(Alpha) String Theory Questions

BenTheMan

Dr. of Physics, Prof. of Love
Valued Senior Member
I want to start this thread so that yall can ask questions about String Theory, if anyone happens to be interested.

I really don't want to defend String Theory from a bunch of random assaults---I will admit fully that String Theory has some very open issues at the moment, and that a growing number of physicists take issue with String Theory. The main reason for this request is that I don't understand the other quantum gravity scenarios that well, and don't really feel comfortable commenting on them.

I have been studying string theory for about two years, specifically the heterotic string (which probably is meaningless to most people). Hopefully I will not make too big a fool of myself in attempting to answer these questions. I will probably not be able to answer all questions, but if there is something that you are interested in asking, perhaps I can give you some ideas about what String Theory says. I will do my best to attempt to answer any questions, including consulting with my colleagues here at OSU.

If yall want a reading list, I would suggest starting with Brian Greene's "Elegant Uiverse". A more general perspective is found in Lee Smolin's "Three Roads to Quantum Gravity", which also explains the Loop Quantum Gravity proposal, as well as other older approaches.

Anyway, I will answer all the questions that I can as long as people ask them. I ask again that we try to stick to questions about String Theory. I am not very familiar with Loop Quantum Gravity, probably the largest competitor to String Theory---think Google and Ask. Strings has a large market share, but Ask has an ambitious business plan:)

It is my hope to generate at least some discussion---I hope this thread doesn't die right away, at least:)
 
WHAT .... then.... is the basis for strings?
the evidense?
the proposals... based on some real empiricall details...

please.. enlighten us, as to why i should believe.

-MT
 
I know next to nothing about string theory, but have read that it is not capable of experimental tests. Is this true? If not, what tests have been done. If answer to that is "none yet," how is it testable? This is similar to MT's questions. Answer both together if you like.
 
at the moment its not really possible to detect strings...They are much to small. So there is no testable way to prove (or disprove) string theory. Although, there was a test which did support string theory. It had to do with super symmetry strings and the theory was that as a photon was pulled into a black hole its super-symmetic partner would then go the opposite direction, therefore making it possible to 'see' that bit of a black hole. this was tested and confirmed, although it does not mean that everything string theory says is true.

note: (i pulled all this from memory of a book i read, so it may not be 100% accurate but thats basically what it said)
 
Ahh yes. Good questions.

I will answer Billy's question, which will lead to MT's quesiton, which I will also answer.

The short answer is that there are currently no experiments that confirm or imply that string theory is correct, and (as it stands now, at least) there are few prospects for such experiments in the future.

There are several things that string theory needs to exist---it needs extra dimensions (7 more, to be exact), and it needs supersymmetry. The problem is that the extra dimensions may be impossibly small to see, and that supersymmetry may be at an impossibly high energy to test. To make matters worse, because string theory is such an ill-defined subject, it may never be able to be proven wrong! To check for sure, we'd have to build a particle physics experiment that is about the size of the galaxy. Considering it takes a photon about 20,000 years to travel around the outer rim of our galaxy, this is an impossible experiment to preform.

The good news is that supersymetry may be found soon. The Large Hadron Collider (LHC) is opening next year in Swizerland at the Center for European Nuclear Research (CERN), and many physicists expect to find evidence of supersymmetry. But there is another problem---if we DO find supersymmetry, it is no guarantee that string theory is right! We can have theories that predict supersymmetry, but that aren't string theory.

There are other proposals for testing string theory---for example, there are some space-based experiments (like WMAP) which may show us signs of string theory in the sky. Signatures of a phenomena called "cosmic strings" would be very good evidence for string theory. Basically, if the universe expanded very rapidly at a very early time, some strings could have expanded in the same manner, showing up in astronomical observations today. The problem is the same---if we don't find cosmic strings, we won't disprove string theory

So, there are ways to test string theory, but no ways to prove (or disprove) it. The question is, why do people study it?

