Discussion:
FAQ's
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Zaz
2005-04-19 08:18:13 UTC
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A first list of FAQ's. We need more such questions -- and
answers. Please contribute. Be brief.

1. Why string theory?
2. Why 10 dimensions? Eleven?
3. What is the difference between the string scale and Planck scale?
4. What is the best result from string theory?
5. What are D-branes?
6. What is M-theory?
7. Can we see strings?
8. What is AdS-CFT?
9. What did Green and Schwarz do?
10. What does string theory say about cosmology?

-ez
Alan
2005-04-19 17:58:04 UTC
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Post by Zaz
A first list of FAQ's. We need more such questions -- and
answers. Please contribute. Be brief.
Two that I would like to see answered:

1. String theory is often described as `incomplete'. What's missing?
2. Do strings generate space-time or simply exist in space-time? If the
former, how do they do it?

regards,
alan
Urs Schreiber
2005-04-19 18:19:05 UTC
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Post by Alan
1. String theory is often described as `incomplete'. What's missing?
A complete nonperturbative definition.
Post by Alan
2. Do strings generate space-time or simply exist in space-time? If the
former, how do they do it?
In the usual worldsheet description of perturbative strings spacetime is a
derived concept.

There is a conformal field theory on the worldsheet and all that matters is
that it is superconformal and of central charge 15.

Every such CFT is said to be a "background", i.e. "spacetime background".
(For a discussion of the term background see this:
http://groups.google.de/groups?selm=Pine.LNX.4.31.0409140346590.31300-100000%40feynman.harvard.edu )

Some such CFTs, the "nonlinear sigma models", can be interpreted as
describing the embedding of the worldsheet into a spacetime manifold on
which possibly certain background fields are turned on.

Other such CFTs, like Gepner models for instance, do not have such an
interpretation. But these are continuously connected to those that have.

By continuously moving around in the space of all admissable CFTs one can
move from backgrounds which describe a classical geometry to those that
don't and back to those that do, from backgrounds with a given topology to
those with another topology, etc.

Perturbative string theory works by computing string scattering amplitudes
with respect to a given background (i.e. using a given worldsheet CFT), but
it can be shown that the choice of background does not matter. (Just as you
can Taylor-expand a given function equivalently about different points and
get consistent results - as long as the radii of convergence overlap.)

In this sense the background geometry in string theory is a derived concept.

For my science journalist friend Ruediger Vaas I have once tried to collect
popular accounts of this fact, see for instance the following links:

http://golem.ph.utexas.edu/string/archives/000330.html#c000878
http://groups.google.de/groups?selm=m3eeob.u4d1.ln%40xor
http://www.damtp.cam.ac.uk/user/rch47/schloessmann.html .
I***@.SYNTAX-ERROR.
2005-04-19 17:58:15 UTC
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What are the physical implications from string theory, that can be
tested with today's equipment?

René

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Urs Schreiber
2005-04-19 18:36:32 UTC
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Post by I***@.SYNTAX-ERROR.
What are the physical implications from string theory, that can be
tested with today's equipment?
If we assume that the string coupling is small enough such that perturbative
string theory is a good description, then there are a couple of universal
predictions of string theory. Most obviously, at high enough energies one
should see the higher excitation modes of the string as well as the extra
dimensions it lives in.

These energies are in reach of current equipment, e.g. of the upcoming LHC
machine, only if the extra dimensions are "large" which means "not to
small".

People have thought about what could be seen in such a case at LHC.

Highly excited (but weakly coupled) strings are called "string balls"
sometimes (see e.g. http://golem.ph.utexas.edu/string/archives/000379.html)
and people have computed the signatures that such string balls would yield
in LHC, see for instance

A. Chamblin & G. Nayak,
Black Hole Production at LHC: String Balls and Black Holes from pp and
Lead-lead Collisions
http://arxiv.org/abs/hep-ph/0206060

K. Cheung,
Black hole, string ball and p-brane productions at hadronic supercolliders
http://arxiv.org/abs/hep-ph/0205033


However, one should be aware that there is no known reason why it should be
"likely" that the extra dimensions are large enough for these effects to be
visible in LHC. I also know of no argument why it should be "likely" that we
find the string coupling to be small.

