrote in message news:<3B69516A.FC1FA863@

"Point particle" is a phenomenological concept anyway.
It means: particles that are described adequately
by a theory containing no structure-related parameters.

Jan

Gordon D. Pusch

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Aug 7, 2001, 10:14:39 PM8/7/01
to fse...@mitre.org
Frederick Seelig <fse...@mitre.org> writes:

Many people have attempt to construct such ``composite'' theories (e.g.,
the class of so-called ``technicolor'' models), but all have failed.

There is a fundamental problem in that the ``natural'' energy scale for
an object of size on the order of R is M*c^2 ~= (\hbar c) / R. The current
experimental upper-bound on the sizes of an electron, muon, or `u' and `d'
quark is less that 1e-18 m, so the natural mass of these quarks and leptons
if they were composite would have been expected to be much greater than
~200 GeV --- which is about five orders of magnitude too large for the
``first generation'' quarks and the electron, three orders of magnitude too
large for the muon and `s' quark, and at least nine orders of magnitude too
large for the electron and muon neutrinos. All attempts at construct a
``composite'' theory of quarks and lepton have foundered on this problem
of being unable to provide a natural reason why the first and second
generation quarks and leptons should be so light compared to the
``natural'' scale for their masses if they were composite.

Another problem most of these composite theories suffer from is the quantum
field theoretic disease called ``anomolies.'' I won't say anything about
anomolies except that they are viewed as Very Bad Things...


-- Gordon D. Pusch

perl -e '$_ = "gdpusch\@NO.xnet.SPAM.com\n"; s/NO\.//; s/SPAM\.//; print;'

Lubos Motl

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Aug 7, 2001, 10:15:42 PM8/7/01
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On Sun, 5 Aug 2001, Frederick Seelig wrote:

> Do electrons have structure? From what I have read, electrons are still
> considered point particles at today's colliders' energy levels.

Yes, according to all the experimental data as well as the Standard Model,
the electrons are point-like. It means that if they have a structure, the
distance between the components must be smaller than 10^{-18} meters or
so.

> But wouldn't it be reasonable to suppose that electrons were composite
> particles, too? If hadrons consist of fractional charge thingies,
> oughtn't one to suppose that leptons do too?

No, it would not be reasonable. First of all, there is absolutely no
motivation to do it. There are hundreds of different hadron species and
the discovery of quarks improved our understanding of physics in many
ways. Instead of hundreds of "elementary" hadrons, people could suddenly
deal with 6 fundamental quarks (and 6 antiquarks).

In the case of electrons, there is one electron only and it looks exactly
as fundamental as a quark. Electron is not as composite as a hadron!
Electron is as elementary as a quark. In the Standard Model, leptons (such
as electron) and quarks (up, down etc.) form families. Protons and
electrons don't! ;-) In the Grand Unified Theories, electrons and quarks
form multiplets that can be transformed into each other, not only
families.

More importantly, there is no observation that suggests a "parton"
structure of electrons. Parts of protons (quarks) can be seen at typical
distances of order 10^{-15} meters. On the contrary, electrons are safely
point-like even at distances of the order 10^{-18} meters or so. This is
the distance that current accelerators can "see".

There is also no new force to be explained. Protons and neutrons attract
each other in the nuclei. Today, we explain this force by a more
fundamental force between the quarks. This force is mediated by gluons.
The fact that quarks can have three colors essentially implies that there
must be a force that guarantees the colors to be equally good (the strong
force described by Quantum Chromodynamics).

On the contrary, electrons interact with the electromagnetic and weak
force (as well as gravity which is negligible in particle physics) and
there is no new force that could justify a substructure of electrons.

To summarize, hadrons are composites of quarks but electrons are not.
Electrons are as fundamental as quarks. It would be easy if we could take
an idea in physics and recycle it 20 times and make some progress 20
times. But physics is fortunately not that simple. You must find something
new to be famous. ;-) The method to explain a particle as a composite of
several smaller point-like particles has been probably exhausted. The only
exception could be the Higgs boson that could be a composite of
techniquarks - but even this possibility seems very unlikely.

Elementary particles have a deeper structure to explain. Perturbative
string theory shows every elementary particle (such as an electron, a
quark or a photon) to be a loop of string in a specific vibrational
pattern. Nonperturbatively, the nature of each particle can be even more
interesting (containing branes of various dimensions etc.) and there may
be several "dual" descriptions of the same thing. However, the simple way
to divide a particle into smaller particles cannot be done indefinitely.
There is a cutoff. Distances smaller than the Planck length do not make
any sense, for example. And therefore this is the last possible scale
where the old strategy must certainly break down.

> Doesn't the Standard Model consider leptons to be elementary particles? Is
> there a simple to understand reason for this? Do any extensions of the
> Standard Model treat leptons as composite particles? At what energy levels
> would electrons show internal structure?

No, there are no extensions like that. For instance, in the Standard
Model, electrons and neutrinos form doublets of SU(2) symmetry. This
symmetry cannot be "broken" in any way because it is a local gauge
symmetry. A composite electron implies a composite neutrino, too.
Neutrinos are almost massless and it is extremely difficult to imagine
that they are composite particles. Such subparticles would probably have
to be very light - but then their wavelength would be too large, it would
determine the "size" of the electrons - and it would contradict the
experiments. The experiments agree with the "point-like" Standard Model up
to energies 1 TeV or so.

In the conventional stringy scenarios, the electrons are loops of
vibrating string as large as 10^{-35} meters or so. In the newer scenarios
with large dimensions, such an electron can be larger. But in both cases
there is no point-like substructure.

Best wishes
Lubos
______________________________________________________________________________
E-mail: lu...@matfyz.cz Web: http://www.matfyz.cz/lumo tel.+1-805/893-5025
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Superstring/M-theory is the language in which God wrote the world.

Frederick Seelig

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Aug 7, 2001, 10:16:30 PM8/7/01
to
"Ralph E. Frost" wrote:
>
> Matthew Nobes <man...@sfu.ca> wrote in message
> news:Pine.GSO.4.30.010804...@fraser.sfu.ca...
> > On Sun, 5 Aug 2001, Frederick Seelig wrote:
> >
> > > Do electrons have structure?
> >
> > To the best of our ablility to test, no.
>
> I think this needs a bit of clarification. Consider that in our local
> region/energy density, ALL imagery is created, conveyed and communicated in
> terms of whole electron units. We can't leak one third of an electron out
> of some membrane and then have that scuttle across into a detector.
>
> In fact, when we power up the fields and equipment used to look for electron
> substructure, folks do so in units of whole electrons. When we develop
> photographs. we do so in terms of whole electrons.
>
> So, since the result of ALL electron substructure queries will ultimately be
> reduced down to ambient energy levels, is it POSSIBLE to have a resolved
> image of a fractional electron?
>
> I don't think so.