A few answers are popular: the first one, which I don't love, is that string theory is so beautiful that it must be right. We are astounded by the mathematical consistency of the theory again and again. Einstein was asked about general relativity---what if it's wrong? He answered "Then I would feel sorry for God". The point is, GR was so simple and beautiful that Einstein felt that it HAD to be right. The same is true for Dirac's theory of the electron.

This argument shouldn't convince you:) The reason that I believe in string theory is the way that it naturally predicts the types of symmetries that we see in nature. We have found that forces are described by certain mathematical structures called Lie Algebras. Indeed, at least two Nobel Prizes (and probably more than that) have been awarded over such discoveries. Well, these mathematical structures naturally appear in string theory. We can know how to make quantum mechanics consistent with such mathematical structures, but we have no reason to do so---that is, the only way that we know such extended symmetries exist is that we looked in nature and found them. Quantum theory is just fine without such mathematical structures, but they are required by string theory.

The other popular answer to "why do it" is that strings is the easiest way to unify quantum mechanics and general relativity. (Other programs, like Loop Quantum Gravity, have potential, and I don't want to short-change these alternative approaches.) Finally, string theory tells us what the dimension of space-time is---eleven. General relativity is perfectly happy in any number of space-time dimension---even Einstein noted this problem early on. String theory only exists in eleven dimensions.

Hopefully I have clarified some questions and raised a few more. If you would like background on any of these topics (supersymmetry, general relativity, cosmic strings...) we can talk about them some, and you may wish to check out wikipedia.
 
Although, there was a test which did support string theory. It had to do with super symmetry strings and the theory was that as a photon was pulled into a black hole its super-symmetic partner would then go the opposite direction, therefore making it possible to 'see' that bit of a black hole. this was tested and confirmed, although it does not mean that everything string theory says is true.

What book did you read? I am not familiar with this test, and I will freely admit that I am not familiar with a lot of experiments in physics:)
 
It was in the book you mentioned, The Elegant Universe. It was at that part regarding black hole entropy and stuff. Like I said in the edit, I just pulled it all from memory, so it's probably not 100% accurate.
 
Does "string theory" offer an explanation or understanding of anything not adequately explained or understood without it? (When asking this, I am not very interested in having the ability to "tie together" in one mathematical structure several now separate and simpler but different math structures. For example, as Maxwell did for electrical and magnetic physics. Doing that is just "changing the description", not any new physics or understanding of the world*. Postulating 7 unobservable dimension merely to unify the math description seems to me to be a step in the wrong direction. )

During the "expansion" phase of the "big bang" is there any reason offered why the 7 still small dimensions of string theory did not also expand?
----------------------------
*I admit Maxwell's efforts did predict EM waves travel at speed of light etc. and advanced the time when this was understood etc. but presumably radio etc could have been discovered and even some discription of it with in "electric theory" worked out. I.e. I was only trying to illustrate "unification" - I see nothing like "radio" (EM waves) being predicted by string theory, but perhaps you can enlighten me.
 
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Does "string theory" offer an explanation or understanding of anything not adequately explained or understood without it? (When asking this, I am not very interested in having the ability to "tie together" in one mathematical structure several now separate and simpler but different math structures. For example, as Maxwell did for electrical and magnetic physics. Doing that is just "changing the description", not any new physics or understanding of the world*. Postulating 7 unobservable dimension merely to unify the math description seems to me to be a step in the wrong direction. )

Well, you're knocking one of my legs out! The unification of particle physics and gravity is a huge thing. In fact, it's the main motivation for string theory---making quantum mechanics consistent with general relativity is probably the biggest problem facing theoretical physics. And I heartily disagree that Maxwell's unified theory of electromagnetism didn't yield any new physics. Maxwell showed that electricity and magnetisme were the same thing!

The first thing that I want to addrress is this:

Postulating 7 unobservable dimension merely to unify the math description seems to me to be a step in the wrong direction.

In a sense, this is more desirable than the situation in GR. Let me explain: GR doesn't predict a number of dimensions. One can use GR to describe ANY number of dimensions---three, four, fifty, 291... String theory is only consistent in 11 dimensions.

