Eva Silverstein has a new preprint out, entitled The Dangerous Irrelevance of String Theory. The title is I guess intended to be playful, not referring to its accurate description of the current state of string theory, but to the possibility of irrelevant operators having observable effects.
The article is intended to appear in the forthcoming Cambridge University Press volume of contributions to the Munich “Why Trust a Theory?” conference held back in December 2015. The impetus behind that conference was a December 2014 article in Nature entitled Scientific method: Defend the integrity of physics. In that article, Ellis and Silk explained the problems with string theory and with the multiverse/string theory landscape.
The organizing committee for the Munich conference was chaired by Richard Dawid, a string theorist turned philosopher who has written a 2013 book, String Theory and the Scientific Method. For a fuller discussion of that book, see the linked blog post. To oversimplify, it makes the case that the proper way to react to string theory unification’s failure according to the conventional understanding of the scientific method is to change our understanding of the scientific method. Much of the Munich conference was devoted to discussing that as an issue in philosophy of science.
One aspect of the Munich conference was that it was heavily weighted towards string theorists, with contributions from Dawid, David Gross, Joe Polchinski, Fernando Quevedo, Dieter Lust and Gordon Kane all promoting the idea that string theory was a success. Polchinski explained a computation that shows that string theory is 98.5% likely to be correct, going on to claim that the probability is actually higher: “something over 3 sigma” (i.e. over 99.7%). The only contribution from a physicist that I’ve seen that argued the case for the failure of string theory was that from Carlo Rovelli, see here. Silverstein’s article says that it was commissioned by Dawid for the proceedings volume, even though she hadn’t been at the meeting. I’m curious whether Dawid commissioned any contributions from string theory critics who weren’t at the meeting.
Silverstein begins her article explaining how physics at a very high energy scale can in principle have observable effects. This of course is true, but the problem with string theory is that, in its landscape version, it has a hugely complicated and poorly understood high energy scale behavior, seemingly capable of producing a very wide range of possible observable effects, none of which have been seen. The article is structured as a defense of string theory, without explaining at all what the serious criticisms of string theory actually are. The list of references includes 53 items, only one critical of string theory, the Ellis/Silk Nature article. Some of the arguments she makes are:
It is sometimes said that theory has strayed too far from experiment/observation. Historically, there are classic cases with long time delays between theory and experiment – Maxwell’s and Einstein’s waves being prime examples, at 25 and 100 years respectively… One thing that is certainly irrelevant to these questions is the human lifespan. Arguments of the sort ‘after X number of years, string theory failed to produce Y result’ are vacuous.
I think the comparison to EM or GR is pretty much absurd. For one thing it’s comparing two completely different things: tests of a particular prediction of a theory (EM or GR) that made lots of other testable, confirmed predictions to the case of string theory, where there are no predictions at all. More relevant to the argument over how long to wait for an idea to pay off is that the real question is not the absolute value of the amount of progress, but the derivative: as you study the idea more carefully, do you get closer to testable progress or farther away? I don’t think anybody can serious claim that, 33 years on, we’re closer to a successful string theory unification proposal than we were at the start, back in 1985. I’d argue that the situation is the complete opposite: we have been steadily moving away from such success (and thus entered the realm of failure).
- About supersymmetry Silverstein writes:
In my view, the role of supersymmetry is chronically over-emphasized in the field, and hence understandably also in the article by Ellis and Silk. The possibility of supersymmetry in nature is very interesting since it could stabilize the electroweak hierarchy, and extended supersymmetry enables controlled extrapolation to strong coupling in appropriate circumstances. Neither of these facts implies that low-energy supersymmetry is phenomenologically favored in string theory.
It is true that Silverstein has never been one of those arguing that the usual string theory scenarios with supersymmetry and 10 or 11 dimensions show that string theory is testable. See for instance her comment here back during a “String Wars” discussion in 2006. Her current take on whether string theory implies supersymmetry is just
Much further research, both conceptual and technical, is required to obtain an accurate assessment of the dominant contributions to the string landscape.
