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What experiment would disprove string theory?

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1894 views

String Theory is often criticised as not falsifiable, i.e. that there is no experiment that can disprove it. Therefore, critics of String Theory often equate it to pseudo-science, etc.  

However, string theorists believe that string theory is falsifiable. 

So could we have a comprehensive list of proposed experiments which could disprove string theory? 

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Albert

asked Nov 2, 2010 in Experimental Physics by Albert (50 points) [ revision history ]
edited Apr 20, 2014 by dimension10
I saw a comment (seems to have vanished) to one of the answers that the view, heavily thrashed out in Smolin's book, that ST assumes a flat background whereas GR clearly accounts for dynamically bending spacetime is a bit moot insofar that GR could be construed of as being in a flat background with some fancy additions. Did this comment have any merit - could Lee Smolin truly have been unaware of such an interpretation? Or could it be something kind of trivial like the curved manifold of a GR solution can be embedded in a higher dimensional flat space (Whitney embedding). Comments anyone?

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user WetSavannaAnimal aka Rod Vance

12 Answers

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One can disprove string theory by many observations that will almost certain not occur, for example:

  1. By detecting Lorentz violation at high energies: string theory predicts that the Lorentz symmetry is exact at any energy scale; recent experiments by the Fermi satellite and others have showed that the Lorentz symmetry works even at the Planck scale with a precision much better than 100% and the accuracy may improve in the near future; for example, if an experiment ever claimed that a particle is moving faster than light, string theory predicts that an error will be found in that experiment

  2. By detecting a violation of the equivalence principle; it's been tested with the relative accuracy of $10^{-16}$ and it's unlikely that a violation will occur; string theory predicts that the law is exact

  3. By detecting a mathematical inconsistency in our world, for example that $2+2$ can be equal both to $4$ as well as $5$; such an observation would make the existing alternatives of string theory conceivable alternatives because all of them are mathematically inconsistent as theories of gravity; clearly, nothing of the sort will occur; also, one could find out a previously unknown mathematical inconsistency of string theory - even this seems extremely unlikely after the neverending successful tests

  4. By experimentally proving that the information is lost in the black holes, or anything else that contradicts general properties of quantum gravity as predicted by string theory, e.g. that the high center-of-mass-energy regime is dominated by black hole production and/or that the black holes have the right entropy; string theory implies that the information is preserved in any processes in the asymptotical Minkowski space, including the Hawking radiation, and confirms the Hawking-Bekenstein claims as the right semiclassical approximation; obviously, you also disprove string theory by proving that gravitons don't exist; if you could prove that gravity is an entropic force, it would therefore rule out string theory as well

  5. By experimentally proving that the world doesn't contain gravity, fermions, or isn't described by quantum field theories at low energies; or that the general postulates of quantum mechanics don't work; string theory predicts that these approximations work and the postulates of quantum mechanics are exactly valid while the alternatives of string theory predict that nothing like the Standard Model etc. is possible

  6. By experimentally showing that the real world contradicts some of the general features predicted by all string vacua which are not satisfied by the "Swampland" QFTs as explained by Cumrun Vafa; if we lived in the swampland, our world couldn't be described by anything inside the landscape of string theory; the generic predictions of string theory probably include the fact that gravity is the weakest force, moduli spaces have finite volume, and similar predictions that seem to be satisfied so far

  7. By mapping the whole landscape, calculating the accurate predictions of each vacuum for the particle physics (masses, couplings, mixings), and by showing that none of them is compatible with the experimentally measured parameters of particle physics within the known error margins; this route to disprove string theory is hard but possible in principle, too (although the full mathematical machinery to calculate the properties of any vacuum at any accuracy isn't quite available today, even in principle)

  8. By analyzing physics experimentally up to the Planck scale and showing that our world contains neither supersymmetry nor extra dimensions at any scale. If you check that there is no SUSY up to a certain higher scale, you will increase the probability that string theory is not relevant for our Universe but it won't be a full proof

  9. A convincing observation of varying fundamental constants such as the fine-structure constant would disprove string theory unless some other unlikely predictions of some string models that allow such a variability would be observed at the same time

The reason why it's hard if not impossible to disprove string theory in practice is that string theory - as a qualitative framework that must replace quantum field theory if one wants to include both successes of QFT as well as GR - has already been established. There's nothing wrong with it; the fact that a theory is hard to exclude in practice is just another way of saying that it is already shown to be "probably true" according to the observations that have shaped our expectations of future observations. Science requires that hypotheses have to be disprovable in principle, and the list above surely shows that string theory is. The "criticism" is usually directed against string theory but not quantum field theory; but this is a reflection of a deep misunderstanding of what string theory predicts; or a deep misunderstanding of the processes of the scientific method; or both.