If the string coupling in our world turns out to be large, we would need a
good non-perturbative formulation of string theory to make any predictions
at all.

In many discussions you will see the expectation discussed that string
theory should predict properties of the standard model, like particle
masses, number of generations, etc. These properties are determined by the
"vacuum" (the background) of string theory that would best approximate the
world we live in.

Once it was hoped that for this question, too, string theory makes a
universal prediction. However, today this does not seem very likely.
Instead, many people today are arguing that there are many,many such vacuua,
called a "landscape" of vacuua.

So in this case we can easily measure something with current equipment, for
instance the number of dimensions, but it is not clear (yet) what, if any,
value string theory would predict here.
Urs Schreiber
2005-04-19 18:40:19 UTC
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many people today are arguing that there are many,many such vacuua, called a
"landscape" of vacuua.
Er, sorry for the typos I do know that the spelling is "vacua". :-)

Everybody please don't hesitate to correct my mistakes or disagree with my
replies.
Peter Woit
2005-04-20 17:22:23 UTC
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Post by Urs Schreiber
Post by I***@.SYNTAX-ERROR.
What are the physical implications from string theory, that can be
tested with today's equipment?
.....
So in this case we can easily measure something with current
equipment, for instance the number of dimensions, but it is not clear
(yet) what, if any, value string theory would predict here.
The answer to this question is very simple: none whatsoever. I think
the kind of obfuscatory answer Urs is giving here does
everyone a disservice. In its current state, string theory predicts
absolutely nothing about anything measurable today. Sure, it would
be nice if it predicted the number of dimensions, but it doesn't. Why
not just give this person a straight answer?

[Moderator's note: Peter Woit is not a string theorist. Whether or
not it means that his answer is reliable depends on the readers'
choice. LM]
Urs Schreiber
2005-04-20 18:06:59 UTC
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Post by Peter Woit
Post by Urs Schreiber
Post by I***@.SYNTAX-ERROR.
What are the physical implications from string theory, that can be
tested with today's equipment?
.....
So in this case we can easily measure something with current equipment,
for instance the number of dimensions, but it is not clear (yet) what, if
any, value string theory would predict here.
Actually, from the context one can deduce that I meant to say "number of
generations" instead of "number of dimensions" here. Ž

But of course the number of large observable dimensions is another thing
that would be determined by the choice of vacuum.
Post by Peter Woit
The answer to this question is very simple: none whatsoever.
As I said, issues which concern the choice of vacuum instead of generic
effects like excited modes of weakly coupled strings, are currently not
known to be predicted by string theory.
Post by Peter Woit
Why not just give this person a straight answer?
I have tried to make quite clear what can be said and what cannot be said.
Post by Peter Woit
In its current state, string theory predicts
absolutely nothing about anything measurable today.
That, too, I did say, giving a little bit more details and qualifiers. With
LHC technology, I said, it would be possible to see perturbative stringy
effects only if there are large extra dimensions and that only if the string
coupling is small.
w***@yahoo.com
2005-04-22 14:38:04 UTC
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[...]
Post by Urs Schreiber
I have tried to make quite clear what can be said and what
cannot be said.
Post by Peter Woit
In its current state, string theory predicts
absolutely nothing about anything measurable today.
This is not often said by string theorists.
Post by Urs Schreiber
That, too, I did say, giving a little bit more details and
qualifiers. With LHC technology, I said, it would be possible
to see perturbative stringy effects only if there are large
extra dimensions and that only if the string coupling is small.
[Moderator's note: Peter Woit is not a string theorist. Whether or
not it means that his answer is reliable depends on the readers'
choice. LM]
I am not a string theorists and I am able to understand only
popularizations. Still I think I know what is a prediction
and so far I choose that Mr Woit and Mr Schreiber
are reliable about the predictions of string theory.

"What is a prediction in physics ?" could be also a good entry for
the string FAQ. It would help people to understand and appreciate
the answer to the FAQ about the predictions of string
theory.



