A commenter in the previous posting pointed to an interview with Lenny Susskind that just appeared at the CERN Courier, under the title Lost in the Landscape. Some things I found noteworthy:
- He deals with the lack of any current definition of what string theory means by distinguishing between “String theory” and “string theory”. “String theory” is the superstring in 10 dimensions somehow compactified to have some large dimensions that are either flat or AdS. This can’t be the real world
I can tell you with 100% confidence that we don’t live in that world.
since the real world is non-supersymmetric and dS, not supersymmetric and AdS. He describes this theory as being “a very precise mathematical structure”, which one might argue with.
Something very different is “string theory”:
you might call it string-inspired theory, or think of it as expanding the boundaries of this very precise theory in ways that we don’t know how to at present. We don’t know with any precision how to expand the boundaries into non-supersymmetric string theory or de Sitter space, for example, so we make guesses. The string landscape is one such guess…
The first primary fact is that the world is not exactly supersymmetric and string theory with a capital S is. So where are we? Who knows! But it’s exciting to be in a situation where there is confusion.
- About anthropics and the landscape, he still thinks this is the best idea out there, but acknowledges it has gone nowhere in twenty years:
Witten, who had negative thoughts about the anthropic idea, eventually gave up and accepted that it seems to be the best possibility. And I think that’s probably true for a lot of other people. But it can’t have the ultimate influence that a real theory with quantitative predictions can have. At present it’s a set of ideas that fit together and are somewhat compelling, but unfortunately nobody really knows how to use this in a technical way to be able to precisely confirm it. That hasn’t changed in 20 years. In the meantime, theoretical physicists have gone off in the important direction of quantum gravity and holography.
- About the swampland, like everyone else I know, he can’t figure out what the argument is that is going to relate it to the real world:
The argument seems to be: let’s put a constraint on parameters in cosmology so that we can put de Sitter space in the swampland. But the world looks very much like de Sitter space, so I don’t understand the argument and I suspect people are wrong here.
His comments on Technicolor strike me as odd:
I had one big negative surprise, as did much of the community. This was a while ago when the idea of “technicolour” – a dynamical way to break electroweak symmetry via new gauge interactions – turned out to be wrong. Everybody I knew was absolutely convinced that technicolour was right, and it wasn’t. I was surprised and shocked.
I remember first hearing about the Technicolor idea around 1979 when Susskind and Weinberg wrote about it. It was a very attractive idea by itself, but the problem was that to match known flavor physics you needed to go to “Extended Technicolor”, which was really ugly (lots of new degrees of freedom, no predictivity). No idea when people supposedly were “absolutely convinced that technicolour was right”, maybe it was for the few months it took them to realize you needed Extended Technicolor.
- About the wormholes, he says:
One extremely interesting idea is “quantum gravity in the lab” – the idea that it is possible to construct systems, for example a large sphere of material engineered to support surface excitations that look like conformal field theory, and then to see if that system describes a bulk world with gravity. There are already signs that this is true. For example, the recent claim, involving Google, that two entangled quantum computers have been used to send information through the analogue of a wormhole shows how the methods of gravity can influence the way quantum communication is viewed. It’s a sign that quantum mechanics and gravity are not so different.
Unclear to me how this enthusiastic reference to the wormholes relates to his much less enthusiastic recent quote in New Scientist:
What is not so clear is whether the experiment is any better than garden-variety quantum teleportation and does it really capture the features of macroscopic general relativity that the authors might like to claim… only in the most fuzzy of ways (at best).
Susskind published today a paper on the Arxiv about de Sitter and holography. Take a look: 2303.00792
Thanks. This seems to be more along the lines of ideas that go back to
and further. The controversial Quanta article about the wormhole stunt ended with a section about this, I guess to show that the AdS problem was being overcome.
An interesting thing about these recent papers is that Susskind now lists his affiliations as not just Stanford, but also Google Research in Mountain View.
Peter, I’m curious about your “…like everyone else I know…” comment regarding the swampland conjectures. A couple of years ago when I was really trying to follow this stuff, it seemed to me that the swampland “program” was clearly targeted at KKLT – saying in effect, “you have a conjecture that DS may emerge from String Theory [note caps] but I have an equally reasonable conjecture that it cannot.” Is this not the right way to think about it?