In science, one can only exclude a theory that contradicts the observations. However, the landscape of string theory predicts the same set of possible observations at low energies as quantum field theories. At long distances, string theory and QFT as the frameworks are indistinguishable; they just have different methods to parameterize the detailed possibilities. In QFT, one chooses the particle content and determines the continuous values of the couplings and masses; in string theory, one only chooses some discrete information about the topology of the compact manifold and the discrete fluxes and branes. Although the number of discrete possibilities is large, all the continuous numbers follow from these discrete choices, at any accuracy.

So the validity of QFT and string theory is equivalent from the viewpoint of doable experiments at low energies. The difference is that QFT can't include consistent gravity, in a quantum framework, while string theory also automatically predicts a consistent quantum gravity. That's an advantage of string theory, not a disadvantage. There is no known disadvantage of string theory relatively to QFT. For this reason, it is at least as established as QFT. It can't realistically go away.

In particular, it's been showed in the AdS/CFT correspondence that string theory is automatically the full framework describing the dynamics of theories such as gauge theories; it's equivalent to their behavior in the limit when the number of colors is large, and in related limits. This proof can't be "unproved" again: string theory has attached itself to the gauge theories as the more complete description. The latter, older theory - gauge theory - has been experimentally established, so string theory can never be removed from physics anymore. It's a part of physics to stay with us much like QCD or anything else in physics. The question is only what is the right vacuum or background to describe the world around us. Of course, this remains a question with a lot of unknowns. But that doesn't mean that everything, including the need for string theory, remains unknown.

What could happen - although it is extremely, extremely unlikely - is that a consistent, non-stringy competitor to string theory that is also able to predict the same features of the Universe as string theory can emerges in the future. (I am carefully watching all new ideas.) If this competitor began to look even more consistent with the observed details of the Universe, it could supersede or even replace string theory. It seems almost obvious that there exists no "competing" theory because the landscape of possible unifying theories has been pretty much mapped, it is very diverse, and whenever all consistency conditions are carefully imposed, one finds out that he returns back to the full-fledged string/M-theory in one of its diverse descriptions.

Even in the absence of string theory, it could hypothetically happen that new experiments will discover new phenomena that are impossible - at least unnatural - according to string theory. Obviously, people would have to find a proper description of these phenomena. For example, if there were preons inside electrons, they would need some explanation. They seem incompatible with the string model building as we know it today.

But even if such a new surprising observation were made, a significant fraction of the theorists would obviously try to find an explanation within the framework of string theory, and that's obviously the right strategy. Others could try to find an explanation elsewhere. But neverending attempts to "get rid of string theory" are almost as unreasonable as attempts to "get rid of relativity" or "get rid of quantum mechanics" or "get rid of mathematics" within physics. You simply can't do it because those things have already been showed to work at some level. Physics hasn't yet reached the very final end point - the complete understanding of everything - but that doesn't mean that it's plausible that physics may easily return to the pre-string, pre-quantum, pre-relativistic, or pre-mathematical era again. It almost certainly won't.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Luboš Motl
answered Jan 17, 2011 by Luboš Motl (10,178 points) [ no revision ]
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Dear Peter, you have just apparently omitted 90% of the methods, not sure why, but OK. At any rate, the falsification is hard but this is true, almost by definition, for any conceivable mathematically consistent hypothesis that addresses the Planck scale - or any other very high-energy - physics. This difficulty is here because of the very problems we're trying to answer - and it has no impact on the validity of string theory whatsoever.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Luboš Motl
Dear Rafael, nope, string theory isn't this general at all. In fact, it doesn't allow any choice of perverse phenomena and regardless of the vacuum, it predicts things about the reality. The goal of my answer was to detonate the misconception of the most uninformed laymen who think that string theory is untestable or unscientific in any sense. I recommend you to read my answer instead of inventing your own nonsense which is pretty much the opposite of what I wrote, and what is the truth.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Luboš Motl
Items 1-5 and 9 seem like they would be pretty shocking whether or not string theory is a true description of the world. Saying "string theory is disproven if QM, or the equivalence principle, or the soundness of mathematics itself is disproven" seems a bit like saying "GR would be falsified if it turns out there's no such thing as gravity." Technically true, but not nearly as useful as being able to say "such-and-such star will have such-and-such apparent position during the next solar eclipse."