Not true. Electrons can resolve the internal structure of hadrons, even
though
quarks have only fractional charge.

Kevin A. Scaldeferri

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Aug 7, 2001, 10:16:44 PM8/7/01
to
In article <tmtp3l9...@corp.supernews.com>,

Ralph E. Frost <ref...@dcwi.com> wrote:
>
>Matthew Nobes <man...@sfu.ca> wrote in message
>news:Pine.GSO.4.30.010804...@fraser.sfu.ca...
>> On Sun, 5 Aug 2001, Frederick Seelig wrote:
>>
>> > Do electrons have structure?
>>
>> To the best of our ablility to test, no.
>
> I think this needs a bit of clarification. Consider that in our local
>region/energy density, ALL imagery is created, conveyed and communicated in
>terms of whole electron units. We can't leak one third of an electron out
>of some membrane and then have that scuttle across into a detector.
>
>In fact, when we power up the fields and equipment used to look for electron
>substructure, folks do so in units of whole electrons. When we develop
>photographs. we do so in terms of whole electrons.
>
>So, since the result of ALL electron substructure queries will ultimately be
>reduced down to ambient energy levels, is it POSSIBLE to have a resolved
>image of a fractional electron?

Of course it is. Hadronic colliders do this all the time.


>
>Er, don't some HEP test smash electrons into pieces?

in some sense, but that's a terribly misleading way of describing what
happens.

--
======================================================================
Kevin Scaldeferri Calif. Institute of Technology
The INTJ's Prayer:
Lord keep me open to others' ideas, WRONG though they may be.

Charles Francis

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Aug 7, 2001, 10:19:43 PM8/7/01
to
In article <3B69516A...@mitre.org>, Frederick Seelig

<fse...@mitre.org> writes
>Do electrons have structure?

They have no substructure. Their structure consists of the manner in
which they interact with photons.

> From what I have read, electrons are still
>considered point particles at today's colliders' energy levels. But
>wouldn't it be reasonable to suppose that electrons were composite
>particles, too? If hadrons consist of fractional charge thingies, oughtn't
>one to suppose that leptons do too?
>
>Doesn't the Standard Model consider leptons to be elementary particles?

Yes.

>Is
>there a simple to understand reason for this?

Dirac originally searched for a relativistic first order equation of
motion to describe a fundamental particle. He found only the Dirac
equation. Then we can conclude that particles which obey the Dirac
equation are actually fundamental.

Regards

--
Charles Francis

Maury Markowitz

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Aug 7, 2001, 10:22:01 PM8/7/01
to
"Matthew Nobes" <man...@sfu.ca> wrote in message
news:Pine.GSO.4.30.010804...@fraser.sfu.ca...
> People have tried to make theories like this. Look up
> ``preons''.

Speaking of preons, can anyone give me a yeah/neah on "PREONS
Models of Leptons, Quarks and Gauge Bosons as Composite Objects"?

Maury

Maury Markowitz

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Aug 7, 2001, 10:22:17 PM8/7/01
to
"Ralph E. Frost" <ref...@dcwi.com> wrote in message
news:tmtp3l9...@corp.supernews.com...

> I think this needs a bit of clarification. Consider that in our local
> region/energy density, ALL imagery is created, conveyed and communicated
in
> terms of whole electron units.

The issue isn't in terms of "whole/half electron units". You can do this
test without any measure of the units at all.

> Er, don't some HEP test smash electrons into pieces?

About a year ago right? Humphrey Maris's conclusions from a liquid-helium
experiment?

If it's the one I'm thinking of I remember reading the reasoning behind it
and thinking "this is completely bogus". Now of course I don't do this for a
living, and it's entirely possible that the entirely bogus part was horrid
simplification needed in order to get it into the form that I was reading.
Still, my BS detector was ringing loud. Here, you tell me:

"I found that the force exerted by the electron was enough to elongate the
bubble until it formed a thin neck," he says. "If the pressure in the liquid
was great enough, there was the possibility of it pinching off the neck so
that the bubble might actually split in two."

Ummm, ok.

There was also some work at KEK, but that was screening experiments and
nothing like fractional charge or anything.

Maury

Aaron Bergman

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Aug 8, 2001, 1:27:52 PM8/8/01
to
In article <m2wv4ie...@pusch.xnet.com>,

gdp...@NO.xnet.SPAM.com (Gordon D. Pusch) wrote:

> Many people have attempt to construct such ``composite'' theories (e.g.,
> the class of so-called ``technicolor'' models), but all have failed.

Technicolor is a composite Higgs theory as I remember it, not a
composite electron theory. They're pretty much ruled out by the lack of
appreciable FCNCs.

Aaron

Gerry Quinn

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Aug 8, 2001, 4:52:16 PM8/8/01
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In article <Pine.SOL.4.10.101080...@strings.rutgers.edu>, Lubos Motl <mo...@physics.rutgers.edu> wrote:
>
>No, there are no extensions like that. For instance, in the Standard
>Model, electrons and neutrinos form doublets of SU(2) symmetry. This
>symmetry cannot be "broken" in any way because it is a local gauge
>symmetry. A composite electron implies a composite neutrino, too.
>Neutrinos are almost massless and it is extremely difficult to imagine
>that they are composite particles. Such subparticles would probably have
>to be very light - but then their wavelength would be too large, it would
>determine the "size" of the electrons - and it would contradict the
>experiments. The experiments agree with the "point-like" Standard Model up
>to energies 1 TeV or so.
>

But surely one can always make the argument (at least above the Planck
scale) that for any given seemingly elementary particle, the
subparticles are heavy and the binding energy is high. Is it that
neutrinos are special because of neutrino oscillations, which presumably
involve any possible sub-particles and therefore speak against them
being 'locked' together in this fashion?