The problem with this is that there is no sign of any possibility of progress towards deciding if the string theory landscape implies low-energy SUSY or not (quite the opposite). If you give up the assumptions of SUSY and 10/11 dimensions, you give up what little hope you had of any connection with experiment. She doesn’t mention the LHC at all, especially not the negative results about supersymmetry and extra dimensions that it has produced. The significance of these negative results is not that they disconfirm a strong prediction of string theory, but that they pull the plug on the last remaining hope for connecting standard string theory unification scenarios to anything observable. Pre-LHC string theorists could make an argument that there was good reason to believe in electroweak-scale SUSY, that such a scenario fit in well with string theory unification, and that LHC discovery of SUSY would point a way forward for string theory unification. That argument is now dead. All that’s left is basically the argument that “maybe a miracle will happen and we’ll be vindicated” which in her version is:
In principle one could test string theory locally. In practice, this would require discovering a smoking gun signature (such as a low string scale at colliders, or perhaps a very distinctive pattern of primordial perturbations in cosmology), and nothing particularly favors such scenarios currently.
- Silverstein’s main argument is basically that string theory is valuable because it leads to the study of models that have various observable signatures that people would not otherwise look for. One example here is supersymmetry, the study of which has had a huge effect on collider physics, strongly shaping the analyses that the experimentalists perform. She gives some detailed other examples from her field of cosmology, in particular about possibly observable non-Gaussianities.
String theory participates in empirical science in several ways. In the context of early universe cosmology, on which we have focused in this article, it helped motivate the discovery and development of mechanisms for dark energy and inflation consistent with the mathematical structure of string theory and various thought-experimental constraints. Some of these basic mechanisms had not been considered at all outside of string theory, and some not quite in the form they take there, with implications for effective field theory and data analysis that go well beyond their specifics.
I think this is the best argument to be made for “phenomenological” string theory research (as opposed to “formal” string theory, where there are other arguments). Yes, coming up with new models with unexpected observable effects is a valuable enterprise. If your speculative idea generates such things, that’s well and good. The problem though is how to evaluate the situation of a speculative idea that has generated a huge number of such models, none of which has worked out. At what point do you decide that this is an unpromising line of research, better to try just about anything else? Silverstein makes the argument that
Whether empirical or mathematical, constraints on interesting regions of theory space is valuable science. In this note we focus on string theory’s role in the former.
Since information theory is currently all the rage, it occurred to me that we can phrase this in that language. Information is maximized when the probabilities are equal for a set of outcomes, since one learns the most from a measurement in that case. The existence of multiple consistent theoretical possibilities implies greater information content in the measurements. Therefore, theoretical research establishing this (or constraining the possibilities) is directly relevant to the question of what and how much is learned from data. In certain areas, string theory plays a direct role in this process.
The problem here is that of what is an “interesting region of theory space”. At this point the failures of string theory unification strongly indicate that it’s not such an interesting region. It seems likely that we’d be better off if most theorists focusing on phenomenology of this failed program were to pick something else to work on.
Update: Will Kinney has a Twitter commentary here.
Update: For another relevant recent Will Kinney Twitter storm, see here and here.
Update: Silverstein gave some lectures to the public about this at Stanford recently, video here and here, slides here. A large part of the lectures were an advertisement for string theory, with the summary at the end
Quantum gravity (string theory) plays a subtle but important role, even contributing to our understanding of empirical measurements of early universe dynamics.
Crediting string theory with “contributing to our understanding of empirical measurements of early universe dynamics” is a peculiar way of saying that string theory produces lots of cosmological models that don’t work (see a better summary by Will Kinney at the end of this presentation).
Update: Renata Kallosh is Silverstein’s colleague at Stanford (and Andrei Linde’s wife). In an interview here she makes much simpler and stronger claims about string cosmology than does Silverstein:
string theory ideas help us to build cosmological models which fit the data from observations. Moreover, we have produced relatively simple predictions from string theory and related theories which will be testable with future detectors of primordial gravity waves.
I’m not sure what specifically she is referring to, but suspect that “prediction” here means something like the “predictions” of string cosmology that Kinney describes (see above) whose failure to be observed has in no way affected string cosmologists enthusiasm for the subject.
Update: For some more context about string theory inflation models and the issue of their testability, you could consult Silverstein’s guest post at Lubos Motl’s blog from 2014, explaining how the BICEP2 observation of that era could provide evidence for “axion monodromy inflation”.