Again, the reason that I like string theory is the appearance of extended symmetries, which are put into quantum theory by hand. String theory naturally incorporates these mathematical structures (which definitely do describe nature).

String theory is finite, in that it gives exact answers. The only way that we know how to do quantum field theory is perturbatively---that is, we can calculate a quantity, then calculate a correction, the calculate a correction to the correction, etc. (Think of it like sighting in your rifle--first you gotta get it on paper, then you can adjust a little bit. Ideally, each time you adjust is a little less than the time before, and you'll be closer to the bulls-eye.) The problem is that doing this in physics leads to infinite answers! For example, if you calculate Compton scattering---the interaction of an electron with the electric field, you get a finite answer at first. But when you compute the correction, you get an infinite answer. We have to do some mathematical hand waving, called renormalization, where we split the infinity into a small correction plus a "new" infinity, and then throw the new infinity away.

If this sounds like a hoax, many physicists feel this way. The problem is that it works! One can get fantastically accurate answers doing this, but nobody really knows why! For example, the most accurate constant in all of physics is the fine structure constant. The calculations (using the renormalizations) and the experiments agree to something like 13 decimal places. It's hard to argue that renormalization isn't valid when you get answers that are that close.

The other thing is that, when one treats gravity this way, one gets infinite answers that can't be renormalized. (I didn't mention this, but there are some specific criteria that a theory must meet before one can use the renormalization prescriptions.) We want to know how to treat gravity quantum mechanically, because, for example, we're pretty sure that there are black holes. But this is just another statement of "string theory presents a unified theory of GR and QM", which you don't want:)

Ok, inflation. There are some proposals as to why four dimensions (or three space + one time) should have been left large, but nothing that is 100% convincing. Getting inflation out of string theory is a very active area of research currently, and I am affraid that I am not very familiar with this program. I do know that one of the original inventors of inflation, Andrei Linde, now at Stanford, has some interesting proposals for getting such dynamics from string theory. I am affraid that I don't know much more than this.

Hopefully this was somewhat helpful!
 
BEN,

WHAT IF I TOLD YOU, there are 28 dimensions... 25 in this space.
what if i told you strings are real.... but they arent strings..
they are spatial tension.... lines of force... one dimensional lines of force manifesting across space.... on and of space..

what if i told you, that strings.... are just like electrostatic lines of force.

lines of force are real.... ben... real strings.... like gravity ben...

gravity is best explained by understanding it as a dense field of nuetral spatial tension... while electric fields... are polar spatial tension.

and that all matter was created into mass... in the first 7 dimensions...
and that the reason you may find supersymetry in mass, has nothing to do with strings... but has to due with the very real... one dimensional lines of spatial tension we do have lots of proof for... electric fields... ben.

what if i told you that?

-MT
 
benTheMan

>> The short answer is that there are currently no experiments that confirm or imply that string theory is correct, >>

What do you say to the claim that modern physics is nothing but fantasy........ obviously baby steps..

I wonder when physics will grow up ?
 
WHAT IF I TOLD YOU, there are 28 dimensions...

Then I'd like to see how one can get all of the anomalies to cancel. String theory predicts 11 dimensions. If you can predict 28 or 25 dimensions, then you should probably publish something.

what if i told you, that strings.... are just like electrostatic lines of force.

lines of force are real.... ben... real strings.... like gravity ben...

This is the original motivation for string theory, to explain the forces between quarks. Now bosons do a much better job. Lines of force ARE real, they show the path of virtual force carrying particles.

What do you say to the claim that modern physics is nothing but fantasy........ obviously baby steps..

I wonder when physics will grow up ?

In order to have a discussion about these things, we obviously have to agree on some common ground, for me I am assuming that a reasonable participant in this discussion would accept the validity of GR and the axioms of Quantum Field Theory, as 100 years of experiments have established these facts.

If you care to discuss something else, please do so in another thread. Off topic posts will be reported to a moderator as per the Alpha rules established in this forum.

