Each summer for nearly a quarter-century there has been a big yearly conference bringing together the string theory community. I’ve often written about these conferences on the blog, see here. This year’s version will be held next week in Vienna, for more information see here.

Taking a look at the program, one thing that stands out is that the string theory community has almost completely stopped doing string theory. Looking at the program, only two out of 44 talks seem to be significantly about string theory. One of three parallel discussion sessions is entitled “Strings and the Real World” and will be chaired by Cumrun Vafa. I’m guessing this will mostly be about the swampland, not string theory.

A tradition at these conferences is one or more public talks designed to publicize string theory. This year’s versions will be given by Netta Engelhardt and Andy Strominger. They have nothing to do with string theory, but they do make very clear what the string theory community has found to replace string theory: black holes. Engelhardt’s title is “The Black Hole Information Paradox: A resolution on the horizon?” and Strominger’s is “Black Holes: the Most Paradoxical Objects in the Universe”.

Looking at the talk titles, the most common words in the titles are “holography” and “black holes”, with the center of gravity of the subject now for a couple decades the effort to use holography to say something about black holes. Maldacena’s title is “What happens when you look at supersymmetric black holes for a long time?” which seems also an interesting question about the field itself.

**Update:** Paolo Bertozzini points out to me that the LQG community has scheduled its big yearly conference LOOPS2022 at exactly the same time as the string theory community one (this week). It’s quite interesting to compare and contrast the two sets of talks. There are some very broad similarities between what both communities are doing, with overlaps in interest around black holes, entanglement, holography (in the form of large symmetry groups at infinity). Another commonality is that both communities are focused on the gravitational field, with nothing to say about particle physics and matter in general. This has been true of LQG since the beginning. In the case of string theory the big selling point originally was that it gave a theory of matter, but the string community has for a long time given up on that. There is a difference in how the communities think about “what are the fundamental degrees of freedom for gravity?” On the string theory side they’ve given up on that, the answer now is that gauge-gravity duality and emergence are supposed to allow you not to care about fundamental degrees of freedom. On the LQG side, people are still hard at work on specific sorts of degrees of freedom and how to quantize them.

Makes sense, black holes are one of the few objects where quantum gravity considerations actually do occur.

When you give a speech on paranormal activities, and you want the audience to believe, you will try to bring them into the most unknown and unprovable ground. In physic, it is…a black hole!!! Anything can happen there….because we have no cleancut theory to have coherent description of the phenomenon.

Would you characterize this as a tacit acknowledgment the LHC has given all it can to “new physics”? With gravitational astronomy still in its infancy, seems like the best (if not only) strategy for plausibly (using that word generously) claiming contact with experiment may be just around the corner.

Andrei Sipos,

Part of what is going on is that long ago most string theorists gave up on saying anything about particle physics and adopted the attitude that “string theory” was now just about quantum gravity. So, then Strings 2XXX started to be Quantum Gravity 2XXX. But this was always not quantum gravity in general, but “quantum gravity approaches that evolved out of string theory, especially holography related”. No loop quantum gravity, no R^2 quantum gravity, etc., etc. The concentration on black holes is a further evolution from this.

LMMI,

For many decades now string theorists and much of the particle physics community have taken the attitude that the limits on energy range of accelerators mean that the thing to do is to move to thinking about astrophysical observations somehow relevant to very high energies, especially very early universe cosmology. “String cosmology” has been featured at Strings 2XXX for a very long time (and never made any sense at all to me, but that’s a different topic, and people seem to have lost interest in it).

Most of the current theoretical activity (solutions to information paradox) around black holes has no plausible relation to astrophysical observations. Maybe there are claims to make contact with things like EHT, but I don’t see how EHT pictures are going to tell you something about the quantized gravitational field.

Okay, I have a question: isn’t this what you wanted, for people to give up working on string theory?

Justin,

Sure, except that first of all I think they should admit they have given up on string theory (for one thing, they should change the name of the conference). Continuing to promote string theory as a unified theory to the public when you have given up working on this since it is going nowhere is pretty problematic.