- Gerry Quinn

Matthew Nobes

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Aug 8, 2001, 5:01:06 PM8/8/01
to
On Wed, 8 Aug 2001, Maury Markowitz wrote:

> "Ralph E. Frost" <ref...@dcwi.com> wrote in message
> news:tmtp3l9...@corp.supernews.com...

[snip]


> > Er, don't some HEP test smash electrons into pieces?
>
> About a year ago right? Humphrey Maris's conclusions from a
> liquid-helium experiment?
>
> If it's the one I'm thinking of I remember reading the
> reasoning behind it and thinking "this is completely bogus".
> Now of course I don't do this for a living, and it's entirely
> possible that the entirely bogus part was horrid
> simplification needed in order to get it into the form that I
> was reading. Still, my BS detector was ringing loud. Here,
> you tell me:
>
> "I found that the force exerted by the electron was enough to
> elongate the bubble until it formed a thin neck," he says.
> "If the pressure in the liquid was great enough, there was
> the possibility of it pinching off the neck so that the
> bubble might actually split in two."
>
> Ummm, ok.

We had a talk on Maris' work a few months back by a student here
at SFU. I'm summerizing from mememory, but here's the essence of
it.

Maris' rigged it up so that the electron's wave function looked
like an ovoid
_______
/ \
| |
\_______/

Then, via some method, which I don't remeber, he managed to
``squeeze'' the wavefunction
__ __
/ \/ \
| |
\__/\__/

Then by increaseing the ``pressure'' via the misremebered
techniqe he managed to ``pinch'' the two lobes off.
__ __
/ \ / \
| | | |
\__/ \__/

I stress that up to this point, *nothing* here violates QM.
Maris invented a clever technique in order to do this, nothing
more. This result actually explains some prior experimental
anomlies IIRC.

The problem lies in Maris' interpretation of what he did. He
claims that he ``split'' the electron into two particles. (at
least, this is what was claimed in the popular press, and the
talk I saw). But as far as I can tell this both violates QM and
hasn't really been tested.

What conventional QM says is quite clear. The electron is now in
a superpostion state (albeit a bizzare one). Say I whip one of
these ``double bubble'' wavefunctions up, then send each bubble
off in different directions

<- B1 B2 -> X

Let's say I put a charge dector at the point marked by the X.
Conventional QM is very clear as to what will happen. Assumeing
a totally even ``split'' 50% of the time my detector will se
nothing, and the other 50% of the time it will see a charge e.
So in that sense the electron has not been ``split'' at all.

The popular reports seemed to be suggesting that Maris claims
that the detector will see 1/2e. I have no idea if they got his
postion correct or not. If they did then what he says violates
conventional QM, if they didn't then he hasn't really ``split''
the electron (though he did come up with an extremely clever way
to put it in a superposition state).

Maury Markowitz

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Aug 9, 2001, 1:27:43 PM8/9/01
to
"Matthew Nobes" <man...@sfu.ca> wrote in message
news:Pine.GSO.4.30.01080...@fraser.sfu.ca...

> We had a talk on Maris' work a few months back by a student here
> at SFU. I'm summerizing from mememory, but here's the essence of
> it.

Excellent summary.

> Then, via some method, which I don't remeber, he managed to
> ``squeeze'' the wavefunction
> __ __
> / \/ \
> | |
> \__/\__/
>
> Then by increaseing the ``pressure'' via the misremebered
> techniqe he managed to ``pinch'' the two lobes off.

This is the basic problem I have. I'd like to avoid the debate on what the
wavefunction "is", but I can't think of any version of it that means that
it's physical and can be sqeezed. The wavefunction isn't a thing, the shape
of the wavefunction isn't physical. Regardless of the shape of the
wavefunction, the electron inside is still a point (after measurement).

It all comes down to this: what does it mean to squeeze a wavefunction? If
you apply pressure to the electron, the wavefunction changes en mass.

> Let's say I put a charge dector at the point marked by the X.
> Conventional QM is very clear as to what will happen. Assumeing
> a totally even ``split'' 50% of the time my detector will se
> nothing, and the other 50% of the time it will see a charge e.
> So in that sense the electron has not been ``split'' at all.

Right.

> The popular reports seemed to be suggesting that Maris claims
> that the detector will see 1/2e.

Yes, but his claim is based on something very different, the fact that the
charge transport rate increased. He explains this by suggesting that these
"double bubble" wavefunctions are 1/2 the size of the originals, and thus
they can move through the He faster than a single large one.

Let's put it this way, what's the difference between a wavefunction with
widely separated lobes, and what he's talking about from a experimental
standpoint? Nothing. If you take the copenhagen-ish approach to
understanding what the wavefunction means , then this split wavefunction is
no mystery at all, the electron is here or there. There doesn't seem to be
any reason to suggest it's a half electron, and I'd be surprised if there
was any way to tell the difference.

Of course this all rests on the interpretation that that bubble size the
electron forms in the He is indeed dependant on the shape of the
wavefunction. I can't say one way or the other.

Maury

Phil Gardner

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Aug 10, 2001, 12:55:44 PM8/10/01
to
nos...@de-ster.demon.nl (J. J. Lodder) wrote in message news:<1exqp5q.18i...@de-ster.demon.nl>...

> Phil Gardner <pej...@oznetcom.com.au> wrote:
>
> > Does the "evidence" that they are point particles include anything
> > more than the facts that:
> > (a) the cross-section for elastic scattering of electrons through
> > angles exceeding (say) pi/2 decreases without limit as the electron
> > energy is increased.
> > (b) in the last 70 years or more no-one has constructed a plausible
> > model of an extended electron with a mass-energy density that is
> > everywhere finite and continuous.
>
> "Point particle" is a phenomenological concept anyway.
> It means: particles that are described adequately
> by a theory containing no structure-related parameters.
>
> Jan


To any classical physicist the best available quantum mechanical
models provide no physical description at all of electrons and other
leptons, let alone an adequate one. They are very successful at
predicting a considerable range of experimental outcomes but that is
all. They can give no answer at all to such simple questions as:

How is the mass-energy of the particle distributed in space? Within
what radius is 50% of it contained?


Phil Gardner (pej...@oznetcom.com.au)

Phil Gardner

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Aug 10, 2001, 12:59:16 PM8/10/01
to
gdp...@NO.xnet.SPAM.com (Gordon D. Pusch) wrote in message news:<m2wv4ie...@pusch.xnet.com>...