Thank you.
 
Hi Ben,

Glad to see this attempt at discussing an interesting topic. I want to ask some questions if you don't mind.

1. Im curious which brand of hetorotic string you study and why you chose the hetorotic string?

2. Mathematical and physical beauty are both often ascribed to string theory. However, I have often been struck by what I would call the "ugly complications" of string theory. For example, the presence of tachyons in the open string spectrum, or the necessity of the GSO projection for removing tachyons from the superstring spectrum. I don't know, maybe the tachyon in the bosonic string condenses to something reasonable just like the Higgs in the Standard Model, but the GSO projection especially has always bothered me. Do you regard such features as taking away from the beauty of string theory or is it perhaps just that we (or I) don't understand things well enough yet?

Thanks!

P.S. I am also a physicist, though not a string theorist.
 
Hi PhysicsMonkey---

Good questions.

For 1), I chose heterotic string theory because that's what the guy that I work for is studying. The heterotic string is nice because it's easy to get good particle physics out of it---that is, one can compactify the (ten dimensional) heterotic string on some subspace (called an orbifold) that is a torus moded by some discrete symmetries and get things like N=1 SUSY, small gauge groups, chiral fermions, etc. Right now I'm working in the E_8 x E_8 heterotic string, but the SO(32) case is not too much different.

As far as 2) is concerned, the GSO projections can be seen as another statement of unitarity. Just like in QM---one has to construct a vacuum state, and then may construct the Fock space. This is essentially what the GSO projection does---it projects out the negative norm states (tachyons) so that we can build a consistent quantum theory. (Sorry if you are already familiar with this!) I personally don't think that the GSO projections are an "ugly" feature of string theory, just one that is required for consistency.

I also think that the GSO projection can be formulated as a statement of modular invariance, i.e. it is implied by the symmetries of the world-sheet, but I am not as familiar with this argument.

Thanks for the good questions!
 
Hi Ben,

Interesting. As you are no doubt aware one meets a similar situation when trying to deal with ghosts in non-abelian gauge theories. There it is also necessary to project out unphysical states, and this can be accomplished in the canonical formalism by requiring that physical states be annhilated by the BRST operator. The whole story of BRST cohomology is certainly a very pretty one as far as the mathematics goes, but these projections still feel tacked on in some sense. In other words, I wish for a more "direct" or "physical" way to see what the proper Hilbert space is. Perhaps the modular invariance argument provides such a motivation.

How about another question then. I went to a talk not too long ago by Wati Taylor where he talked about the role (or really lack thereof) that string theory had to play in particle physics. Basically the message was we aren't gonna find anything essentially stringy at the LHC. Nevertheless, its almost a foregone conclusion amongst many high energy people that we will find supersymmetry at the LHC. But what if we don't? (Let's say we just see a naive Higgs sector.) Given that world sheet supersymmetry is necessary as far as we know, how easy is it to cook up a phenomonology that doesn't have weak scale supersymmetry?
 
I do agree that the GSO projection seems artificial in the same sense that the ghosts seem artificial, in the same sense that renormalization seems artificial... It certainly would be nice if the theory GAVE us a positive definite Hilbert space.

Actually the question about low energy SUSY is an interesting one---the only real motivation for SUSY from the particle physics side is that it protects the electroweak hierarchy. Further, SUSY isn't really where it should be---the b-mixing experiments at FermiLab last Spring-Summer were supposed to find SUSY, but they didn't.

The way we get low energy SUSY in string theory is to compactify on some manifold (or orbifold) with SU(3) holonomy. This tells us that the six dimensional manifold that we must compactify on is a Calabi Yau. (There are some more general seven dimensional manifolds that one can use when starting with eleved dimensional MTheory.)

But there is no reason why we can't choose some other six dimensional manifolds (or orbifolds) that don't sarisfy the SU(3) holonomy requirement. I think that, if one were willing to live with a hierarchy problem, one could build perfectly consistent string models without low energy supersymmetry. The SUSY, then, is broken at the string scale, and we'd never be able to see it.