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Tim Goodman
In other words, I'd think the predictions of a theory which aren't part of previously accepted physics are the ones we'd really like to test. Items 6-8 seem more in this vein, although 8 sure isn't coming any time soon.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Tim Goodman
Lubos, when writing down a string action, doesn't it begin, as Lagrangians usually do, by imposing Lorenz invariance and the invariances for the fields that are to be reproduced? If not, I'd love to read up on how that works. Could you suggest a reference for me?

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user user349653
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this comment might get lost in the shuffle, but by QFT I assume you mean "The standard model" - you can't pick arbitrary fields + gauge group and expect them to reproduce observations. I have always thought this was an important distinction, and a minor pet peeve of mine.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user levitopher
@DiMension10: I hate to be an idealogue, but lucky I don't need to be since research in LQG is published regularly in extremely high-quality journals. It's obviously not "mathematically inconsistent" in the way you are using it.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user levitopher
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Since many people seem to have very odd ideas about this, let's address this from a much simpler point of view.

Let's suppose you have a friend who only knows math at the level of arithmetic of positive integers. You try to tell him about the existence of negative numbers, and he tells you,

That's stupid, there's obviously no such thing as "negative" numbers, how can I possibly measure something so stupid? Can you have negative one apple? No, you can't. I can owe you positive one apple, but there's clearly no such thing as negative apples.

How can you start to argue that there is such a thing as negative numbers?

A very powerful first step is mathematical consistency. You can list all of the abstract properties you believe to characterize everything about positive integer arithmetic:

  • For all a,b,c, a(b+c) = ab+ac
  • For all a,b, a+b=b+a, ab=ba
  • There exists a number, called 0, such that, for all a, a+0=0+a=a, a0=0a=0
  • There exists a number, called 1, such that, for all a, 1a=a1=a

(note that, in sharp contrast to the case of real numbers, the first property can be proved with induction, and need not be an axiom. Similarly, other listed properties can be proved from other ones designated to be more basic if one wishes, which can not be done in the case of the reals.)

So, once you both agree that these axioms characterize the positive integers completely, you can show that these hypothetical negative numbers, based on their formal properties, are consistent with the above axioms. What does this show?

The positive integers, with the addition of the negative integers, can do at least as much as the positive integers by themselves.

(STOP At this point, pause to realize how powerful this constraint is!! How many other ways could one generalize arithmetic, at this level, to something else that is consistent with the properties you want? Zero. There is absolutely no other way to do it. This is incredibly suggestive, and you should keep this in mind for the rest of the cartoon argument, and see how every argument that follows is secretly an aspect of this one!)

Your friend responds:

Sure, you can write down toy models like that, and they may be consistent, but they don't correspond to reality.

Now, what else do you need to demonstrate to your friend to convince him of the validity of the negative numbers?

You find something else they can do that you can't do with the positive numbers alone. Simply, you can state that every positive-integer-valued algebraic equation does not have a solution:

x + 1 = 0

does not have a solution.

But, it is a trivial fact that extending to the negative numbers allows you to solve such equations. Then, all that's left to convince your friend of the validity of negative numbers is to show that this is equivalent to solving an ("a priori") different problem which only involved arithmetic of positive integers:

x+1=0 <=> y + 1 = 1

So, y = 0, and y=x+1 is equivalent to the other problem.

To be complete, we also have to consider problems that are "unique" to the negatives, such as (-1)(-1) = 1, but in the realm of integers, these are trivial matters that are reducible to the above. Even in the case of reals, given the other things we've shown, these consequences are almost "guaranteed" to work out intuitively obviously.

Now, assuming your friend is a reasonable, logical, person, he must now believe in the validity of negative numbers.

What have we shown?

  • Consistency, both with previous models and with itself
  • The ability to solve new problems
  • The reduction of some problems in the new language to problems in the old language

Now, to decide if this a good model for a particular system, you must look at the subset of problems that did not have a solution before, and see if the new properties characterize that system. In this case, that's trivial, because the properties of negative numbers are so obvious. In the case of applying more complicated things to describe the details of physical situations, it's less obvious, because the structure of the theory, and the experiments, is not so simple.