We Pretty

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Urs Schreiber
2005-04-22 14:57:58 UTC
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On Fri, 22 Apr 2005 ***@yahoo.com wrote:


[Peter Woit wrote:]
Post by w***@yahoo.com
Post by Peter Woit
In its current state, string theory predicts
absolutely nothing about anything measurable today.
This is not often said by string theorists.
That's because many people feel that saying so would be a drastic
oversimplification of a complex issue.

Theoretical physics is currently in a state where experimental data is
badly missing. In such a situation every theoretician has to decide on a
somewhat subjective basis which approach looks interesting and most likely
to be useful once data is flowing in again.

String theory is popular currently because to many it seems attractive in
this sense. Without further experimental data it is hard to say if string
theory is right, but the same would hold for every other approach.

Peter Woit thinks that string theory is not attractive and that too many
people are spending their time with it. In order to change that he is
trying to convince people of the uselessness of working on string theory
by saying things like "ST makes no predictions at all."

I believe a more promising approach to make people study something else
than string theory would be to come up with an idea that many people will
find more attractive than string theory.


The best one can do with respect to upcoming experiments is to think of
as many interesting models as one can think of, work out their properties
and wait if they will be confirmed or not by LHC.

There is truly no lack of such models. Have a look at hep-ph
to get an impression. Many or even most such models are inspired by string theory in
one way or another. It is these models that make concrete predictions that
can be falsified.

All these models are based on some assumptions, though. Once it was hoped
that by deriving everything from a fundamental theory like string theory
all but precisely one of such models would be theoretically inconsistent.

Today it is recognized that string theory is not well enough understood
yet to decide whether there is a unique such model ("vacuum") or more than
one. Or even many of them.
Post by w***@yahoo.com
Still I think I know what is a prediction
and so far I choose that Mr Woit and Mr Schreiber
are reliable about the predictions of string theory.
In that case I hope you have realized a crucial difference in the answers
both of us gave. Peter Woit answered: "ST makes no predictions at all.",
while I discussed what amazing "retrodictions" ST has already made
and under what conditions which predictions are currently possible.
Peter Woit
2005-04-23 10:12:48 UTC
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Post by Urs Schreiber
[Peter Woit wrote:]
Post by w***@yahoo.com
Post by Peter Woit
In its current state, string theory predicts
absolutely nothing about anything measurable today.
This is not often said by string theorists.
That's because many people feel that saying so would be a drastic
oversimplification of a complex issue.
Whether or not string theory might ever be able to predict anything is
a complex issue which reasonable people might disagree about. Whether
it can predict anything right now is not.
Post by Urs Schreiber
In that case I hope you have realized a crucial difference in the
answers both of us gave. Peter Woit answered: "ST makes no predictions
at all.", while I discussed what amazing "retrodictions" ST has
already made and under what conditions which predictions are currently
possible.
It might be worth noting the inherent incompatibility of the claims

1. string theory can't make any testable predictions because it is not
well enough understood.

2. string theory makes "amazing 'retrodictions'".

More technically: string theorists are happy to admit that they don't
understand the theory well enough to determine its vacuum state. But they
continually claim that consequences of the perturbative expansion about
the perturbative vacuum state are well-founded predictions
of string theory. This makes no sense. You need to understand your
theory well enough to understand its vacuum state before you can say
anything about how the excitations about the vacuum state will behave.
Saying "string theory predicts spin-two massless graviton excitations
about its ground state" may be just as wrong as saying "QCD predicts
spin-one massless excitations about its ground state". We know enough
about QCD to know this is wrong and why. Until we know enough about string
theory to determine its vacuum state we have no idea whether it has
gravitons in its spectrum.

Proposed entries for the FAQ:

What is a scientific prediction?

A scientific prediction of a theory is a logical consequence derivable
from the theory that tells us what an experimenter will see if he or she
does a certain experiment.

Does string theory make any scientific predictions?

No.
Urs Schreiber
2005-04-25 13:24:24 UTC
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Post by Peter Woit
It might be worth noting the inherent incompatibility of the claims
1. string theory can't make any testable predictions because it is not well
enough understood.
2. string theory makes "amazing 'retrodictions'".
True, these two claims are incompatible. If you replace the first one by

1'. One currently cannot use string theory to predict things that depend
on strong coupling behaviour or questions of vacuum selection.


then 1' and 2 are compatible.