>
> There is a fundamental problem in that the ``natural'' energy scale for
> an object of size on the order of R is M*c^2 ~= (\hbar c) / R. The current
> experimental upper-bound on the sizes of an electron, muon, or `u' and `d'
> quark is less that 1e-18 m,
>
> -- Gordon D. Pusch
>

Do these meaurements of the "size of the electron" measure anything
more than the size of the scattering cross-section and how it varies
with the kinetic energy of the electron. What grounds have we for
assuming they tell us any more about the real size of the electron
than the Rayleigh scattering cross-section (one that goes to zero as
the photon energy) tells us about the size of a molecule or dielectric
sphere, ie nothing at all?


Phil Gardner <pej...@oznetcom.com.au>

Kevin A. Scaldeferri

unread,
Aug 13, 2001, 11:54:39 PM8/13/01
to
In article <744cb7cd.01081...@posting.google.com>,

Phil Gardner <pej...@oznetcom.com.au> wrote:
>nos...@de-ster.demon.nl (J. J. Lodder) wrote in message news:<1exqp5q.18i...@de-ster.demon.nl>...
>> Phil Gardner <pej...@oznetcom.com.au> wrote:
>>
>> "Point particle" is a phenomenological concept anyway.
>> It means: particles that are described adequately
>> by a theory containing no structure-related parameters.
>
>To any classical physicist the best available quantum mechanical
>models provide no physical description at all of electrons and other
>leptons, let alone an adequate one.

I don't know what you mean by "adequate" if not:

> They are very successful at
>predicting a considerable range of experimental outcomes

At any rate, this is wrong:

> They can give no answer at all to such simple questions as:
>
>How is the mass-energy of the particle distributed in space? Within
>what radius is 50% of it contained?

The models claim the particles are pointlike.

Experiment tells us that the radius is less than 10^-18 m

Lubos Motl

unread,
Aug 14, 2001, 6:07:47 PM8/14/01
to
On Fri, 10 Aug 2001, Phil Gardner wrote:

> To any classical physicist the best available quantum mechanical
> models provide no physical description at all of electrons and other
> leptons, let alone an adequate one.

Well, most of the developments of 20th century physics would sound useless
or absurd to any "classical physicist". The energy of any radiation is
discrete; the atoms are stable; the electrons in atoms are described by
wave functions; time and space are perceived differently by observers in
relative motion. Fortunately, it is the classical physicist, not modern
physics, who is missing something. ;-) And modern physics also knows much
better which questions are well-defined and which questions are not.

> They are very successful at predicting a considerable range of
> experimental outcomes but that is all. They can give no answer at all
> to such simple questions as:
>
> How is the mass-energy of the particle distributed in space? Within
> what radius is 50% of it contained?

Well, similar questions have often a refined meaning in quantum field
theory; we can read answers from the correlators of the energy-momentum
tensor etc. While the electrons in path integrals etc. are truly
point-like, the correlators including quantum corrections show some
nontrivial smoothed behaviour at distances like the Compton wavelength and
the classical radius of the electron. But one would have to make the
question quantitative before we can deduce some quantitative answers. The
question formulated above neglects the uncertainty principle and does not
make much sense.

If we consider a state in the Hilbert space containing one electron, we
cannot localize it to a space smaller than the Compton wavelength and
therefore the expectation value of the energy density will be distributed
over a comparable volume and will depend on the precise shape of the wave
packet. A more localized idea about the "shape" of the electron's mass
density can be calculated from electron-graviton scattering.

Lubos Motl

unread,
Aug 13, 2001, 5:58:35 AM8/13/01
to
On Wed, 8 Aug 2001, Gerry Quinn wrote:

> But surely one can always make the argument (at least above the Planck
> scale) that for any given seemingly elementary particle, the
> subparticles are heavy and the binding energy is high.

Yes, there exists a limited possibility of heavy fundamental particles
whose binding energy is huge and negative and therefore the resulting
composite particle is light. This possibility requires a strongly coupled
theory. For example, pieces of string have essentially Planckian mass and
the cancellations between the kinetic energy and the tension of the string
(one of them can be negative due to the quantum zeta function miracles)
can lead to exactly massless particles (such as the photon) or
approximately massless ones (such as the electron); see the Chapter 6 of
The Elegant Universe. ;-)

But this logic is not true in theories similar to QCD. If the concept of
"partons" is meaningful, they should be weakly coupled at some scale. In
the case of QCD, this is the QCD scale 300 MeV corresponding to the
distances 1 fermi or so (the size of a small nucleus etc.). It is the same
scale that determines both the size (1 fermi) and the mass (1 GeV) of the
composite particles (such as the proton). All the scales agree, everything
is natural. Proton's mass is what you expect from such an interaction that
becomes weakly coupled at sub-fermi distances.

I wanted to say that a corresponding "compositeness scale" in the case of
electrons must be greater than approximately 1 TeV - because we know that
they look point-like (as in QED) at distances 10^{-18} meters. Then it is
extremely unnatural to get particles as light as electrons and especially
neutrinos (fractions of eV) from such a theory that is strongly coupled at
all the distance scales longer than 10^{-18} meters, a scale corresponding
to 1 TeV. This would require a cancellation which is extremely unlikely in
the case of quantum field theories.

Compositeness is fine, but in the case of electrons and neutrinos,
compositeness from several point-like subparticles is unreasonable. The
capacity of this simple paradigm has been exhausted. We must study strings
to see deeper into the particles that we call "elementary" today.

J. J. Lodder

unread,
Aug 16, 2001, 5:53:47 AM8/16/01
to
Phil Gardner <pej...@oznetcom.com.au> wrote:

> To any classical physicist the best available quantum mechanical
> models provide no physical description at all of electrons and other
> leptons, let alone an adequate one. They are very successful at
> predicting a considerable range of experimental outcomes but that is
> all. They can give no answer at all to such simple questions as:

> How is the mass-energy of the particle distributed in space? Within
> what radius is 50% of it contained?

Guess I should take you up on that:
The classical theories of the electron did not do anything like that
either. For the state of the art you should consult:
H. A. Lorentz: "Theory of Electrons"
(last decades of the 19th century, final form ą 1905)
(real heavy stuff; never underestimate the ancient massters :-)
More up to date textbooks, Jauch's for example,
are easier going, but do not added major conceptual advances.

Lorentz considered extended electrons, with an assumed charge
distribution, and calculated it all using Maxwell's eqns,
including the selfinteractions,
and inventing the Lorentz transformations on the way.