Yes, that’s the right way to understand what they are doing (although, as Susskind notes, the landscape idea of dS is string theory, not String theory). But then, where I think I join Susskind and many others in being confused, is in trying to figure out what the implications of this are supposed to be if it is successful. If you can show string theory can’t give you dS, and you know we live in dS, the obvious implication is that string theory can’t describe the real world. But, the people doing this claim to be string theorists devoted to making string theory work. Their plan instead seems to be to argue that since string theory has to be true, the world cannot actually be dS, it’s just an illusion, with the real story quintessence or whatever. But there seems to be no good argument for this.
Susskind’s makes the following comment regarding the recent “wormhole” simulation:
“…two entangled quantum computers have been used to send information through the analogue of a wormhole…”. What are the “two entangled quantum computers” in reality?
Peter, your last comment is extremely illuminating. I’ve always been confused by the same thing: all evidence points to us existing in a dS universe, so if one could show that (S/s)tring theory is a sort of theoretical closure of the set of all Theories of Everything for AdS (and only AdS) universes, then that would constitute the closest thing you could get to an unassailable disproof of string theory. As magically energetic colliders will forever be out of our reach, this kind of no-go theorem is the only way I can imagine string theory being falsifiable in practice. I never could understand why they’d be so eager to detonate their own field of study by proving such a damning conjecture—a conjecture which all evidence points to being true, so far.
If their goal is talk themselves out of the universe being dS, then I truly don’t know what hope there is for the future. We’ve gone from “Oh, these useless 1+1 AdS toy models of QG are just stepping stones to more realistic models” to demanding that the universe actually be such a model. I’ll continue disregarding the entire subfield until the day they can describe the universe we actually live in.
Something that strikes me as a theme in all of this is the repeated use of towers of conjectures. The arxiv links of Susskind in the comments are replete with conjectures based on other conjectures. It is difficult to find the motivations for such conjectures as they are often not spelled out and seem to resort to general agreement among a group of mathematically talented superstring physicists like Witten, Maldecena et al.
This makes me wonder about the claim (which I’m entirely unqualified to understand) that there is this dichotomy between S vs s superstring theory. I would *assume* that the distinction is based on some mathematically proven line of definitions/theorems in the capital S, but Susskind’s fondness for Nth degree reliance on conjectures gives me pause.
Maybe time will be kind to these towers of conjectures with future generations of mathematicians/physicists following up with rigorous and formal methods, but it seems a shame that this generation is not as interested in going back and putting in the work of justifying the conjectures at the root of the tower.
I was also puzzled by the technicolor comment. It seemed like practically no one was talking about that when the LHC turned on. It was all sparticles lighting detectors up like Christmas trees. And now the world isn’t supersymmetric at all? Why? I thought the whole “lesson” of SUSY heaving nothing to do with EW symmetry breaking is that there’s a Landscape and SUSY could be broken at any scale you could imagine. Because there is no string (capitalized or not) without SUSY. I can’t keep it all straight anymore. What goalpost is even left to move at this point?
I’d be wary of any belief in illumination re the goals of the Swampland program. The only things clear to me about the motivations for this program are
1. they would like to challenge the idea that KKLT-type vacua are consistent, implying no predictivity.
2. they would like to claim that having a consistent theory of quantum gravity implies constraints on unified theories, string theory qualifies, so they have “string theory predictions”.
Beyond this though, I don’t understand what their vision is for a predictive theory, and Susskind seems to share my confusion about this.
The distinction between string and String theory is exactly Susskind’s attempt to address this. I think he’s acknowledging that “string theory” now just means some very speculative “string-inspired” scenario, yes, with conjecture built on conjecture. As he says, the way to characterize this is
“Who knows! But it’s exciting to be in a situation where there is confusion. ”
The problem for me is that I don’t see any way to have a sensible discussion about this sort of “Who knows!” string theory. Nothing wrong with people working this way if it leads somewhere, but all the evidence is that for decades this has gone nowhere.