How does this apply to string theory? What must we show to convince a reasonable person of its validity? Following the above argument, I claim:

  • String theory reproduces (by construction) general relativity
  • String theory reproduces (by construction) quantum mechanics (and by the above, quantum field theory)

So string theory is at least as good as the rest of the foundations of physics. Stop again to marvel at how powerful this statement is! Realistically, how many ways are there to consistently and non-trivially write a theory that reduces to GR and QFT? Maybe more than one, but surely not many!

Now the question is--what new do we learn? What additional constraints do we get out of string theory? What problems in GR and QFT can be usefully written as equivalent problems in string theory? What problems can string theory solve that are totally outside of the realm of GR and QFT?

Only the last of these is beyond the reach of current experiments. The "natural" realm where string theory dominates the behavior of an experiment is is at very high energies, or equivalently, very short distances. Simple calculations show that these naive regions are well outside of direct detection by current experiments. (Note that in the above example of negative numbers, the validity of the "theory" strictly in the corresponding realm didn't need to be directly addressed to make a very convincing argument; pause to reflect on why!)

However, theoretical "problems" with the previous theories, such as black hole information loss, can be solved with string theory. Though these can't be experimentally verified, it's very suggestive that they admit the expected solution in addition to reproducing the right theories in the right limits.

There are two major successes of string theory that satisfy the other two requirements.

AdS/CFT allows us to solve purely field theory problems in terms of string theory. In other words, we have solved a problem in the new language that we could already solve in the old language. A bonus here is that it allows us to solve the problem precisely in a domain where the old language was difficult to deal with.

String theory also constrains, and specifies, the spectrum and properties of particles at low energies. In principle (and in toy calculations), it tells us all of the couplings, generations of particles, species of particles, etc. We don't yet know a description in string theory that gives us exactly the Standard Model, but the fact that it does constrain the low-energy phenomenology is a pretty powerful statement.

Really, all that's left to consider to convince a very skeptical reader is that one of the following things is true:

  • It is possible for string theory to reproduce the Standard Model (e.g., it admits solutions with the correct gauge groups, chiral fermions, etc.)
  • It is not possible for string theory to reproduce the Standard Model (e.g., there is no way to write down chiral theories, it does not admit the correct gauge groups, etc. This is the case in, e.g., Kaluza-Klein models.)

I claim, and it is generally believed (for very good reasons), that the first of these is true. There is no formal, complete, mathematical proof that this is the case, but there is absolutely no hint of anything going wrong, and we can get models very similar to the standard model. Additionally, one can show that all of the basic features of the Standard Model, such as chiral fermions, the right number of generations, etc, are consistent with string theory.

We can also ask, what would it mean if string theory was wrong? Really, this would signal that,

  • The theory was mathematically inconsistent (there is no reason to believe this)

  • At a fundamental level, either quantum mechanics or relativity failed in some fairly pathological way, such as a violation of Lorentz invariance, or unitarity. This would indicate that a theory of everything would look radically different than anything written down so far; this is a very precarious claim--consider what would happen in the example of arithmetic in the above if there were something "wrong" with addition.

  • The theory is consistent, and a generalization of GR and QFT, but is somehow not a generalization in the right "limit" in some sense. This happens in, e.g., Kaluza-Klein theory, where chiral fermions can't be properly written down. In that case, a solution is also suggested by a sufficiently careful analysis (and is one potential way to get to string theory).

Of these three possibilities, the first two are extremely unlikely. The third is more likely, but given that it is known that all the basic features can show up, it would seem very strange if we could almost reproduce what we want, but not quite. This would be like, in the arithmetic example, being able to reproduce all the properties we want, except for 1+ (-1) = 0.

If you're careful, you can phrase my argument in a more formal way, in terms of what it precisely means to have a consistent generalization, in the sense of formal symbolic logic, if you like, and see what must "fail" in order for the contrapositive of the argument to be true. (That is, (stuff) => strings are true, so ~strings => ~(stuff), and then unpack the possibilities for what ~(stuff) could mean in terms of its components!)

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Mr X
answered Jan 17, 2011 by Mr X (200 points) [ no revision ]
Your answer is a tour de force. I give it +1, with the caveat that I disagree with your statement that "String theory reproduces (by construction) quantum mechanics". I do not know of any aspect of string theory or of any claims (speculative or otherwise) which could substantiate such a statement. We start of with the action (Nambu-Goto/Polykaov) for a string. This action describes a classical object. We go on to "quantize" this action following the standard prescriptions of quantum mechanics. In no way does QM arise from a stringy basis, IMHO. Please correct me if you think I'm mistaken.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user user346
Well it doesn't from an operator theory point of view, fix the quantization procedure or anything, so you can't really quite construct an "axiomatic qft" based off of it. But it is still designed to be appropriately quantum mechanical in the right limits. But knowing, and using, quantum mechanics in advance is not an impediment to constructing it or to my statement ;).

