Ethan Siegel today has a new article at Starts With a Bang, entitled Why String Theory is Both a Dream and a Nightmare. For the nightmare part, he writes:
its predictions are all over the map, untestable in practice, and require an enormous set of assumptions that are unsupported by an iota of scientific evidence.
which I think just confuses the situation, which could be much more accurately and simply described as “there are no predictions”. The fundamental reason for this is also rather simply stated: the supposed unified theory is a theory in ten space-time dimensions, and no one has figured out a way to use this to get a consistent, predictive model with four space-time dimensions. If you don’t believe this, try watching the talks going on in Santa Barbara this week, which feature, after 17 years of intense effort, complete confusion about whether it is possible to construct such models with the right sign of the cosmological constant.
Siegel gets a couple things completely wrong, although this is not really his fault, due to the high degree of complexity and mystification which surrounds the 35 years of failed efforts in this area. About SUSY he writes
For one, string theory doesn’t simply contain the Standard Model as its low-energy limit, but a gauge theory known as N=4 supersymmetric Yang-Mills theory. Typically, the supersymmetry you hear about involves superpartner particles for every particle in existence in the Standard Model, which is an example of an N=1 supersymmetry. String theory, even in the low-energy limit, demands a much greater degree of symmetry than even this, which means that a low-energy prediction of superpartners should arise. The fact that we have discovered exactly 0 supersymmetric particles, even at LHC energies, is an enormous disappointment for string theory.
Like everything else, there’s no prediction from string theory about how many supersymmetries will exist. The special role of N=4 supersymmetric Yang-Mills theory has nothing to do with the problem of low energy SUSY, instead it occurs as the supposed dual to a very special 10d superstring background (AdS5 x S5). This is of interest for completely different reasons, one of which was the hope that this would provide a string theory dual to QCD, allowing use of string theory not to do quantum gravity, but to do QCD computations. This has never worked, with one main reason being that it can’t reproduce the asymptotic freedom property of QCD. Siegel tries to refer to this with
And when you look at the explicit predictions that have come out for the masses of the mesons that have been already discovered, by using lattice techniques, they differ from observations by amounts that would be a dealbreaker for any other theory.
including a table with the caption
The actual masses of a number of observed mesons and quantum states, at left, compared with a variety of predictions for those masses using lattice techniques in the context of string theory. The mismatch between observations and calculations is an enormous challenge for string theorists to account for.
He’s getting this from slide 31 of a talk by Jeff Harvey, but mixing various things up. The table has nothing to with lattice calculations, those are relevant to the other part of the slide, which is about string theory predictions for pure (no fermions) QCD glueballs. These are not physical objects, thus the comparison to lattice computer simulations, not experiment. The table he gives is from here and about real particles. The “predictions” are not made as he claims “using lattice techniques in the context of string theory.” There are no lattice techniques involved.
Normally Siegel does a good job of navigating complex technical subjects. The subject of string theory is now buried in a huge literature of tens of thousands of papers over forty years with all sorts of claims, many designed to obscure the fact that ideas haven’t worked out. It’s fitting that the name chosen for the kind of discussions going on at Santa Barbara this week is “The String Swampland”. String theory verily is now deep in a trackless swamp…