Next L. dealt with the classical divergencies
like in the 'modern' renormalization group methods:
keep only terms independent of the assumed charge distribution
to obtain the physically relevant observable term,
like electromagnetic mass, mass dependence on velocity, etc.

And he obtained the correct result for the mass/velocity relation first,
which was originally known as the Lorentz-Einstein mass formula.

As far as physics is concerned there is no difference
between 'real' point particles and particles with a structure
that is far too small to observe.

Best,

Jan

Jim Carr

unread,
Aug 18, 2001, 2:06:42 PM8/18/01
to
Frederick Seelig <fse...@mitre.org> wrote
in message news:<3B69516A...@mitre.org>...
}
} Do electrons have structure? From what I have read, electrons are still
} considered point particles at today's colliders' energy levels.
<... snip rest ...>

In article <744cb7cd.01080...@posting.google.com>
pej...@oznetcom.com.au (Phil Gardner) writes:
>
>Does the "evidence" that they are point particles include anything
>more than the facts that:
>(a) the cross-section for elastic scattering of electrons through
>angles exceeding (say) pi/2 decreases without limit as the electron
>energy is increased.
>(b) in the last 70 years or more no-one has constructed a plausible
>model of an extended electron with a mass-energy density that is
>everywhere finite and continuous.

Yes.

It includes the fact that the energy and momentum transfer
dependence of scattering data agree (to within experimental
uncertainties) with the predictions of QED without any need
for a size correction (called a form factor). This is much
more than the qualitative statement you gave, and places
a quantitative limit around 10^{-18} m for substructure.

It also inlcudes plausible models (some named in other replies),
but you do not need a model to measure a model-independent
quantity like a form factor.

--
James Carr <j...@scri.fsu.edu> http://www.scri.fsu.edu/~jac/
SirCam Warning: read http://www.cert.org/advisories/CA-2001-22.html

e-mail info: new...@fbi.gov pyr...@ftc.gov enfor...@sec.gov

John Baez

unread,
Aug 20, 2001, 5:26:46 PM8/20/01
to
In article <tmtp3l9...@corp.supernews.com>,
Ralph E. Frost <ref...@dcwi.com> wrote:

>Er, don't some HEP tests smash electrons into pieces?

No: if they did, we wouldn't all be telling you that there's
no evidence for substructure in electrons!

John Baez

unread,
Aug 20, 2001, 5:37:44 PM8/20/01
to
In article <Pine.GSO.4.30.01080...@fraser.sfu.ca>,
Matthew Nobes <man...@sfu.ca> wrote:

>Maris rigged it up so that the electron's wave function looked
>like an ovoid
_______
/ \
| |
\_______/

>Then, via some method, which I don't remember, he managed to


>``squeeze'' the wavefunction
__ __
/ \/ \
| |
\__/\__/

>Then by increaseing the ``pressure'' via the misremembered
>technique he managed to ``pinch'' the two lobes off.


__ __
/ \ / \
| | | |
\__/ \__/

>I stress that up to this point, *nothing* here violates QM.

>The problem lies in Maris' interpretation of what he did. He


>claims that he ``split'' the electron into two particles.

Ugh! I guess he just wanted to get on the front cover of
some popular science magazines. Do an experiment, get the
result quantum mechanics predicts... but then make up a completely
crazy way of explaining it in words, and you can have your
15 minutes of fame. It seems by now to be an established
technique - for example, all the results which supposedly
demonstrate "superluminal communication", but which actually
rely on the confusion between group velocity, phase velocity
and signal velocity:

http://www.netspace.net.au/~gregegan/APPLETS/20/20.html

As long as reporters keep falling for scientists who try to
make their work sound cooler than it is, certain scientists
will keep doing it.

John Baez

unread,
Aug 20, 2001, 5:41:01 PM8/20/01
to
In article <abergman-5142DA...@cnn.princeton.edu>,
Aaron Bergman <aber...@Princeton.EDU> wrote:

>Technicolor is a composite Higgs theory as I remember it, not a
>composite electron theory. They're pretty much ruled out by the lack of
>appreciable FCNCs.

What the f**k is a FCNC?

Frederick Seelig

unread,
Aug 20, 2001, 6:11:56 PM8/20/01
to

Jim Carr wrote:

[snip]


> It includes the fact that the energy and momentum transfer
> dependence of scattering data agree (to within experimental
> uncertainties) with the predictions of QED without any need
> for a size correction (called a form factor). This is much
> more than the qualitative statement you gave, and places
> a quantitative limit around 10^{-18} m for substructure.
>
> It also inlcudes plausible models (some named in other replies),
> but you do not need a model to measure a model-independent
> quantity like a form factor.

Jim,

Current experimental evidence aside, would you care to speculate
on what physicists in 10 or 50 years will see? In 2050, will we
be taught that electrons are still point particles? Or will they
have substructure? What do you guess?

Fred

Aaron J. Bergman

unread,
Aug 21, 2001, 1:09:26 PM8/21/01
to

Flavor Changing Neutral Current. The bane of all attempts to go beyond
the standard model.

And I might be wrong about the last statement, too. It might be that
technicolor is ruled out by neutron dipole moment experiments. I can't
remember now. It's probably both, although (does some checking)
hep-ph/0007304 seems to lean towards FCNCs, but I only glanced. It's a
shame that technicolor is pretty much ruled out, though -- it's quite
the pretty theory.

Aaron
--
Aaron Bergman
<http://www.princeton.edu/~abergman/>

Jeff Berryhill

unread,
Aug 21, 2001, 1:11:08 PM8/21/01
to
In article <9ls05d$gnq$1...@glue.ucr.edu>,

Flavor Changing Neutral Current interactions, usually pertaining to
quarks. The experimental data regarding them severely restrict any
kind of model which directly connects two up-like quarks (or two
down-like quarks) of differing flavors. Examples of FCNC phenomena
are

K^0, B^0, or D^0 mixing
K^0, D^0 or B^0 \rightarrow \ell\ell
B \rightarrow K^* \gamma or \rho \gamma
D \rightarrow \rho \gamma
B \rightarrow K^* \ell\ell or \rho \ell\ell

\mu \rightarrow e \gamma is an FCNC involving leptons.

