It used to be that string theorists would respond to arguments that string theory predicted nothing with the claim that it predicted supersymmetry. For example, in an interview with Witten done for the PBS Elegant Universe series, one sees:
NOVA: It seems like the standard criticism of string theory is that it isn’t testable. How do you respond to that criticism?
Witten: One very important aspect of string theory is definitely testable. That was the prediction of supersymmetry, which emerged from string theory in the early ’70s. Experimentalists are still trying to test it. It hasn’t been proved that supersymmetry is right. But there is a very precise relationship among the interaction rates of different kinds of particles which follows from supersymmetry and which has been tested successfully. Because of that and a variety of other clues, many physicists do suspect that our present decade is the decade when supersymmetry will be discovered. Supersymmetry is a very big prediction; it would be interesting to delve into history and try to see any theory that ever made as big a prediction as that.
Of course the problem with this was always that supersymmetry had to be broken somehow, and string theory said nothing about how to break it, not even the scale of the breaking. Back in 2004 when the anthropic landscape business began, Susskind was enthusiastic about the idea that it could be used to predict the scale of supersymmetry breaking, and Michael Douglas started working on computations counting string vacua that were supposed to say something about this (I’ve followed this story in several blog postings, an early one was here). The bottom line quickly became clear: a host of problems make this impossible, string theory remains incapable of predicting anything about this.
Today at the Simons Center, Douglas gave a talk entitled Does String Theory Predict Low Energy Supersymmetry? (video available here), and not surprisingly the conclusion is still that string theory predicts nothing about this. Amusingly, someone in the audience took exception to Douglas saying that string theory doesn’t now make predictions, and one gets to hear Douglas try and explain to his fellow string theorist what a real prediction is. The video quality is great, but the sound doesn’t work so well when two people are loudly trying to talk over each other.
This particular talk was held indoors, for a report on what the outdoor ones have been like, see here.
I am surprised that someone of Witten’s calibre would commit this elementary logical error so publicly. To claim, as Witten did, that the hunt for experimental supersymmetry is a “test of string theory” is an example of the fallacy known as “affirming the consequent”, given the existence of supersymmetric quantum field theories.
I suspect that, given the opportunity to elaborate, Witten would have explained that what he meant was just that discovering supersymmetry would have provided support for the ideas behind string theory (but not proved string theory, since you can have supersymmetry without string theory). To pin him down, the interviewer should have asked about the falsifiability question: would a failure to find supersymmetry provide evidence against string theory?
Anon, your diagnosis of Witten’s error is wrong. If theories A and B both predict unobserved phenomenon X, the search for X is still a test of A, even though B also predicts it.
Does anybody know what Witten mean when he said “But there is a very precise relationship among the interaction rates of different kinds of particles which follows from supersymmetry and which has been tested successfully?”
Isn’t supersymmetry an assumption of string theory as opposed to a prediction? If this is the case it seems like no physicists care to make this distinction any more even though there is a clear logical difference between the two, basically what Anon is saying I think.
I guess it must mean gauge coupling unification, and what that says about the relative size of the three gauge couplings at the weak scale.
@AV: since consistent string theories appear to require supersymmetry for their internal validity, I think it is fair to say that supersymmetry *at some energy scale* is a prediction of string theory. Supersymmetry at the TeV scale is clearly not a prediction of string theory – its absence does not falsify the subject – although its presence would suggest that string theory is on the right track.
As piscator comments, what Witten is referring to is a supersymmetric GUT calculation, where coupling constant unification works better than in non-supersymmetric GUTS. This assumes, among other things, a “desert hypothesis” of no new relevant physics between the weak and GUT scales. While supersymmetric GUTs push up the GUT scale and thus make the proton lifetime longer, some classes of these models are ruled out by proton lifetime limits. Of course, as usual, there are lots and lots of models and choices to be made, so one can evade experimental limits.
Douglas correctly says that the importance of Low SUSY for String theory has been diminished after the latest theoretical developments, mainly because the theory has now other mechanisms to solve the hierarchy problem like warped compactifications, large extra dimensions or even anthropic reasoning via the string landscape. Low SUSY was very important in the early days of String theory when only the Heterotic model could produce the standard model and Low SUSY was the only available mechanism to solve the hierarchy problem. Now on the other hand there is no such restriction you can get the standard model from D-branes in IIB or IIA where the aforementioned mechanisms could be used to deal with the hierarchy problem and thus do not need Low SUSY (they do not preclude it though). In this case SUSY could break at larger scales since it doesn’t have to solve the hierarchy problem anymore.
Just to add something to my previous comment.
Of course large hierarchies (large enouph to solve the hierarchy problem) produced by large extra dimensions or strong warping, means new physics at TeV scale that should be detected by LHC. So far though this doesn’t seem to be the case.
So if they found just a light Higgs at LHC and nothing else at TeV scales (no new physics) the stringy landscape may be the only explanation for the stability of the EW breaking (unless of course someone comes up with a better idea).
The LHC is discovering that there is no supersymmetry — at least none of the sort that had been widely predicted. The string theorists are like a doomsday cult that desperately needs an alternate story to cover up their failed predictions.
Mitchell Porter, I guess it depends on the semantics of “test of string theory” that Witten could have reasonably taken his audience to understand. In the context of the interview question, it is clear to me that the average educated listener would have taken this to mean “something that can distinguish string theory from other serious proposals such a field theories”. In this sense, even if you may be right in your statement that Witten did not commit an error of logic in a strict technical sense, in my opinion he certainly did so given the context. I am pretty sure that he was bloody well aware of the loophole at the time and made a conscious decision to take advantage of it for political reasons, which I find somewhat immoral.
In any case, you will no get many physicists today still seriously claiming that the LHC is testing string theory. Why do you think that is?
“In this case SUSY could break at larger scales since it doesn’t have to solve the hierarchy problem anymore.” – Giottis
Now that the hierarchy problem has been magically airbrushed from the scene don’t forget about Split Supersymmetry. Or my own personal favourite, Supersplit Supersymmetry.