There is a grey-scale in between "predicts nothing at all" and "predicts
all parameters of the standard model".


How amazed one is by the fact that quantizing the Polyakov action leads
to gravity and Yang-Mills theory of course depends on one's temper.
Post by Peter Woit
More technically: string theorists are happy to admit that they don't
understand the theory well enough to determine its vacuum state. But they
continually claim that consequences of the perturbative expansion about the
perturbative vacuum state are well-founded predictions
of string theory. This makes no sense. You need to understand your theory
well enough to understand its vacuum state before you can say anything about
how the excitations about the vacuum state will behave.
Whatever the vacuum state is, it must be a solution to the background
equations of motions. These include, to lowest order, those of GR.

It can also be checked non-perturbatively that string theory knows about
gravity. Graviton scattering amplitudes have been computed with Matrix
Theory.

AdS/CFt also demonstrates that strings know about gravity.
Peter Woit
2005-04-25 17:52:06 UTC
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Post by Urs Schreiber
Post by Peter Woit
It might be worth noting the inherent incompatibility of the claims
1. string theory can't make any testable predictions because it is
not well enough understood.
2. string theory makes "amazing 'retrodictions'".
True, these two claims are incompatible. If you replace the first one by
1'. One currently cannot use string theory to predict things that
depend on strong coupling behaviour or questions of vacuum selection.
then 1' and 2 are compatible.
Everything depends upon the question of vacuum selection. If you don't
know what the vacuum is, you don't know what
the fundamental excitations about it are, much less anything about their
interactions.
Post by Urs Schreiber
There is a grey-scale in between "predicts nothing at all" and
"predicts all parameters of the standard model".
Yes, but string theory is not in the grey area.
Post by Urs Schreiber
Whatever the vacuum state is, it must be a solution to the background
equations of motions. These include, to lowest order, those of GR.
The problem is not that that you know the equations which the vacuum
state must satisfy and are just having trouble
solving them. The problem is that you don't know what these equations
are. One thing you do know is that
they're not just the ones determined by lowest order string perturbation
theory. If that were the case, you would
know what the vacuum state is (of course it would give you the wrong
physics).
Post by Urs Schreiber
It can also be checked non-perturbatively that string theory knows
about gravity. Graviton scattering amplitudes have been computed with
Matrix Theory.
AdS/CFt also demonstrates that strings know about gravity.
AdS/CFT and Matrix theory have nothing to say about the case at issue
here: quantum gravity in four large space-time dimensions, 6 or 7
compactified (or dealt with
in some brane-world scenario).
Lubos Motl
2005-04-25 18:03:22 UTC
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Post by Peter Woit
Post by Urs Schreiber
Post by Peter Woit
1. string theory can't make any testable predictions because it is not
well enough understood.
2. string theory makes "amazing 'retrodictions'".
True, these two claims are incompatible. If you replace the first one by
1'. One currently cannot use string theory to predict things that depend on
strong coupling behaviour or questions of vacuum selection.
I think that this is a rather accurate description, at least if you added
"except for things that are protected by supersymmetry".
Post by Peter Woit
AdS/CFT and Matrix theory have nothing to say about the case at issue
here: quantum gravity in four large space-time dimensions, 6 or 7
compactified (or dealt with in some brane-world scenario).
In the case of AdS/CFT, it is very easy to show that your statement is
incorrect. The simplest example is AdS4 x S7 - the near horizon geometry
of an M2-brane - that is dual to the 2+1-dimensional superconformal field
theory which is the infrared limit of the 2+1-dimensional maximally
supersymmetric gauge theory.

Admittedly, that's not a too realistic case. With many people, we have
discussed the conjectured CFT duals of the KKLT AdS4 vacua. There should
be a huge number of these CFT3 theories if holography of AdS spaces is
universal and if the KKLT vacua exist. Various people show examples why
many such theories may exist.

On of the complaints Cumrun had about the CFTs is that they should have
many operators of dimensions comparable to one, which should correspond to
KK modes of the momentum comparable to the inverse AdS radius. Frederik
Denef argues that he has convinced Cumrun that these operators are dual to
the Kahler moduli whose masses are also comparable to the radius... But
I'm getting off-topic now.