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Mr X
I'm not accepting string theory as a consistent well-defined theory of physics until string theorists agree on the answer to the question: How does information escape from a black hole? Right now, I'm not even sure string theorists can correctly formulate the conditions that a string theory state is a black hole.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Peter Shor
There are many papers addressing this. And there are interesting statements you can make in, e.g., AdS/CFT by thinking about how the field theory corresponds to black holes in the gravity dual. I believe Lubos's blog has a few discussions of this in fairly simple terms. Of course, many details are not yet understood, but the general idea certainly is.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Mr X
I like the metaphor of the extension of the positive integers to the signed integers as a model for the extension of relativity+quantum theory to string theory. But I think a better metaphor is the extension of Euclid's axioms. It turned out there were multiple consistent (even "correct") extensions of the axioms. But of course, only one extension is "true" (or "real) for any given universe. This is rather the question being asked here: is there any experimental way to know which extension reflects reality?

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Mark Beadles
Bravo! This has given my scepticism a strong blow.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Karl Damgaard Asmussen
+ 7 like - 0 dislike

String theory, was constructed with the idea that at low energies it should reduce to the quantum mechanical and particle world that we see every day. This is analogous to the correspondence principle in quantum mechanics.

In some sense, any experiment that discredits quantum mechanics will cause a serious re-examination of string theory, however, as many experimentalists will joke, a theorist will always find a way to fix his theory to match the observations. In any case, quantum theory seems very well supported and is unlikely that it will be discredited any time soon

Direct tests of string theory, however, will have to wait until we can probe much higher energies.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user crasic
answered Nov 2, 2010 by crasic (140 points) [ no revision ]
...or someone finds a much more powerful and clever "magnifier" for string theoretical effects then is currently known.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user dmckee
And what are those effects? What string theory predicts for high energies that other theories do not?

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Anixx
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The only way to 'test' string theory, is to actually figure out what it predicts first, which is ambigous at the moment.

Contrary to most claims, String theory is actually a unique theory in that there are no adjustable free parameters. However it has a large or possibly infinite amount of classical solutions or 'vacua'. Most of these vacua look nothing like the real world, some (by some I mean it could be ~ 10^500) are very similar (in that they seem to correspond to low energy physics like the standard model), and at most one corresponds to the real world.

When you have specified a vacua, you then have fixed the predictions identically -- for everything (thats what it means to be a theory of everything). So for instance, you could compute the mass of the electron to however many decimal places and then test it in the lab and also the vacua might make an unambigous prediction for cosmological objects (eg the existence of cosmic strings or domain walls). We don't know exactly.

Of course absent a way to figure out which vacua corresponds to reality, we are left with the same problem that quantum field theory has. Namely you actually have to go out and measure certain things first in order to make predictions (so for instance in the standard model we need to pin down the 26 adjustable free parameters, like the Yukawa couplings, masses of elementary particles and so forth).. But once you have done that, you can then predict infinitely many other things, like scattering cross sections.

So in string theory, that would mean some way of wittling down 10^500 vacua to a more human number, like 50, 10 or better 1 or 0 (0 means the theory is falsified). Which requires doing scattering experiments at the Planck scale, which means a particle accelerator roughly on the scale of the Milky Way.

Of course, it may turn out that string theory makes unambigous falsifiable predictions, but that really requires a great deal of theoretical work b/c it implies knowing something about not just the vacua that corresponds to the real world, but also the 10^500 other ones if you get my meaning. Still, we know that it does make some predictions. For instance, the existence of the quanta of gravity is universal across all solutions. Likewise, the fact that quantum mechanics and special relativity must hold is another robust prediction.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Columbia
answered Jan 17, 2011 by Columbia (160 points) [ no revision ]
Nice answer @Columbia. My question is about your last sentence: the fact that quantum mechanics and special relativity must hold [in string theory] is another robust prediction. I've heard this said before and I don't see how these are "predictions" of string theory. The string action is covariant so relativity is obeyed. Quantization follows the rules of quantum mechanics. These ingredients are part of the setup from the get go. Perhaps I am misinterpreting the statement.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user user346
@SpaceCadet. It is true that if you start learning st from GSW in the usual way that world sheet LI and Dirac QM are automatically there from the beginning. So in a sense it is not surprising that they stay that way, although you could in principle imagine Lorentz breaking by anomalies. However if you start from equally valid formulations of st where LI is not manifest ex: Light cone gauge, then it is always the case that it is recovered. These are non trivial consistency checks. Similarly with quantum mechanics, you often recover Dirac rules whenever you might have forgotten them