In the standard model the quark flavor transitions b to d, b to s, c
to u, t to c, etc., are forbidden at tree level but they can sneak in
via second order electroweak diagrams like "boxes" and "penguins"
(don't ask me to draw one in ASCII). Being second order in the
electroweak coupling, the standard model rates are quite small, so
*any* new physics which introduces FCNCs at tree level will
substantially enhance the rates above the SM predictions, even if it's
suppressed by some large mass scale, as high as tens of TeV.

Lots of mesons of the types listed above have been produced and
studied in laboratories, and no deviations from SM predictions have
been observed. Charged higgses, technimesons, squarks, just about any
physics beyond the standard model may contribute to FCNCs and so
phenomenologists have to go through all kinds of (otherwise
unmotivated) contortions to evade the experimental constraints placed
by FCNCs. They're a very powerful probe of the electroweak scale.


--Jeff Berryhill

Paul D. Shocklee

unread,
Aug 21, 2001, 1:12:24 PM8/21/01
to

Flavor-Changing Neutral Currents.

Like s -> d + X^0.

--
Paul Shocklee
Graduate Student, Department of Physics, Princeton University
Researcher, Science Institute, Dunhaga 3, 107 Reykjavik, Iceland
Phone: +354-525-4429

Kevin A. Scaldeferri

unread,
Aug 21, 2001, 1:13:34 PM8/21/01
to

Flavor changing neutral current

And, while the above it true of the most simple technicolor models,
the proponent of technicolor are as clever as any one else about
finding variations that aren't ruled out.

John Baez

unread,
Aug 21, 2001, 10:26:44 PM8/21/01
to
In article <9kmkuh$k0t$1...@news.state.mn.us>,
Moataz Emam <em...@physics.umass.edu> wrote:

>The only known reasonable theory of electronic structure is within the
>context of String theory, where electrons are extended strings in
>spacetime. That is only apparent on the level of the Planck energy, or
>length scale, about 10^-33 cm.

I consider preon models to be almost as reasonable as string
theory, and in these models, quarks and leptons are bound states of
"preons", with compositenss becoming manifest at length scales
far exceeding the Planck length. Sure, preon models have their
problems, but so does string theory! Anyway, my point is not to
advocate these models so much as to remind people of their existence.
They are not popular these days, but they do have their charms.
One can probably read about them in Mohapatra's or Ross' books on
grand unified theories.

Matthew Nobes

unread,
Aug 21, 2001, 11:10:27 PM8/21/01
to

Flavour changing neutral current.

Ralph E. Frost

unread,
Aug 26, 2001, 9:27:53 PM8/26/01
to
John Baez <ba...@galaxy.ucr.edu> wrote in message
news:9lrvam$gdj$1...@glue.ucr.edu...

What was I thinking? Gellman said things are made of quarks, electrons, and
photons. I guess he must have meant there were structured out of quarks,
electrons and photons, huh?

Oops, what are the neutrino things then, debris that's caught in
back-eddies?

Lubos Motl

unread,
Aug 27, 2001, 5:27:17 PM8/27/01
to
John Baez:

> I consider preon models to be almost as reasonable as string
> theory, and in these models, quarks and leptons are bound states of
> "preons", with compositenss becoming manifest at length scales
> far exceeding the Planck length.

Preon models are simple quantum field theories based on a simple idea that
leptons, quarks and even gauge bosons (!!) can be composite particles. All
of them should compose of spin-1/2 preons. There is nothing such as "preon
theory". This class of models does not contain gravity; they look less
natural than the Standard Model itself. The models not solve a single
serious problem of physics today - with a possible exception of the chance
to explain some hierarchies in QFT. As far as I know, they essentially do
not offer anything that the Standard Model cannot. In my opinion, it
sounds funny to compare the preon proposal with string theory. To see how
unrealistic and unorthodox the models are, see

http://arXiv.org/abs/hep-ph/9909569

> Sure, preon models have their problems, but so does string theory!

No, you certainly cannot compare them.

> Anyway, my point is not to
> advocate these models so much as to remind people of their existence.
> They are not popular these days, but they do have their charms.

Could you please write more details about the charms that they are
supposed to have? Your text seems as another attempt to spread the
illusion that string theory is not the unique path to unification -
without having any evidence whatsoever. But string theory probably *is*
the unique path. Preons are something close to technicolor and they share
similar problems; the absence of realistic models is an important example.
String theory certainly do not suffer from a similar kind of troubles.

Toby Bartels

unread,
Aug 27, 2001, 12:52:48 AM8/27/01
to
Ralph E. Frost wrote:

>What was I thinking? Gellman said things are made of quarks, electrons, and
>photons. I guess he must have meant there were structured out of quarks,
>electrons and photons, huh?

>Oops, what are the neutrino things then, debris that's caught in
>back-eddies?

2 possibilities:

* Gell-Mann was being elliptical,
listing only *examples* of the items that things are made of.
Besides quarks, electrons, photons, and neutrinos, the list includes
muons, tauons, gluons, and W, Z, and Higgs bosons,
as well as presumably something along the lines of gravitons.
So far, it appears that none of these have substructure.

* Gell-Mann was talking only about ordinary things.
To a high level of precision, the number of
neutrinos, muons, tauons, and W, Z, and Higgs bosons
in ordinary matter is 0.
Even among quarks, only 2 of the 6 flavours show up significantly.
He could easily have left out gravitons as too speculative.
However, this possibility doesn't explain his omission of gluons,
which are quite common in ordinary stuff.


-- Toby
to...@math.ucr.edu

Lubos Motl

unread,
Aug 28, 2001, 2:59:46 PM8/28/01
to
Ralph E. Frost <ref...@dcwi.com> wrote:

> Er, don't some HEP tests smash electrons into pieces?

HEP experiments can smash one positron and a single electron (with a very
huge energy) and produce 25 protons, 20 antiprotons, some neutrons,
neutrinos, pions etc. If this event does not violate the universal laws
(conservation of momentum, angular momentum, energy and the electric
charge), essentially anything is possible. But this fact does not imply
that electrons are made of 25 protons etc. If we want to say that a
particle is made of some constituents, there should be a way to see them.
For example the deep inelastic scattering experiments "proved" quarks
inside nucleons.

Ralph E. Frost

unread,
Aug 27, 2001, 11:38:23 PM8/27/01
to

Lubos Motl <mo...@physics.rutgers.edu> wrote in message
news:Pine.SOL.4.10.101082...@strings.rutgers.edu...