Secondly, while I’m in favor of giving up on string theory, I’m not in favor of giving up completely, which is more or less where the string theorists have ended up.

Peter, one of my colleagues suggests this analogy in favor of String Theory (ST): that just as in GR there was a quiet 30-year period (from 1925-1955 with the exception of Oppenheimer and Snyder (1939)), during which the general framework of GR was acclaimed but not as an active field of research–and without further experimental data–so too may we see a 30-year quiescent period for ST (save for its use in studies of Black Holes and as a source of mathematic techniques in condensed-matter) followed by an explosion of renewed activity. This analogy presupposes that ST is the right long-term framework, as GR has proven to be today, i.e., in our ongoing “golden age” of quantitative astrophysics and cosmology. I guess long-term confidence in ST, despite the limited progress in the past 20 years, is a matter of faith for some and not for others.

Michael Weiss,

The problem with that analogy is that comparing GR to string theory is just delusional. Pre-1925, GR was a huge success: a completely well-defined, simple (when expressed in terms of Riemannian geometry) theory that made a huge number of predictions, including several dramatic ones about small corrections to Newtonian gravity that were quickly confirmed. This is something completely different than “string theory”, where you have no predictions and no well-defined theory, just a hope that someday you might find one.

I was thinking more gravitational wave (laser interferometry) observatories than VLB interferometers. I recall there being some hype around stringy signatures in the spectra of colliding black holes when the first observations were coming in (circa 2016, I think). Haven’t heard much about that lately. Maybe someone has since decided we’ll need LISA to see extra dimensions or something along those lines.

LMMI,

I should have mentioned LIGO/LISA as well as EHT. But the story is always the same: “X will see effects of string theory” claims are all completely bogus, for every value of X.

Not sure which LIGO hype you are thinking of, I mentioned one such here,

https://www.math.columbia.edu/~woit/wordpress/?p=9409

may have not mentioned some others, like this

https://physics.aps.org/articles/v14/s110

The “just wait for LISA” thing has been going on since before my students were born, often as part of string theory hype. For one example from 2006, see here

https://www.math.columbia.edu/~woit/wordpress/?p=418

Note that back then the story was that LISA would launch within ten years…

You write:

Secondly, while I’m in favor of giving up on string theory, I’m not in favor of giving up completely, which is more or less where the string theorists have ended up.

Perhaps you should explain this a little.

James Smith,

The initial selling point of string theory was that it was supposed to unify particle physics and gravity, explaining the standard model. String theorists have given up on anything to do with particle physics, with one justification that the string theory landscape is so vast that you can’t predict anything with it, so there is no way to say anything new about particle physics, you just have to give up.

For quantum gravity, the fundamental problem has always been the non-renormalizability of our standard quantum theory of space-time degrees of freedom. Recently what the string theory community has done has been to restrict study to “emergent space-time”, with no theory of what degrees of freedom space-time emerges from. In that sense they’ve also given up on the main problem of quantum gravity.

I loved your last paragraph. No more anthropic, but antropological effects of string theory.

@Peter,

I’m confused why it’s a good idea to directly quantize GR? I mean I would never quantize pressure and energy density which it what the theory spits out for the FLRW metric. I’m curious what your thoughts are?

Okay, I kind of see. There is much that I don’t understand in your second paragraph but the gist of what you are saying is that you have not given up on the fundamental problem of quantum gravity as you call it, but the string theorists have.

One further question if you don’t mind. Is it that the standard quantum theory of space-time predicts the standard model, so to speak, but does not encompass quantum gravity, and solving this fundamental problem would extend that theory or at least produce another that would encompass quantum gravity? Solving this fundamental problem is what you have been working on, right? If so, could the quantum gravity part of such a theory be verified by experiment?

More anonymous,

The part of our fundamental theory now that explains gravitational forces is a classical dynamical theory of space-time (GR). One way to describe the fundamental degrees of freedom of this theory is as the metric field, but there are many other formulations (e.g. using vierbeins and the spin connection). Applying standard quantization methods to this theory leads to the non-renormalizability problem.