Best
Lubos
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Peter Woit
2005-04-25 22:02:12 UTC
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Post by Lubos Motl
In the case of AdS/CFT, it is very easy to show that your statement is
incorrect. The simplest example is AdS4 x S7 - the near horizon
geometry of an M2-brane - that is dual to the 2+1-dimensional
superconformal field theory which is the infrared limit of the
2+1-dimensional maximally supersymmetric gauge theory.
Admittedly, that's not a too realistic case. With many people, we have
discussed the conjectured CFT duals of the KKLT AdS4 vacua. There
should be a huge number of these CFT3 theories if holography of AdS
spaces is universal and if the KKLT vacua exist. Various people show
examples why many such theories may exist.
Hi Lubos,

I've always wondered about this. Forgetting KKLT vacua,
if strings on AdS4 really give a 4d theory of quantum gravity
in terms of a holographic dual 3d superconformal field theory
why isn't everyone happily calculating the answer to any
question about 4d quantum gravity using this 3d QFT?
What's not realistic about it?

Peter
Lubos Motl
2005-04-25 22:20:31 UTC
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Post by Peter Woit
I've always wondered about this. Forgetting KKLT vacua,
if strings on AdS4 really give a 4d theory of quantum gravity
in terms of a holographic dual 3d superconformal field theory
why isn't everyone happily calculating the answer to any
question about 4d quantum gravity using this 3d QFT?
What's not realistic about it?
Peter
Hi Peter, of course that one can in principle calculate any question about
quantum gravity in AdS4 x S7 from the dual CFT - which is the
2+1-dimensional superconformal field theory (SCFT) of the M2-branes, the
IR limit of the 2+1 dimensional N=8 gauge theory. And in fact, many things
have been checked. Because of the words "IR limit" earlier in this
paragraph, it is not too easy to work with the 3-dimensional SCFT. It's
more convenient to work with the four-dimensional conformal field theories
(directly gauge theories) which describe quantum gravity in AdS5. A lot of
stuff has been learned from the dual CFT. Thermodynamics of large black
holes have been verified qualitatively and in some other AdS cases
quantitatively, graviton scattering has been calculated from the CFT
correlators, branes have been found as baryons, various phase transitions
on both sides were matched (Hawking-Page vs. confinement-deconfinement)
etc.

Even in AdS4, one can learn a lot from the 3-dimensional conformal field
theory. What's not realistic about AdS4 x S7? For example, the radius of
AdS4 is (up to factors of two) equal to the radius of S7, so claiming that
S7 is negligibly small also implies assuming that the AdS space is
infinitely curved. AdS4 x S7 has 32 supercharges - too much for a
realistic theory. Realistic theories can start from 4 supercharges at
most.

"Quantum gravity in d=4" is not such a unique notion. Quantum gravity -
namely string theory - has many vacua (and our friends who believe the
anthropic haystack would say that 10^{350} vacua) that describe four large
dimensions, but the details of the remaining degrees of freedom (the extra
6 or 7 dimensions, speaking about the most famous examples) may be found
in many solutions (compactifications, inclusion of branes etc.) - and many
detailed results of quantum gravity will of course depend on the
compactification of the other dimensions etc.

I think that the only universal features of 4D theories of quantum gravity
are those that can be extracted from the classical and semiclassical
treatment of the Einstein-Hilbert action and non-stringy considerations.
Everything else is background-dependent.

Also, it is pretty difficult to extract very local physics (of quantum
gravity) from the boundary CFT (such as subplanckian behavior) - it's
easier to use the CFT to calculate the properties of "large" objects.
Nevertheless, a lot of local physics (like in the pp-wave limits) has been
extracted from the CFTs, too.
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w***@yahoo.com
2005-04-25 14:18:37 UTC
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Post by Urs Schreiber
In that case I hope you have realized a crucial difference
in the answers both of us gave. Peter Woit answered: "ST
makes no predictions at all.", while I discussed what
amazing "retrodictions" ST has already made and under what
conditions which predictions are currently possible.
What is the answer to the question:
Is there an experiment now or in near future such that
the results according to string theory would differ from
the results according to current accepted theories such
that if the predictions of string theory were not observed
it would imply that string theory is false ?

