Big-shots losing faith in string theory

String theorist Lubos Motl

Lenny Susskind's most eye-catching comments were summarized by Matt Strassler as follows:

In fact, we heard this from none other than Lenny Susskind (famous for his efforts, along with those of ‘t Hooft and many others, to oppose Hawking’s view that black holes require no revision of quantum mechanics, but now himself deeply puzzled by the firewall problem — the failure of what Susskind called `complementarity’). Susskind stated clearly his view that string theory, as currently understood, does not appear to provide a complete picture of how quantum gravity works.

Bizarre. And it's not just Susskind; Joe Polchinski said very similar things recently. ...

Again, let me point out that the error that Susskind, Polchinski, and others were doing in recent years is a special case of Einstein's error in the EPR papers. Einstein was assuming that once the two parts of an entangled photon pair separate, they must have objective properties in the classical sense which, via Bell's-theorem-like reasoning (if I use the equivalent "newer" toolkit), implies that the correlations can't be too large or too universal. In the same way, the "firewall problem" advocates think that the properties of a black hole such as its positions may be treated as classical observables once the black hole is created. While Einstein (and EPR) would think about small systems and pairs of particles, their point was much more general and the "firewall problem" champions' mistake (or two related mistakes, in the counting done above) is not just analogous but it is a special case of Einstein's error.

As you can see, I think that most of these misunderstandings, especially by the big shots, boil down to their subtle (?) misunderstandings of quantum mechanics, more precisely attempts to treat certain questions classically even though it is totally paramount to treat them quantum mechanically to avoid "paradoxes" they want (?) to avoid in their final understanding of quantum gravity.

Lumo defends quantum mechanics and string theory, of course.

Peter Woit has more detail on the failure of string theory.

With all the good reasons to give up on string theory, the black hole firewall paradox is an odd one. They all seem to have an Einsteinian preconception of a complete unified theory.

Physicist Matt Strassler writes:

Over the weekend, someone said to me, breathlessly, that they’d read that “Results from the Large Hadron Collider [LHC] have blown string theory out of the water.” ...

Now what’s so silly about this notion that the LHC has ruled out string theory is that the whole reason a lot of people hate string theory is that it doesn’t make any testable predictions! So obviously you can’t rule it out with current experiments… that would require testable predictions!

Be sure and read the knowledgeable comments. There are pretty much two possibilities -- that string theory makes no testable predictions, and that the LHC has disproved the theory.

All the string theorists seem to have believed that there ought to be supersymmetric (SUSY) particles in the range of energies accessible to the LHC. The LHC has not found any such particle, and is systematically eliminating the possibility. There are different reasons for believing in SUSY. For some of those reasons, the LHC should have found a SUSY particle.

Strassler defends string theory, but is not a string theorist himself. For a hard core enthusiast and hater of anyone else, see Lumo:

When Michio Kaku or even Brian Greene were explicitly or at least implicitly promising you time machines that will produced because of advances in string theory, they oversold the practical power of string theory. When someone would "promise" that it's guaranteed that an experiment that has already been performed would have to observe some beyond-the-Standard-Model physics, they surely oversold the "urgency" of string-theoretical predictions as well – simply because no BSM physics has been observed yet.

However, when Edward Witten "guessed" that the single right string theory compactification capable of predicting all particles' properties would be found within weeks back in 1985, it was just a guess that turned out to be overly optimistic but that reflected this top scientist's best judgement at the moment. There were very good reasons to think so.

Lumo brags about how well the theory works, but cites no published papers to back it up.