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Columbia
The point I think is that String theory is very tightly constrained, perhaps moreso than any other physical theory invented and its a bit of a culture shock to pause for a minute and appreciate just how much we took for granted in other theories. There is no wiggle room really for theorists to come in and insert something by hand, b/c if one thing breaks then everything breaks and it usually happens in a very obvious and violent way.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Columbia
From where this number of variants came from, 10^500? Why you do not know whether the number of possibilities finite or not?

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Anixx
@Annix the number of solutions to the dynamical equeations IS finite and well and well defined.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Dilaton
+ 7 like - 0 dislike

It is very hard to probe the scales of quantum gravity experimentally so there are very few possibilities for testing if string theory is incorrect as a unified theory encompassing quantum gravity. One of the few observations that has probed the Planck scale was the Fermi gamma ray telescope observation which showed that photons of different energy travel very close to the same speed over cosmic distances. If the result had shown a dispersion of speed it would have disproved string theory and encouraged physicist to look at other ideas.

Of course any observation that disproves quantum theory or general relativity will also disprove string theory, but the real interest is in observations that relate directly to quantum gravity because the separate theories are already well established in their own regimes.

Although it is hard to get direct experimental input for any theory of quantum gravity the constraints on any theory from the logical requirement to combine quantum theory and gravity into one consistent theory for physics are already tremendously strong. In particular there should be some perturbative low energy limit that describes gravity in terms of gravitons which interact with matter. Despite much effort string theory is the only approach that can accomplish this and it is very hard to conceive of a second way. In fact it is very surprising that there is this one way because it requires almost miraculous cancellations of anomalies to make it work. This gives many people the confidence that string theory is the correct path to follow.

Ultimately there needs to be some definitive observation of a quantum gravitational effect that supports string theory. As I said, there are not many possibilities for such observations at present, but this would be a problem for any alternative theory of quantum gravity too.

It is possible that we might get lucky and observe large extra dimensions at the LHC, but there is no reason to expect that. Another possibility that I think is a little more plausible is that supersymmetry would be observed and found to take a form that supports a supergravity origin. Still we have no moral right to expect the universe to hand us such an easy clue and string theory does not promise one.

Such difficulties do not mean that string theory is wrong as some opponents say. It just means that it is going to be difficult to explore quantum gravity empirically.

There has still been constant progress in understanding string theory from the theoretical side and that is continuing. More work needs to be done on the non-perturbative side of string theory so that we can better understand its implications for cosmology. There is some hope that an observation of relic gravitational waves or even low frequency radio waves left over from the big bang may have a characteristic signature dependent on quantum gravitational effects. Again we have no moral rights to demand that such an observation will be forthcoming but it might if we are lucky.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Philip Gibbs
answered Jan 17, 2011 by Philip Gibbs (650 points) [ no revision ]
Your indication of the importance of quantum gravity to this question is a great point.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Mark Beadles
+ 6 like - 0 dislike

If understood correctly (?) something Lubos once said, string theory requires torsion (in GR) to be zero. There are experiments underway/planned right now to measure torsion. Therefore not only is there an experimental disproof of string theory, but we should get the data soon.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Michael C Price
answered Jan 30, 2013 by Michael C Price (120 points) [ no revision ]
@Lubos Motl Could you please confirm whether Michael understood you right. Nonzero torsion is something we nonstringtheoretical mere mortals can readily understand, and, if Michael's understanding is right, then this is an extremely important and concrete test for string theory.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user WetSavannaAnimal aka Rod Vance
could you please give a link or a summary of the experiments. Nonzero torsion would in one way be a strict telling against GR, but this is not "serious" in the sense that we would then have Einstein–Cartan theory: different in form but not really different in the grounding ideas of GR. Maybe the same could be said of string theory? Please, someone might like to comment, since I know a bit about GR, but nothing about ST - is there an ST that reduces to Einstein Cartan, rather than plain GR? Or is such a thing fundamentally different?