I know that when the question was asked with relativity the
answer was a definite yes. The answer was not something like
"yes, _if_ the speed of light is low enough or if the hyper
space field is strong enough".

Perhaps a question "What is a non-trivial prediction of a theory"
could be in FAQ and the answer something that would result yes in
the previous question.

I'm almost sure that when physicists cannot agree on whether or
not string theory can make predictions it is not because they do
not understand the theory but because they do not agree on what
is a prediction.

I do realize the difference between the answers to the question
about the predictions of string theory. I think that string
theory cannot make non-trivial predictions defined as I did but
I still would be happy to finance the research of string theory
with my tax dollars if I'd pay my taxes in dollars. I have got
an impression that string theory is cool and therefore should
be researched. I have nothing more to say to this thread and
won't post to it anymore.

We Pretty
Lubos Motl
2005-04-25 14:32:12 UTC
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Post by w***@yahoo.com
Is there an experiment now or in near future such that
the results according to string theory would differ from
the results according to current accepted theories such
that if the predictions of string theory were not observed
it would imply that string theory is false ?
I think that this is a pretty sharply formulated question, and my opinion
is that the answer to this strong question is unfortunately No, there is
no such known experiment, and string theory is not falsifiable in this
strong sense. If someone thinks that I'm wrong, it will be great to learn
why.
Post by w***@yahoo.com
I know that when the question was asked with relativity the
answer was a definite yes. The answer was not something like
"yes, _if_ the speed of light is low enough or if the hyper
space field is strong enough".
Fair enough. We no longer live in the world where things are as easy as
special relativity. Special relativity is based on 1-10 papers that are
probably still more important than the 15,000 papers of string theory. But
that's how the life goes. The society is also much richer than it was 100
years ago, so it's not shocking that it typically can afford to fund
research where the expected gain per dollar is smaller than from the
salary for the Swiss patent clerk.
Post by w***@yahoo.com
Perhaps a question "What is a non-trivial prediction of a theory"
could be in FAQ and the answer something that would result yes in
the previous question.
There are many striking predictions that assume certain assumptions -
assumptions whose validity we're not able to verify with the current
understanding of the theory - but nevertheless these are predictions such
that if they're confirmed, they will go very far to support the theory.
Post by w***@yahoo.com
I'm almost sure that when physicists cannot agree on whether or
not string theory can make predictions it is not because they do
not understand the theory but because they do not agree on what
is a prediction.
I wish the world were as simple as you think. There are many contexts in
which our understanding is so shallow that even string theorists do not
agree whether something is a prediction of string theory or not - they
disagree whether it follows from string theory, regardless of the
terminological detail whether one calls it a "prediction".
Post by w***@yahoo.com
I do realize the difference between the answers to the question
about the predictions of string theory. I think that string
theory cannot make non-trivial predictions defined as I did
As I said, I think you're right.
Post by w***@yahoo.com
but I still would be happy to finance the research of string theory with
my tax dollars if I'd pay my taxes in dollars. I have got an impression
that string theory is cool and therefore should be researched.
I really think that we should be more honest in saying what can be
expected from some direction of research, and what cannot. String theory
is indeed cool. It's a source of great and often unexpected ideas that
seem to connect virtually all good ideas in high-energy theoretical
physics and many branches of mathematics.

All the best
Lubos
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E-mail: ***@matfyz.cz fax: +1-617/496-0110 Web: http://lumo.matfyz.cz/
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A.J. Tolland
2005-04-19 20:01:55 UTC
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Post by I***@.SYNTAX-ERROR.
What are the physical implications from string theory, that can be
tested with today's equipment?
Supersymmetry might be the best experimental signature. IIRC, all
known supersymmetric extensions of the Standard Model can be embedded in
string theory. So, although string theory doesn't predict supersymmetry
(i.e. it admits non-supersymmetric solutions), finding supersymmetry in an
accelerator laboratory CERN would be good evidence for string theory. If
nothing else, it could be useful to have string theory handy for
computational purposes.