> John Baez:

> > I consider preon models to be almost as reasonable as string
> > theory, and in these models, quarks and leptons are bound states of
> > "preons", with compositenss becoming manifest at length scales
> > far exceeding the Planck length.

> Preon models are simple quantum field theories based on a simple idea that
> leptons, quarks and even gauge bosons (!!) can be composite particles. All
> of them should compose of spin-1/2 preons. There is nothing such as "preon
> theory". This class of models does not contain gravity; they look less
> natural than the Standard Model itself. The models not solve a single
> serious problem of physics today - with a possible exception of the chance
> to explain some hierarchies in QFT. As far as I know, they essentially do
> not offer anything that the Standard Model cannot. In my opinion, it
> sounds funny to compare the preon proposal with string theory. To see how
> unrealistic and unorthodox the models are, see

How come the same can't be said for the standard model of, um, a couple
months ago before it began to SNO? HOW did it describe neutrinos??

I think, given the massive intellectual deposits into the string theory
account, that if someone was going to pull the rabbit out of that hat,
they would have done it by now. It's been several dog-years since Gellman
suggested it was the last great hope for the traditionalists.

Also, I am not too sure that striving to maintain agreement with the
standard model is a wise objective function. Or, did string theory predict
the neutrino "oscillation"/massness BEFORE it was measured?

More likely, given that all the less unified models (aka the traditional
imagery) do not BEGIN with a primary overt tenet that says, "things are
unified", there is rough sledding ahead for SM version xx++ die-hards.

The one world, many descriptions model image sort of guarantees that string
theory must be, or have been useful in the overall transition, but to make
more of it seems presumptuous when staring at such a dearth of, you know,
things that jibe with experiment.

Oops, I guess the same can be said about the prior version of the standard
model (again) (and again.) (..).

Let me guess. You disagree.

> http://arXiv.org/abs/hep-ph/9909569

> > Sure, preon models have their problems, but so does string theory!

> No, you certainly cannot compare them.

> > Anyway, my point is not to
> > advocate these models so much as to remind people of their existence.
> > They are not popular these days, but they do have their charms.

> Could you please write more details about the charms that they are
> supposed to have? Your text seems as another attempt to spread the
> illusion that string theory is not the unique path to unification -
> without having any evidence whatsoever. But string theory probably *is*
> the unique path. Preons are something close to technicolor and they share
> similar problems; the absence of realistic models is an important example.
> String theory certainly do not suffer from a similar kind of troubles.

Life is hard. We all got our problems, don't we?

Do you ever think about synthesizing? I happen to think that's where the
useful trial is. As for there being a "unique trail", I ain't no
mathematician, but even I know there isn't only one unique trail. There is
the <first> trail. There is the <first> really helpful, popular trail. But
there is not just the singular trail.

I feel certain you are aware of that.

John Baez

unread,
Aug 30, 2001, 7:02:20 PM8/30/01
to
In article <Pine.SOL.4.10.101082...@strings.rutgers.edu>,
Lubos Motl <mo...@physics.rutgers.edu> wrote:

>John Baez:

>> I consider preon models to be almost as reasonable as string
>> theory, and in these models, quarks and leptons are bound states of

>> "preons", with compositeness becoming manifest at length scales

>> far exceeding the Planck length.

>In my opinion, it sounds funny to compare the preon proposal with
>string theory.

Of course I said this mainly to see how you'd respond...
but I also had some other reasons, too.

>To see how unrealistic and unorthodox the models are, see
>
> http://arXiv.org/abs/hep-ph/9909569

I'm not sure why you picked this particular model to talk about -
as far as I know, it's not one of the most popular ones. Did you
pick it just because it seems weird? That wouldn't be very fair...

Anyway:

To me, being "unorthodox" is fine - we'll never figure out
the fundamental laws of physics by trying to be "orthodox".

On the other hand, being "unrealistic" is not good. You'll have
to explain to me why the above preon model is unrealistic: I've
never seen this one before, so it would take me a while to see
what physical predictions it gets wrong.

(I assume by saying that a theory is "unrealistic" you mean
that it makes wrong predictions. If you mean something else,
please explain what you mean.)

>> Sure, preon models have their problems, but so does string theory!

>No, you certainly cannot compare them.

Of course the two are very different. String theory is much more
ambitious: it's trying to explain gravity along with the forces
and particles in the Standard Model, and it's trying to be a
"theory of everything", good to arbitrarily high energy scales -
or the string energy scale, whichever comes first. To live up
to what's claimed for string theory, it needs to be pretty much
perfect. Preon models are only trying to serve as a next step
after the Standard model, and they're only trying to handle
physics up to energies of roughly 100 TeV or so. They're not
supposed to be the last word in physics. So, the demands we're
entitled to make on string theory are much higher.

But my point was that as theories of "what happens in particle
physics after the Standard Model", both preon models and string
theory have their problems. The problems of string theory are
well-known. For example:

1) Despite decades of work and over ten thousand papers on the
subject, string theory has not made a single experimentally
verified prediction. One reason is that:

2) There are zillions of different perturbative superstring vacua,
giving zillions of different theories of real-world particle physics.
Nobody knows which one is right, so we cannot use string theory to
make specific predictions about particle physics at low energies.
All we get for sure are very general results such as: there are
forces described by gauge fields, fermions have spin 3/2 or less...
and the following more surprising thing:

3) String theory predicts that every boson has a corresponding fermion
of the same mass! This is clearly wrong. The only way out is for
supersymmetry to be spontaneously broken. Unfortunately, nobody
understands how this works. For this reason, anyone wishing to use
string theory to make predictions about particle physics must break
supersymmetry "by hand" - that is, by penciling in dozens of "soft
supersymmetry breaking terms" in the field theories that arise as
low-energy limits of string theory.

Preon models also have their problems, and they are certainly far
less pretty than string theory. However, given the above problems of
string theory, it is far too soon to rule out alternatives like preon
models.

>> Anyway, my point is not to
>> advocate these models so much as to remind people of their existence.
>> They are not popular these days, but they do have their charms.

>Could you please write more details about the charms that they are
>supposed to have?

Sure! I'll start with something nontechnical that all the lurking
layfolkd can enjoy, and then mention a couple of more technical things.

