String Theory – Dead Again?
The other day I came across an old clipping from the December 2005 issue of Physics World. It’s from an article called What will they think in 2105? looking forward from 2005 at likely developments in the next 100 years of physics, given the context of the centenary of Einstein’s “year of miracles” (1905) in which he came up with, among other things, Special Relativity which I start teaching today.
The article asks what present-day discoveries would be remembered in a hundred years. Many of those asked the question said string theory. My response was somewhat less enthusiastic:
I got quite a lot of stick at the time from senior physicists for this statement! My use of the phrase “dead again” was based on the observation that the popularity of string theory has waxed and waned several times over the years. It may not have died in 2015 as I predicted, but it does seem to me to be in a moribund state, in terms of its impact (or lack thereof) on physics.
I’m mindful of the fact that many mathematicians think string theory is great. I’ve had it pointed out to me that it has a really big influence on for example geometry, especially non-commutative geometry, and even some number theory research in the past few decades. It has even inspired work that has led to Fields medals. That’s all very well and good, but it’s not physics. It’s mathematics.
Of course physicists have long relied on mathematics for the formulation of theoretical ideas. Riemannian geometry was `just’ mathematics before its ideas began to be used in the formulation of the general theory of relativity, a theory that has since been subjected to numerous experimental tests. It may be the case that string theory will at some point provide us with predictions that enable it to be tested in the way that general relativity did. But it hasn’t done that yet and until it does it is not a scientifically valid physical theory.
I remember a quote from Alfred North Whitehead that I put in my PhD DPhil thesis many years ago. I wasn’t thinking of string theory at the time, but it seems relevant:
There is no more common error that to assume that, because prolonged and accurate mathematical calculations have been made, the application of the result to some fact of nature is absolutely certain.
My problem is not with string theory itself but with the fact that so many string theorists have become so attached to it that it has become a universe in its own right, with very little to do with the natural universe which is – or at least used to be – the subject of theoretical physics. I find it quite alarming, actually, that in the world outside academia you will find many people who think theoretical physics and string theory are more-or-less synonymous.
The most disturbing manifestation of this tendency is the lack of interest shown by some exponents of string theory in the issue of whether or not it is testable. By this I don’t mean whether we have the technology at the moment to test it (which we clearly don’t). After all, many predictions of the standard model of particle physics had to wait decades before accelerators got big enough to reach the required energies. The question is whether string theory can be testable in principle, and surely this is something any physicist worthy of the name should consider to be of fundamental importance?
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In the Dark 
December 5, 2022 at 3:34 pm
String theory was only ever proposed because nobody could find a renormalisable quantum field theory of gravity, or one that unified gravity with other fundamental interactions. So let us ask how much definite advance string theory has given us on that front before proceeding any further.
I am wholly dissatisfied with the present situation in which I am supposed not to ask the question “Will the next electron [locked in a silver atom orbital] emerge from the spin-up or the spin-down exit of a Stern-Gerlach apparatus oriented along Ox, given that all atoms entering my apparatus have flown in vacuo from the spin-up exit of a Stern-Gerlach apparatus oriented along Oz?” The only rationale I have ever been offered for not asking this question is that macroscopic objects as Stern-Gerlach apparati do not have a description in quantum field theory, which is regarded as the fundamental ontology. But I can use a further apparatus of some sort to prepare a physical system which has distinct yet well-defined descriptions in quantum field theory, according to which exit of my x-Stern-Gerlach apparatus the next electron flies out of. If I wrap this apparatus and my x-Stern-Gerlach apparatus which is immediately upstream of it in a black box then both the input and outputs of this box have well defined quantum field descriptions, yet quantum field theory cannot describe what goes in in the black box. That is – or should be – unacceptable. Even less do I accept arguments that one should not ask the question. If you are not prepared to ask questions about nature then you have no right to call yourself a physicist.
So yes, bring on the hidden variables. Even if we know (from Bell) that a hidden-variable theory has to be acausal, we have a *reason* why we cannot predict the future deterministically, and we might be able to get the basic equations of such a theory even if we cannot test them in situations in which acausality is involved. That would still be a huge advance. And advances have a way of leading to further advances – in contrast to the self-censorship involved in not asking certain questions. Perhaps it would be fruitful to ponder the restraints on a hidden variable theory due to the fact that so much of physics *is* local.