--A.J.
Urs Schreiber
2005-04-20 12:13:38 UTC
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Post by A.J. Tolland
Post by I***@.SYNTAX-ERROR.
What are the physical implications from string theory, that can be
tested with today's equipment?
Supersymmetry might be the best experimental signature. IIRC, all
known supersymmetric extensions of the Standard Model can be embedded in
string theory. So, although string theory doesn't predict supersymmetry
(i.e. it admits non-supersymmetric solutions), finding supersymmetry in an
accelerator laboratory CERN would be good evidence for string theory. If
nothing else, it could be useful to have string theory handy for
computational purposes.
Deciding which amount of target space susy perturbative string theory
predicts is much like solving the vacuum selection problem.

In this context I would like to emphasize an often neglected, apparently
minor, point:

While there is no solid prediction of spacetime susy, there is a solid
prediction from perturbative string theory that there must be (at least) N=1
worldsheet susy. The closed bosonic string does not have a stable
background, only the superstring does. This predicts that every consistent
string background (susy or not) must contain fermions, since the superstring
and only the superstring has spacetime fermions in its spectrum of
excitations.

So perturbative string theory predicts the existence of fermions, if you
wish.

In ordinary field theory the Klein-Gordon particle has N=0 susy on the
worldline while the Dirac particle has N=1 susy on its worldline. Since
worldline susy is so trivial, neither choice has drastic consequences and
from the worldline point of view a world consisting only of Klein-Gordon
excitations would be just as consistent as a world containing fermions.

When the parameter space dimension is increased just one notch the situation
changes dramatically. Worldsheet susy is still relatively simple, but now it
makes a big difference.

Some people will find the statement that "string theory predicts fermions"
silly. But I think that while it may be useless for practical purposes
(since we knew about fermions before) it is not empty.

In the same spirit perturbative superstring theory predicts that physics
is about

1) gravity

and about

2) Yang-Mills theories.

Furthermore, while maybe not quite predicting, it makes very
plausible that

3) the spacetime signature is (-++...+) and not anything else.

This are quite a lot of correct qualitative features of the real world that
drop out of perturbative strings (i.e. out of just quantizing the Polyakov
action) without having been put in by hand.

If one thinks about it carefully, this is rather amazing. To me at least.
Zaz
2005-04-20 23:20:41 UTC
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Thanks to those picking up this thread. I want to recall that
the intent is to collect LISTS OF QUESTIONS -- to be answered later
(the answers will be entire threads of their own, I imagine).
I would also like to restate the "rules" for posting FAQ's:

1. Be concise.
2. Be specific.
3. Refrain from editorializing.

These apply to moderators and posters alike.
Thanks, and I look forward to reading more FAQ's.

-ez
Urs Schreiber
2005-04-21 10:40:53 UTC
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Post by Zaz
Thanks to those picking up this thread. I want to recall that
the intent is to collect LISTS OF QUESTIONS -- to be answered later
(the answers will be entire threads of their own, I imagine).
1. Be concise.
2. Be specific.
3. Refrain from editorializing.
These apply to moderators and posters alike.
Thanks, and I look forward to reading more FAQ's.
I'll give it a try. I have taken your original questions and incorporated
them in the following tentative proposal for a rough outline of a list of
possible FAQs that currently come to my mind. Even though this list looks
long, I am certain to have forgotten lots of important points. The
formatting also needs some more work. And probably there is some
repetition, too.

Anyway, here goes:


1. Why string theory?

1.1 What problems are there with non-stringy fundamental physics?

1.1.1 Why does gravity have to be quantized?
1.1.2 What is the problem with quantizing gravity?
1.1.3 What are effective field theories and is the standard model one?
1.1.4 Why is the standard model thought to be "not the last word"?
1.1.5 What is (are) the hierarchy problem(s)?
1.1.6 Why is susy considered an attractive idea?
1.1.7 What is the cosmological constant problem?