If you want to argue that these degrees of freedom are not fundamental, that they are just effective low energy degrees of freedom coming from some more fundamental theory, that’s fine (this is what string theorists were originally doing, the metric was just a low energy mode of the more fundamental string theory). But you need to explain what your more fundamental theory is. The current fashion is to announce that “space-time is doomed”, that the metric is just an effective field, but not to give a new proposal for a consistent fundamental theory which exhibits this behavior. It’s this move that I would call “giving up”.

James Smith,

The “standard quantum theory of space-time” doesn’t say anything about the standard model, where the fundamental degrees of freedom are “internal”, not space-time degrees of freedom. It also has the renormalization problem.

A conventional approach to unification is to extend one’s notion of space-time by adding more dimensions, then trying to interpret these new degrees of freedom as the internal degrees of freedom of the standard model. I’m trying to do something a bit different, exploiting different aspects of geometry that appear when one works in Euclidean space-time and with twistors. I’m still very enthusiastic about these ideas (now writing up a new version of part of this), but there’s a lot to be done to get these ideas to work, and still to be seen what new implications one might get from a fundamental theory formulated this way.

Thank you for the clarification.

“I’m guessing this will mostly be about the swampland, not string theory.” The Swampland papers (apart from the heuristic pheno papers) are very stringy. By construction of what the Swampland means. I invite you to look at the average paper on the topic and you will strings everywhere

thomas van riet,

If “swampland” now divides between “formal swampland” and “phenomenological swampland”, I was referring to the latter, since the session is about “String theory and the real world”. The attempts I’ve seen to relate the “swampland” to the real world have little relation to string theory, basically are very speculative conjectures about “any quantum theory of gravity”.

It seems that words as “unified field theory” or “super-symmetry” have no meaning, or any meaning that suits you, in pop culture now, see this article in The Guardian:

http://www.theguardian.com/film/2022/jul/18/empire-of-the-drones-terrence-howard-takes-us-where-no-actor-has-gone-before

Peter, in the Loops2022 conference the have invited Neil Turok (who although is not a core string theorist) has worked on cosmic strings and branes.

Also does any one have any data on amount on NSF/DOE funds on LQG vs string theory (in US)?

Shantanu,

I don’t think this kind of question has made any sense for a long time now. “String theory” long ago stopped referring to a specific theoretical framework. Most “string theorists” stopped working on string theory and “string theorist” now is just a name for a tribal affiliation, with a wide spectrum of intensities of that affiliation. Turok is definitely not now significantly affiliated with the tribe, especially given the kind of things he has been saying about string theory since 2016, such as

“[String theory has] almost self-destructed, I would say because it turned out to be not just one theory but this vast collection of theories which could all give different descriptions of the world.”

See https://www.math.columbia.edu/~woit/wordpress/?p=8361

At this point it would be surprising to see him at Strings 2022, not at LOOPS22.

My general impression is that there’s relatively little funding of LQG research in the US right now, much more in Europe. One reason for that is possibly that the string theory tribe in the US has always been quite hostile to LQG, seeing it as a heresy.

Hi Peter,

“Another commonality is that both communities are focused on the gravitational field, with nothing to say about particle physics and matter in general.”

The Loops22 conference has had 4 talks (out of a total of 100 or so) that were about matter, in one way or another. So formally there is nonzero interest about matter in the LQG community. That said, I completely agree that mere 4% of the talks represent nowhere near enough interest that IMHO the community should have about the topic. So I agree it is a bit sad state of affairs.

Best, 🙂

Marko

I don’t see why the concentration only on the gravitational aspect from the part of the LQG community is something supposedly bad.

The idea that in order to do QG you need to include matter and actually have a unified ToE is old and coming mainly from the particle physics thinking of the 70s and one of the main motivations that gave us string theory (from that same community).

From the beginning, the LQG community has explicitly rejected such hypothesis and considers it as not necessary for a QG theory. That’s their hypothesis. I don’t know if it’s right or wrong, but if we start to argue over my hypothesis vs yours, we go back all the way to the string wars again.

I think LQG has more important problems to deal with (in particular, empiral testing issues etc.) than its publicly acknowledged hypotheses!

