Friday, August 27, 2021

Supersymmetry and String Theory Are In Bad Shape

Woit makes his latest and still convincing case that Supersymmetry and String Theory are in deep trouble in his latest blog post reviewing some recent publications

He opens with a quote from the Economist in an article directed at educated laymen:

But, no Susy, no string theory. And, 13 years after the LHC opened, no sparticles have shown up. Even two as-yet-unexplained results announced earlier this year (one from the LHC and one from a smaller machine) offer no evidence directly supporting Susy. Many physicists thus worry they have been on a wild-goose chase… 
Without Susy, string theory thus looks pretty-much dead as a theory of everything. Which, if true, clears the field for non-string theories of everything.

This is right on target. 


neo said...

David Gross:

So you don’t think that other approaches like loop quantum gravity have . . .

Loop quantum gravity is total BS. I mean, it’s really not worth discussing it. Don’t put that in the book. But, it really isn’t.

what is your opinion on lqg ?

Mitchell said...

Sorry, but whoever wrote that article for The Economist seems to be a fan of alternative physics, rather than someone who can actually judge.

In a part that Peter Woit doesn't quote, they mention what these new non-string theories of everything say about reality:

"All of these are mind-bending. One modestly seeks to overturn the principle of causality. Another suggests that everything in the universe really is connected to everything else, and that it is from this simultaneous connection of all with all that the fabric of reality emerges. Time and space are, on this view, not fundamentals of nature, but merely the effects of deeper processes.

Such ideas are in the grand tradition of physics upsetting what seems, to the limited outlook of the human intellect, to be common sense."

I mean, if that's what they care about... the Born rule of quantum mechanics "overturned causality" over 80 year ago! Or if they want something more fundamental than space-time, their best best is probably string theory itself, e.g. its matrix models.

andrew said...

@neo LGQ is one of multiple viable research programs. It is a small group of researchers, but they've made a lot of meaningful intermediate steps and might be right. Worth monitoring but it takes a lot of work because it is such a different paradigm.

@Mitchell Proper string theory, to the extent that the folk wisdom that it needs a SUSY low energy approximation, is in trouble. It might contribute bits of concepts and methods, but the whole is almost surely wrong.

Mitchell said...

Regarding LQG, many recent papers by Martin Bojowald, who worked for years in the area, criticize common claims.

neo said...

@ Mitchell

isn't string and susy also debunked ?

andrew said...

There is criticism, but it isn't exactly a fighting words rejection. For example, his most recent pre-print and abstract are as follows:

"arXiv:2108.11936 [pdf, ps, other] gr-qc hep-th
Space-time physics in background-independent theories of quantum gravity

Authors: Martin Bojowald

Abstract: Background independence is often emphasized as an important property of a quantum theory of gravity that takes seriously the geometrical nature of general relativity. In a background-independent formulation, quantum gravity should determine not only the dynamics of space-time but also its geometry, which may have equally important implications for claims of potential physical observations. One of the leading candidates for background-independent quantum gravity is loop quantum gravity. By combining and interpreting several recent results, it is shown here how the canonical nature of this theory makes it possible to perform a complete space-time analysis in various models that have been proposed in this setting. In spite of the background-independent starting point, all these models turn out to be non-geometrical and even inconsistent to varying degrees, unless strong modifications of Riemannian geometry are taken into account. This outcome leads to several implications for potential observations as well as lessons for other background-independent approaches.

Submitted 26 August, 2021; originally announced August 2021."

Mitchell said...


The idea that has really suffered, is that there would be particles beyond the standard model at low energies, in order to make the theory "natural". There are many versions of that idea; one of them is the great pre-2012 synthesis that includes low-energy supersymmetry, a WIMP from the MSSM as dark matter, grand unification, and string theory. None of the first three have shown up. Part of the appeal of the fourth item, string theory, was that the first three seem to fit naturally into it.

However, as we all know, string theory can also produce many other kinds of outcome. In fact, it is quite remarkable that a single "theory" can produce a landscape of outcomes, which on the one hand is as broad as string theory, but which on the other hand has so much extra content compared to field theory. e.g. quantities which in field theory are just free parameters, in string theory can only take the values determined by some geometric ground state (one of the vacua).

There are many reasons to think that string theory is the right approach to a final theory. That is not in itself a reason to disdain alternative approaches, and there's also the fact that the possibilities of string theory itself are far from completely known. We may have to go all the way with Urs Schreiber and fully understand M-theory as mathematics, before we really know all the theory's forms.

So even for a string theory fan, it is worth being flexible and e.g. giving Deur's quantum gravity a chance (after all, if Deur can get those effects from quantum general relativity, they should also exist in those limits of string theory that are approximated by quantum general relativity), Rivero's supersymmetry already hidden in the standard model, and many other ideas.