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user WetSavannaAnimal aka Rod Vance

There is no propagating torsion in GR or string theory (it would be a new field of a kind which is not coupled to the string worldsheet or any brane), but spinning matter produces localized torsion naturally, localized at the location of the spinning particle, as you can see from doing parallel transport of a vector field in a background which is the infinitesimal limit of the Kerr solution. The formal version of this observation is the Einstein-Cartan theory, which is just GR with spinning sources, so that there is an angular momentum density in space. In this theory, you have a torsion proportional to the classical spin-density, but it's not propagating, it's sitting on top of the spinning matter, so, for example, in the interior of a ferromagnet.

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String excitations, rspt lack thereof. Problem is however that unless you believe in a theory with a lowered Planck scale, you'll have to go to energies we'll never be able to reach in the lab to test this regime. But at least in principle it's falsifiable.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user WIMP
answered Jan 21, 2011 by WIMP (150 points) [ no revision ]
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As suggested by other posters, the key question is energy.

At very high energy levels, approaching some of the quantum gravity 'limits', the string-like nature of fundamental particles would become increasingly apparent. (In terms of an experiment, at a high enough energy level, for instance, there would likely be new specific 'resonances' of the material that could be identified.)

You might also find this article interesting.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user Joseph Weissman
answered Nov 20, 2010 by Joseph Weissman (80 points) [ no revision ]
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To disprove the string theory I would say is not very likely; but there are two ways that would cause trouble in string theory: one theoretical and one by experiment. Both unlikely to happen soon, (an both unlikely to ever disprove it, because the string theory seems to be in the right way, at least until now by far.) - If one would like to "dissprove it" at least in an initial approximation to cause some scepticism, should find a theory which solves at least as many problens as the string theory solves, and deals with at least some of the remaining problems of the string theory. (Even if this other theory exists, it will take several decades after its discovery to evolve anyway..)

-Experimentally as everyone else said we need to achieve higher energies on earth, or some better way to locate and detect these higher energies in universe possibly. To achieve or locate somewhere, the energies where string theory effects are detectable will take many many years from now-at least that is the common belief.

-The "argument" that sometimes I listen the string theory is not a theory or is wrong because it can not be verified or disproved is competely wrong. Simply because this is not an argument. If we have to reach higher energies to see the string theory, then we just have to; is very possible that this is the physics. In any case even simpler physics stories like the one of neutrino which critised (with the critics of that time to be:'not even wrong' or the most optimistic "there is no practical way of observing the neutrino"); with Pauli and Fermi doing the theoretical prediction in ~1930 and the discovery of the Cowan and Reines in 1956 can teach us some things...

In one sentence the answer to your last line is: Until now there is no experiment or detector that can verify the string theory. In the future it is a belief that this will happen when we will be able to achieve or locate somewhere these higher energies that the stringy effects become detectable.

This post imported from StackExchange Physics at 2014-03-30 15:55 (UCT), posted by SE-user dgalaxy
answered Jan 17, 2011 by dgalaxy (60 points) [ no revision ]
+ 0 like - 1 dislike

I was a little surprised that no one mentioned, a collider with a minimum energy of 1018 eV would be required to smash a hypothetical string on the Planck scale of length into smaller bits to see what, if anything, it is made of.

Collider energy is specified by the strength of the magnetic field (maximum about 25 Tesla using the most advanced high temp superconductors, for dipole and quadrupole magnets), and the circumference.   There just isn't enough Yttrium in the Earth, enough room for the collider ring, nor enough money with the combined resources of this small globe, for us to be able to build a collider with that much energy.

I think that answers the question as directly as possible.

This analysis discounts several predictions already made by string theory which did not work out.  Mind you, I do not mean to imply that the effort to develop string theory was wasted.  It most definitely was worth the trip.  All of the results or predictions do not need to be verifiable by experiment, whether it is practical to do so or not.

answered Apr 21, 2014 by danshawen [ revision history ]
edited Apr 21, 2014
People are working on new technologies that allow to build smaller colliders too. And yes, at present without these technologies available, huge colliders as you discribe would be necessary to directly access the string or even the quantum gravity scale. However, what many people in particular in the course of some discussions dont know or ignore, that very high energy physics can (it does not have to ...) give indirect hints for its presence in low energy measurements and data. The recent BICEP results and their interpretation are a nice example of this.

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