Gross doesn’t believe this nonsense. He’s known to start quoting Churchill, shouting “Never, never, never, never give up!” when people like Susskind (or Srednicki) start talking about how great it would be to have a theory that is compatible with anything. At his CUNY talk and elsewhere he emphasized that it is the role of particle theorists to find a theory that explains the things about particle physics that the standard model doesn’t, not to congratulate themselves for having found a theory that is consistent with anything.

Gross now stakes his hopes on the idea that the presently known version of string theory is inconsistent and that when a consistent theory is found it will have better uniqueness properties. I (and many others, including Susskind) think this is wishful thinking. I also think he should ditch his powerpoint slide that says that string theory is a finite and consistent theory of quantum gravity, when later in his talk he is pinning his hopes on its inconsistency.

Of course, this won’t lead us immediately to the “theory of everything,” whatever that might mean. But it could be an important step. We could try to test, for instance, whether the real world looks like some intersecting brane scenario.

First of all, even if you believe in finiteness of higher loops in string perturbation theory, perturbative string theory is not a finite theory. All evidence is that the expansion in the string coupling constant is a divergent series. What you’ve got is presumably an asymptotic expansion. This may give a useful approximation if the string coupling constant is very small, a useless one if it isn’t. Saying that perturbative string theory is a “finite, consistent theory of quantum gravity” is simply not true. You can’t sum the series to get something finite, and if you cut it off at a finite order your theory is inconsistent (not unitary).

So if you can get any QFT + gravity out of perturbative string theory + a choice of background, what have you actually got? You’ve got an exceedingly complex theory that predicts absolutely not a single thing about anything and is inconsistent to boot. Would this be a “stunning triumph”?

If you want to engage in extreme wishful thinking and believe that string theorists will come up with a finite, non-perturbative string theory that could have any QFT as its low energy limit, calling such a thing a “stunning triumph” would still be complete hype. It would predict nothing about anything. Theorists should save terms like “stunning triumph” for the day they actually manage to make a single solid prediction that in anyway goes beyond the standard model.

I strongly disagree with this. The extreme case of the discretuum is that *every* QFT is realizable as the low-energy limit of string theory. This probably isn’t right, but suppose it is. Then we could take *any* QFT (with some exceptions: no Landau poles below the Planck scale, for example), include gravity, and have a finite quantum theory (assuming string finiteness holds up, which it may not). This should be considered a stunning triumph!

Furthermore, probably not *every* QFT is realizable in this way. Some are, some aren’t. Which class is the Standard Model in? It seems to me that this is a very important (though also very hard) question.

The great dream was that there would be only *one* QFT realizable as the low energy limit of string theory, and that it would turn out to be the Standard Model. It would’ve been great, but right now it doesn’t seem likely. That certainly doesn’t mean that string theory is vacuous.

The interesting thing about Gross’s talk was that he was kind of going out on a limb on the issue of supersymmetry at the LHC. I’m more and more convinced that he and a lot of others are getting discouraged about string theory and no supersymmetry at the LHC will be the final straw.

The worst possible thing for particle theory would be if the standard model Higgs shows up, behaving like a standard elementary scalar field with a certain mass. Then we would still be in the situation of having no idea where the Higgs potential or couplings come from, and no prospects for doing experiments in our lifetime to find out.

I’m also hoping the LHC doesn’t find a standard Higgs field, but evidence for some more interesting way of breaking electroweak gauge symmetry, one that we haven’t thought of yet.

If they don’t find any SUSY at LHC, I doubt it will be the death knell of String Theory. Alas, there is enough degrees of freedom there to just argue that it breaks at a higher scale, no big deal!

The nasty thing of course is that it just adds more fine tuning to minimal SUSY, and much of the original point would be lost.

I’m more interested though, in the (seemingly absurd) case that the LHC doesn’t discover the Higgs! AFAICS, all reasonable models put it firmly in reach of the LHC.. If we don’t find it, well, something drastic has got to give. I don’t see many people talking about that, so entrenched is the SM gospel in our minds. What a strange higgs sector that would imply.

Have you decided where to go to grad school yet?

Today I’ve put up somewhat more positive posts, and in general hope to mix the positive and negative. Unfortunately even the positive posts will probably be so speculative as to be “Not Even Wrong”. When I figure out a unification idea that really works I’ll dump the blog and start instead enjoying the fruits of fame and fortune that accrue to super-star physicists.

Your idea of pursuing applications of string theory to better understand strongly interacting gauge theories is quite reasonable. I don’t have any problem with that part of the string theory program. My problem very specifically is with the continual hyping of the failed idea of unifying the standard model and gravity in a 10/11d supersymmetric string/M-theory.

I’m not so convinced that new information about strongly interacting gauge theories coming from string theory duals will solve the problem of spontaneous gauge symmetry breaking. The technicolor idea is quite beautiful, but all implementations of it seem to involve adding a huge amount of new structure to one’s theory, while not being able to actually calculate anything one wants to calculate. On the other hand, maybe one can use string duals to
find a way of calculating things, and if this is the way the world works, there should be real evidence for it a few years from now.