Landskepticism
Lubos Motl
we should perhaps stimulate it. Let me start a thread about
landskepticism.
http://motls.blogspot.com/2004/12/landskepticism.html
Tom Banks, my (former) PhD advisor, has submitted a new paper
* hep-th/0412129
Tom argues that the landscape is not a well-established feature of string
theory. In a conceptual paper which does not bother you with too many
equations (the Wheeler-DeWitt equation being an exception), he argues that
* one must distinguish the 1PI and the Wilsonian effective actions
more carefully: the 1PI action describes the whole field content, and you
get the same action regardless of the point around which you expand
* on the other hand, the Wilsonian effective action contains the
low-energy degrees of freedom that depend on the point in the
configuration space - and this is the type of the effective action that
one obtains in string theory
* Tom re-uses his statements that one cannot think about the bubble of
another vacuum (such as a de Sitter bubble) as an excitation of the
original vacuum, and interprets the Guth-Farhi results in this fashion -
one cannot verify the existence of a dS bubble inside the bottle because
the external observer sees that it is surrounded by a black hole
If I understand well, Tom does not propose new arguments that the numerous
KKLT-like vacua don't exist. Instead, he says that they cannot be
effectively used as vacua of the same theory, i.e. they cannot be combined
into a big multiverse, and therefore there is no cosmological mechanism
that would connect them and create a realistic ensemble of the Universes.
I would agree that this "disconnected feature" of the vacua prevents us
from making a scientific claim about the probability measure of different
ones. He argues that the "democratic" measure on the space of vacua is
unjustified, and a more precise measure cannot be defined - and I agree
with these statements. On the other hand, if the large number of stringy
vacua exist (just imagine that!), we may perhaps be living in either of
them, regardless of the existence of cosmological solutions (such as the
eternal inflation) that interpolate between them. Of course that I tend to
agree that even in this case, the full rules of string theory are more
likely to pick some "special" vacua rather than the numerous ones, but we
don't have any proof either way. In order to kill the current versions of
the landscape idea completely, one would have to find a general enough and
serious problem with the construction of the KKLT-like vacua.
Nima Arkani-Hamed, who has been converted to a landscape supporter much
like many other people have been converted to Christianity ;-) (but
otherwise he studies it much more scientifically than other landscapers!),
argues very correctly that the anthropic idea is either colossally correct
or colossally wrong. It is a bifurcation point that can direct physics in
the next 10 years in vastly different directions, and one cannot decide
which answer is correct by cheap attacks.
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Urs Schreiber
news:Pine.LNX.4.31.041215...@feynman.harvard.edu...
> Tom argues that the landscape is not a well-established feature of string
> theory.
I find Bank's comment interesting, because I was always surprised with how
little time and space landscape papers devote to clarification of basic
issues - issues like if the problem to be tackled is well defined and
exists, for instance. Essentially, landscaping is a flavor of quantum
cosmology, which has always been conceptually very difficult.
When we discussed here a Susskind paper a while ago I voiced my surprise
about how the author seemed to gloss over a couple of well-known subtleties
concerning the role the Hamiltonian (constraint) plays in cosmology and what
this implies for the "energy" that he used in his canonical ensemble over ST
vacua.
Apart from the issue of what to do with knowledge of landscape numbers (e.g.
whether or not knowing that there are N vacua with a given property implies
anything about physics) it seems to be clear that what is known about the
landscape is on rather more shaky grounds than usual for "non-rigorous"
physics fact.
In my humble opinion it appears as if it is way to early to say much in
particular about the space of vacua of string theory with certainty, and
that the best would be if people kept their ideas about the landscape at the
same place where they keep their feelings about the interpretation of
quantum mechanics - until, maybe, some advancement in our understanding of
that vast edifice called string theory has advanced to a point where the
analysis of the landscape really becomes a well-defined scientific
enterprise.
Lubos Motl
> I find Bank's comment interesting, because I was always surprised with how
> little time and space landscape papers devote to clarification of basic
> issues - issues like if the problem to be tackled is well defined and
> exists, for instance. Essentially, landscaping is a flavor of quantum
> cosmology, which has always been conceptually very difficult.
I agree. And one of the reasons why these basic questions are not given
enough time is exactly because many people don't consider the landscape
proposals to be a part of quantum cosmology. But of course that if one
talks about the vacuum selection - or at least about the question which
vacua should be preferred and looked at - then he or she does study
questions of quantum cosmology, and this field is subtle.
It may be useful if the landscape people tried to respond to these
proposals and questions that Tom raised - whether the effective actions
are Wilsonian or 1PI, whether it's important, and so on.
> When we discussed here a Susskind paper a while ago I voiced my surprise
> about how the author seemed to gloss over a couple of well-known subtleties
> concerning the role the Hamiltonian (constraint) plays in cosmology and what
> this implies for the "energy" that he used in his canonical ensemble over ST
> vacua.
Well, you must allow Lenny a special degree of freedom in his creativity -
it has been useful so many times! ;-)
> Apart from the issue of what to do with knowledge of landscape numbers (e.g.
> whether or not knowing that there are N vacua with a given property implies
> anything about physics) it seems to be clear that what is known about the
> landscape is on rather more shaky grounds than usual for "non-rigorous"
> physics fact.
Right.
> In my humble opinion it appears as if it is way to early to say much in
> particular about the space of vacua of string theory with certainty, and
> that the best would be if people kept their ideas about the landscape at the
> same place where they keep their feelings about the interpretation of
> quantum mechanics - until, maybe, some advancement in our understanding of
> that vast edifice called string theory has advanced to a point where the
> analysis of the landscape really becomes a well-defined scientific
> enterprise.
For me, the interpretation of quantum mechanics is a much more settled
subject - of course, the exception are exactly the interpretational issues
involving quantum cosmology which are poorly understood. But as far as
normal quantum theories in a well-defined background go, you know that it
must be about the probabilities, and that quantum decoherence explains the
emergence of "classical" from "quantum" and the boundary. Is there
anything wrong with taking the consistent histories (Copenhagen done
right) - or many-worlds, if you prefer - as the complete resolution of the
quantum puzzles that don't require any quantum gravity? In reality,
they're scientifically equivalent and differ in philosophy only.
The vacuum selection in string theory looks much more shaky to me - there
are no convincing, justified proposals.