If the infamous string wars showed us something, it’s that nobody is going to change their mind by such discussions. The only reason why string theory is now falling into some disgrace is because many string theorists themselves have given up due to the issues dealing with reproducing the SM and the non-discovery of susy in the LHC. So, let the LQG community work with what they have and believe and see where that goes. Perhaps they will also give up, perhaps they will realize that the formalism itself is asking for non-gravitational fields to be included, etc.

Personally, I see no chance of connection with the empiricial realm for any QG theory candidate at least in the coming 100 years, so I predict a long QG winter after the final fall of string theory in the near future (yes, I know that they are all doing “QG” now, but how far goes their influence beyond Princeton? They are beyond recovery now, they lost the most important thing for such speculative endeavors: credibility. It’s a bit like the stock market, and their stocks are certainly going down after a big bubble).

Alex,

“I don’t see why the concentration only on the gravitational aspect from the part of the LQG community is something supposedly bad.”

There’s nothing intrinsically wrong with doing QG without matter, but the issue is that there is only so much you can do with pure gravity. It is similar to doing vacuum GR — there are some interesting solutions and nontrivial geometry, but if you want to study the really cool phenomena and model realistic physics, you need matter in your theory. My overall impression from listening to all the talks at Loops-22 (especially the panel discussion at the end) was that we have sort-of constructed a well-defined theory of QG, and now we don’t know what to do with it.

The most interesting set of problems in QG require coupling of matter. For example — what is the equation of state for matter fields at the center of the black hole? What is the corresponding curvature? What is the state of the gravitational field sourced by matter in quantum superposition? What are the effective EoMs for matter in a gravitational field in quantum superposition? How does matter emit and absorb individual gravitons? How does one properly derive QFT in curved spacetime as an approximation to a full theory of QG? Can we resolve the CC problem? Is there a limit to the validity of the equivalence principle? Etc… None of these questions can be answered without coupling matter to QG.

And if you try to couple it, you immediately face the question what kind of matter, leading you to the SM, dark matter, their various extensions and beyond to ToE. That said, I agree that the question of matter spectrum (i.e. what are fundamental matter fields and their coupling constants) may or may not be related to the ability to couple matter to gravity. So far we do not know of any restrictions on matter spectrum coming from gravity. But we cannot tell either way if nobody does the research.

Best, 🙂

Marko

Marko,

Yes, I definitely agree that those are important problems and the matter issue will have to be addressed at some point for that.

What I was thinking is that vacuum QG is conceptually more interesting and complicated than vacuum GR, since you now have to deal with things like: are distances, areas, volumes, proper times, discrete and probabilistic? What happens to the continuum manifold? What’s even a quantum worldline? It’s like going to GR 101 and start all over again with the presentation of the very basic stuff, like manifolds and curves, but now with their quantum versions.

Then it would be interesting to have phenomenological models that take those predictions and some effective model of the rest of matter, and see if we can at least test that. Of course, the full coupling between QG and quantum matter would be even better, but I think it’s just too much to ask for the current state of development.

In the case of LQG, I don’t think it has even solved in a completely satisfactory way any of those problems I mentioned. So, I maintain my prediction of a QG winter period starting in the near future (like the one that happened to AI in the 70s and 80s). I actually think it will be good for the field. Many people that still are prominent need to retire, since their current “research” is utter pseudo science (in both camps, strings and loops). Furthermore, technology has to evolve. Who knows, maybe in a couple of decades or even in 100 years, a fresh new generation will be able to make the breakthrough.

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String phenomenology has its own conference, String Pheno 2022. There are plenty of would-be applications of string theory to particle physics there.

Mitchell Porter,

The point of my post was not that Strings 20XX talks no longer have any connection to the real world (that happened long ago), but that now they no longer have any connection to strings either. I haven’t written in a while about “String Pheno 20XX” conferences, since the subject has become so bizarre that it’s hard to figure out what to say. At least in the US, almost all leading “string theorists” except Vafa have now long ago abandoned any interest in “string phenomenology”, realizing that it’s a hopeless failed program. Why there are so many people and conferences devoted to a subject that almost everyone regards as a complete failure is something I’ll never understand.