It is very possible that string theory is the answer, but that we'll never be able to check it in the back of the book. The problem with any extra-dimensional theory is that all we'll ever be able to see is an effective theory, and we'll only be able to measure effective coupling constants. So, one can always add terms to the lagrangian that mimick, say, Kaluza Klein modes and their interactions. There will be little coming from the particle physics side that will be able to tell us that we're only seeing an effective field theory, unless we find something like proton decay. The best hope for proving string theory will probably come from the cosmology experiments, though, detecting cosmic string networks or something.
 
BenTheMan said:
..The short answer is that there are currently no experiments that confirm or imply that string theory is correct, and (as it stands now, at least) there are few prospects for such experiments in the future...

I've got some questions:

Can you give or link to a description of string theory that a layman can understand?

What are strings "made" out of?

If it's going to be a struggle to verify String Theory, can it really be considered to be science?

Is there a danger that people "believe" in it so much that they are unreceptive to other ideas offering valuable insights that can be verified?

As an aside, I thought this was interesting. It's a rating of a Briane Greene article on string theory using the Baez "crackpot" index:

http://www.kuro5hin.org/story/2006/10/31/161746/39

"I feel a little dirty having done this. Part of me feels compelled to point out that I know this is a newspaper editorial for general consumption, rather than a "scientific" document. But I think the fact that a comparison with a crackpot index has any traction at all says something important and unpleasant about string theory's role in physics..."
 
Then I'd like to see how one can get all of the anomalies to cancel. String theory predicts 11 dimensions. ...
IMHO, this is not the "correct" answer to MT question, because MT may not accept "string theory" as correct.
Better answer:
"I would ignore you, until you provide some evidence or equally elegant theory with 28 dimensions."
 
Well, you're knocking one of my legs out! The unification of particle physics and gravity is a huge thing. In fact, it's the main motivation for string theory---making quantum mechanics consistent with general relativity is probably the biggest problem facing theoretical physics. And I heartily disagree that Maxwell's unified theory of electromagnetism didn't yield any new physics. Maxwell showed that electricity and magnetisme were the same thing!
In my foot note I admited that Maxwell's unification did immediately allow the undestanding of light as EM wave, predicted Radio waves etc. I.e. addmited my example was not very strong or false. Perhaps my point is better made only within QM: In some analyis of observation light must be treated as packets of energy (Photovoltatic effect in dim light) and in others as waves, even in the same experiment if the detector is an array of photo detectors behind a double slit. I am not bothered by the need to use one "theory" for one partof the observation and another theory for another part, if these separate theories are more simple (easily applied) than an integrated theory few can understand. In some ways the current trends in physics, seem to be headed towards a “priesthood” as in ancient Egypt. I am concerned in that if this is too extreme then society will not provide the funds needed to learn more.
However, that said, you have satisfied my objection to Unity for unity's sake by noting the string theory does predict something (11 dimension) that the current set of separate theories does not. I just wish the prediction were more observable, as Maxwell's prediction of radio waves were - Hertz went off and found them. Etc.

As I know nothing about any calculations in string theory, I take you word for the fact that they do not require a sequence of approximations, but am not sure that is a NECESSITY in you Compton scattering example (or others). To be specific: I of course ignore the recoil momentum and energy of the nucleus the electron was bound to when conserving photon's original energy and momentum, but think this is only to keep the set of equations to be solved smaller. Sort of like ignoring the effect of Saturn when calculating Earth's orbit. Not a NECESSITY, just a convenience. Perhaps string theory's "one step exactness" is also the reason why few can understand it?

Earth’s orbit could be given by a very complex set of equations in “one step exactness” also. Fact that it is computed by set of approximations with perturbations is not fundamental lack of understanding. (I hope I am being clear, despite my ignorance of string theory.)

SUMMARY: Perhaps "divide and conquer" is better for most people and should not be dropped for beauty, if that is all there is to an untestable, but unified theory.
 
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