1.2 How does string theory shed light on the problems with non-stringy
fundamental physics?

1.2.1 How does gravity arise in string theory?
1.2.2 How do branes arise in string theory?
1.2.3 How does Yang-Mills theory arise in string theory?
1.2.4 How does supersymmetry arise in string theory?
1.2.5 In which sense does string theory unify gravity with gauge forces
as well as forces with matter?
1.2.5 How are standard-model-like models found inside string theory?
1.2.6 How does AdS/CFT help to understand ordinary QCD?
1.2.7 Does string theory help with the cosmological constant problem?
1.2.8 Is perturbative string theory sufficient to address issues in
cosmology?

1.3 How can string theory be motivated from ordinary field theory?

1.3.1 What did historically lead to string theory?
1.3.2 What is the "worldline formalism" in field theory?
1.3.3 What replaces Feynman diagrams in string theory?
1.3.4 How is the Klein-Gordon action like 1D gravity coupled to scalar
fields on the worldline?
1.3.5 How is it that ordinary fermions have N=1 SUSY on their worldline?
1.3.6 How does field theory compare to string field theory?
1.3.7 Can we consider even higher-dimensional objects than strings?


2. What are the principles of string theory?

2.1 What are the Polyakov and the Nambu-Goto actions?
2.2 Why conformal invariance?
2.3 What is the BRST formalism?
2.4 What is the spectrum of the string?
2.5 How can we think of particles as being strings?
2.6 What is the difference between the Planck scale and the string
scale ?
2.6 Why does one consider 10 dimensional backgrounds?
2.7 Where do the background equations of motion come from?
2.9 How do D-branes follow from strings?
2.10 What is the Dirac-Born-Infeld action?
2.11 What is the meaning of the tachyon?
2.12 Why do we need worldsheet supersymmetry?
2.13 Why do there appear to be five different "string theories"?
2.14 What did Green and Schwarz do?
2.15 What are dualities?
2.16 How does the eleventh dimension come into the game?
2.17 What is meant by "M-theory"?
2.18 What is Matrix Theory?
2.19 What is String Field Theory?
2.20 What is AdS/CFT?
2.21 What are *the* equations of string theory?


3. What are the implications of string theory?

3.1 What are the experimental consequences of string theory?

3.1.1 What does string theory predict?
3.1.2 How could string theory be tested?
3.1.3 What would happen when a realistic string vacuum were found?
3.1.4 What does string theory say about cosmology?
3.1.5 What is the vacuum selection problem?
3.1.6 What is meant by the "landscape"?

3.2 What impact does string theory have on general theoretical physics?

3.2.1 What is meant by "embedding" a field theory in string theory?
3.2.2 What can be learned about ordinary field theories from string
dualities?
3.2.3 What does AdS/CFT teach us about ordinary field theories?
3.2.4 What is the twistor string formalism and what is it good for?
3.2.5 What does string theory say about black hole entropy?
3.2.6 What is meant be "holography" in string theory?
3.2.7 What is the topological string and what is it good for?
3.2.8 What is meant by "topological M-theory"?

3.3. What is the interplay between string theory and pure mathematics?

3.3.1 What are index theorem and their relation to ST?
3.3.2 What is mirror symmetry and its relation to ST?
3.3.3 What is the monster group and its relation to ST?
3.3.4 What is K-theory and its relation to ST?
3.3.5 What are derived categories and their relation to ST?
3.3.6 What are Kac-Moody algebras and their relation to ST?


4. What are the big open questions in string theory?

4.1 What is known about the non-perturbative definition of the theory?
4.2 What is string theory, really?
4.3 How can I read all of today's papers before lunch?
katerina
2005-05-02 13:15:48 UTC
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Urs, I am looking forward to the answers for FAQ.
Until that time I would like to read something.
I have read The Greene's Elegant Universe (thank you, Lubos, for the
translation), and started to read hep-th/9702155v1.
I doubt that it was the best choice to start with that, but I don't know
how to choose. So I will continue reading with half understanding.

I suggest another question to FAQ -

What can I read about strings?
What mathematics is needed and what should I read to understand?

Thanks
Katerina
Urs Schreiber
2005-05-04 19:10:38 UTC
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Post by katerina
What can I read about strings?
What mathematics is needed and what should I read to understand?
Have a look at this:

http://superstringtheory.com/

(But stay away from the discussion forum there.)

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