First of all, everyone who has ever thought about particle physics
has wondered this:

"Molecules are made of atoms,
atoms are made of electrons and nuclei,
nuclei are made of protons and neutrons,
protons and neutrons are made of quarks and gluons....
what if it keeps on going like this?"

In fact, back when I used to read sci.physics, it seems like I'd
see a post about this every few months! For some reason the
people who post these articles usually jump to the conclusion
that particle physics is futile - a silly conclusion, in my view.
The universe is the way it is, and no matter how it is, we should
try to understand it!

Anyway, preon models are an attempt to study the possibility that
the "elementary" particles we know and love are built from more
basic constituents. I am glad people are looking into this sort
of scenario.

Of course, such a scenario is irritating if you want to jump from
the Standard Model to the theory of everything in one fell swoop!
However, there is no terribly strong reason to think this "one
fell swoop" approach is bound to work. It's an incredible
extrapolation which could easily fall flat on its face. So,
we should hedge our bets and also consider alternatives.

More technically, here is a preon model whose charm should
be visible by any particle physicist. I think this one was
cooked up by Pati, Greenberg and Sucher. (I'm no expert on
this stuff and all I know comes from the books on grand unified
theories by Rabindra Mohapatra and Graham Ross.)

The idea here is to lump all the fermions in a given generation into
a single irrep of SU(4) x SU(2) x SU(2). For example:

(u_R u_G u_B nu_e)
(d_R d_G d_B e )

The SU(4) group acts to mix up the columns of this matrix in the
obvious way. There's an obvious SU(3) subgroup mixing up the
red, green and blue quarks; this is the usual strong force SU(3).
But there are also transformations that mix up the quarks and leptons.
Thus, in this model, the distinction between quarks and leptons
arises from the spontaneous breaking of the symmetry from SU(4)
down to SU(3).

The two copies of SU(2) act to mix up the rows. To see how
this works, we need a more detailed picture of the above matrix,
where we separate out the left-handed and right-handed fermions.
We get a picture like this:

(u_r u_g u_b nu_e) <--
LEFT-HANDED QUARKS AND LEPTONS
(d_r d_g d_b e ) <--

(u_r u_g u_b nu_e) <--
RIGHT-HANDED QUARKS AND LEPTONS
(d_r d_g d_b e ) <---

We have one copy of SU(2) acting on the left-handed guys in
the obvious way, another acting on the left-handed ones.
Thus, in this model, the chiral nature of the weak force
arises from the spontaneous breaking of the symmetry from
SU(2) x SU(2) down to the left-handed copy of SU(2).

Now, I think this SU(2) x SU(2) x SU(4) theory goes back to
Pati and Salam in 1974. The new twist in the preon model is
to build the above fermions as bound states of more fundamental
fermions and bosons. The idea is to have the more fundamental
fermions transform nontrivially only under SU(2) x SU(2), and
the bosons under SU(4). The fermions look like this:

F_u <--- LEFT-HANDED "UP" FERMION

F_d <--- LEFT-HANDED "DOWN" FERMION

F_u <--- RIGHT-HANDED "UP" FERMION

F_d <--- RIGHT-HANDED "DOWN" FERMION

That is, they lie in C^2 x C^2 and transform under SU(2) x SU(2)
in the obvious way. The bosons look like this:

(B_r B_g B_b B_l)

and transform under SU(4) in the obvious way. Thus, bound states
consisting of one fermion and one boson will transform under
SU(4) x SU(2) x SU(2) in exactly the way that quarks and leptons do!

Of course, we need something to bind our fundamental fermions
and bosons together. We can do this with an SU(N) gauge field,
analogous to the strong force and usually called "technicolor",
which confines particles together in technicolor-neutral bound
states, just as the strong force binds quarks into hadrons.

To do this, we should make our F and B particles transform under
the fundamental representation of SU(N) and its dual, respectively.
They will then bind together in pairs - one F with one B - a bit
like how a quark and antiquark bind together to form a meson.

In short, besides our F and B particles, we have SU(N) gauge fields
carrying the technicolor force as well as SU(4) x SU(2) x SU(2)
gauge fields carrying the rest of the forces.

There is more to say about how the spontaneous symmetry breaking
goes, but I think I'll stop here.

Now: my point is *not* that I think this model is correct. My
point is just that particle physicists should continue to tinker
with such models, along with many other possibilities - including
string theory.

>Your text seems as another attempt to spread the
>illusion that string theory is not the unique path to unification -
>without having any evidence whatsoever.

String theory is obviously NOT the unique path that people are
taking in the attempt to understand the real world of particle
physics. Whether it's the unique path that succeeds, or whether
it succeeds at all, only time will tell. Once any theory starts
making predictions that are verified by experiments, you can be
sure that everyone will fall in line with that one! But in the
meantime we need people taking all sorts of different paths and
reporting back from time to time on how they're doing. In fact,
there *are* people taking all sorts of different paths, so all
we need to do is read about what they've done. It's not hard to
do, and it's actually fun.


Aaron J. Bergman

unread,
Aug 30, 2001, 9:48:34 PM8/30/01
to
In article <9mmglr$o5i$1...@news.state.mn.us>, John Baez wrote:

>But in the
>meantime we need people taking all sorts of different paths and
>reporting back from time to time on how they're doing. In fact,
>there *are* people taking all sorts of different paths, so all
>we need to do is read about what they've done. It's not hard to
>do, and it's actually fun.

I was bored the other day (there's frightfully little interesting stuff
in strings these days...) and was looking at the first few papers to
show up on lanl. I found this:

hep-th/9109002

The second paragraph of the introduction, in particular. The more things
change....

Ralph E. Frost

unread,
Aug 30, 2001, 11:08:57 PM8/30/01
to
John Baez <ba...@galaxy.ucr.edu> wrote in message
news:9lrvam$gdj$1...@glue.ucr.edu...

Er, pardonne moi, but aren't electrons known and proven to exist as both
particles and waves?

theos ek mechanes

unread,
Aug 31, 2001, 3:37:13 PM8/31/01
to
Yeah this is where I was going with that "What is spacetime" thread
I started...particularly, the final sentence of the second paragraph
of the introduction...

"As I will describe, there are indications that quantum GR provides
a natural cut-off at the Plank scale."

Best

aber...@Princeton.EDU (Aaron J. Bergman) wrote in message news:<slrn9otr7h....@phoenix.Princeton.EDU>...

Charles Francis

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Sep 1, 2001, 8:42:48 PM9/1/01