December 11, 2022 at 3:22 pm
Tbh, I’m getting a little annoyed at this seemingly endless backlash against string theory and the people who work in the field. Though I’m not currently in academia, I did my PhD on string/SUSY based model building and phenomenology [insert overused joke here], so obviously I have some skin in the game.
I totally understand the anger/frustration at the media attention/hype given to string theory, and I’m more than a little embarrassed by grandiose claims – such as the recent “wormhole creation” brouhaha. I’ve never worked on AdS/CFT related stuff, but my PhD institution had a large group who did and while I found some of that interesting, it did feel like every high-energy seminar was on entanglement entropy, “dictionaries” and deformations.
I started in the field a few years after the so-called “string wars”, and I got the sense that the string community were largely humbled by the experience, though of course external observers may disagree. The failure to detect an LSP or other strong hints of SUSY during the early LHC runs also had a dampening effect. The people I’ve worked with have constructed many string models in which the Standard Model, dark matter candidates, and possible inflaton candidates can be found, but few of them think these constructions will be the “Theory of Everything” that will be the final say in the matter. Though everyone recognizes the “landscape” of string vacua, most people I knew were not at all comfortable with the idea of a physically realized multiverse.
I do agree with the sentiment that a large part of the string communities activities is more mathematics than physics – classifying and understanding various complex manifolds, calculating the dimensions of cohomology groups of bundles on such manifolds, other questions in algebraic geometry.
But I would contend that whether or not this work counts as physics is perhaps now not the relevant question. [I can hear the screams of those who’ve half-skimmed Popper already.] For me, the relevant question has to be: is this useful in advancing our understanding? I think the answer is broadly yes.
To give an analogy from an adjacent sub-field, an open area of research in particle theory/HEP is confinement and the understanding from first principles of the mass gap in Yang-Mills/QCD. A different theory, the CP^N model – which doesn’t describe quark-gluon interactions – displays these properties and allows us to better appreciate the distinctive issues that arise in QCD.
This is the role that I feel string theory now plays in physics – a “theoretical laboratory”, where we can poke and prod many string-descended models, to understand what possible patterns of Yukawa couplings, GUT gauge groups, discrete symmetries, that are possible/allowed in a theory containing gravity that is UV safe. The work on the so-called “Swampland” over the last decade exemplifies this, teasing out what general features a sensible low energy theory with a UV completion should contain.
(You may raise the question: yes, we can learn all these things about models that arise from string theory, but is this the only UV completion of gravity we could have? That would be an excellent point, which we could discuss at length …)
As for the media hype, well, I don’t have experience dealing with science journalists but however well-meaning they are, the general media environment promotes more sensational stories over complicated technical ones. Even Quanta, which has set out to be more careful and to pay more attention to incremental advances in fundamental research, seems to have fallen prey to this. If I could, I would urge my former colleagues to be more reticent in their statements and flesh out all the assumptions they make, but at the same time there’s only so much you can do.
To some extent, [putting on my tinfoil hat] I feel that there’s a deliberate over-hyping of news from the string theory community in order to drive up engagement by the now sizable anti-string, anti-SUSY groupings online. A lot of the figureheads of this loose movement are people who feel slightly aggrieved by the past dominance of string theory/SUSY and have become cast as renegade truth-tellers, attracting the usual “intellectual dark web” fanboys who pop up in YouTube comments and Twitter replies. (I of course note that one notable exception to this is/was a person who works(ed?) in string theory and had a blog which was … very distasteful, to say the least. If that was your only window into the field, I’m very sorry.) It’s given people a righteous cause to rally around and could end up hurting the wider HEP field.
A question I would put to you is: is there anything in HEP/particle physics that comes close to the media attention that string theory gets? Maybe this is down to the hype generated by Green et al. of 20 years ago, maybe this is down to the vast sprawl of topics that string theory now covers. Maybe it’s just that the subfield hasn’t really had any big breakthroughs in the past 20-30 years. (Or maybe all the funding gets sucked up by strings … which I don’t believe.) For whatever reason, other research (asymptotically safe gravity, spin-foams; Higgs portal models, dark sectors; CP violation, neutrino masses) just haven’t caught on in the public imagination, and it feels like there’s a group of people who want to bash particle physics because of it.