He doesn’t say exactly what “String theory” is, but it is the sort of thing I used to spend a lot of time trying to understand and argue with people about exactly what is going on. His claims that this is completely well-defined mathematics that mathematicians build on and get Fields medals for I don’t think is true. But, in any case, he agrees this cannot give you a theory of the real world.
By the time the LHC had turned on, the problems with the technicolor idea were well-known and most people were skeptical that this could work. It remained possible that something was being missed, that the LHC would turn up not the standard Higgs, but something that was a variant of the technicolor idea, perhaps an unexpected variant (I would have loved that to have happened…).
Susskind clearly isn’t claiming that everyone thought technicolor was right until the LHC. He seems to say there was some point at which it “turned out to be wrong”. Not sure what that would be, except it happened long ago.
My understanding of capital S “String theory” as a mathematician is that there are rigorous definitions of objects that could be called “topological strings” of “type IIA and type IIB”. Relevant work here is due to Kontsevich-Soibelman, Costello, and Lurie. These are 2d oriented topological field theories valued in chain complexes, meaning that there are operations on some chain complex parametrised by chains on the moduli space of Riemann surfaces with disjoint homomorphically embedded discs, and some compatibilities with sewing these Riemann surfaces along embedded discs. But this seems to be a far cry from making rigorous sense of the finer structure of N=2 d=2 SCFT associated to Calabi-Yau threefolds, which seems to be the minimum of what a string theorist would mean about “String theory” of type II.
Yes, the things mathematicians can make sense of and that have had significant impact in pure mathematics are things like topological strings, not anything like the String theories Susskind is referring to which try to model the real world but require exact SUSY.
Not much point though in arguing about this with Susskind though, since he is conceding that the theories he is making these dubiously optimistic claims about can’t be a theory of the real world.
Peter, since you’ve mentioned SUSY in the comments, you might be interested to know that Ben Allanach of Cambridge University UK gave an interview two years ago, which can be found on Youtube: Supersymmetry and Particle Physics | Ben Allanach.
“up to last year I spent all my career working on the supersymmetry theory… it was a bridge to everything…so yeah I had to rethink last year and now I’m working in a different mode, I’m looking at the data, and looking for glitches, and so we predict with the Standard Model of particle physics”
The LHC has done a very successful hatchet job, and not just on Susskind’s Technicolor.
The “killing” of Technicolor was carried out by precision ElectroWeak measurements at SLD and the detectors at LEP1 . Peskin and Takeuchi created a set of variables (S, T) which parametrized the isospin conserving and isospin breaking part of BSM physics. If you put the SM at 0,0 in an S,T plot then Technicolor is off in the upper left somewhere. The various EW asymmetries (and also atomic parity violation measurements) shut the door on any large deviations in S,T space. By the time the LHC turned on Technicolor was long dead.
(Full disclosure: my PhD thesis was the measurement of the Left-Right Asymmetry of Z bosons made with polarized electrons).
Composite Higgs models and Walking Technicolor models are some natural evolution of the original technicolor. These were seriously probed at the LHC, where the mass value of the Higgs boson, the production rate of Higgs signals and the exclusion of top partners were the main tests. Perhaps Susskind meant these generalized technicolor models.
Trying to grasp the precise difference between “big s” and “small s” string theory. Is this distinction along the lines of what Susskind is referring to:
1. String theory: Perturbative superstring theory on a flat Minkowski background. This is a well-defined theory, at least up to physical rigor.
2. string theory: Some conjectured nonperturbative background-independent thing which reduces to String theory in the appropriate limit.
I think the much earlier SLD/LEP1 results Amitabh Lath mentions are more likely what Susskind is referring to. I don’t remember him (or anyone else) having much enthusiasm for Technicolor by the time the LHC turned on.
Susskind seems to be specifically pointing to the AdS/CFT picture of non-perturbative String theory as defined as the dual of N=4 SYM on the boundary, something that goes much beyond perturbative String theory. It’s debatable whether that’s as well-defined as Susskind claims, but since he agrees it can’t give you the real world, not worth debating.
The distinction he is making centers more on supersymmetry, where he’s making the (correct…) argument that the world is not supersymmetric, but there exists no well-defined non-supersymmetric string theory.
An older (2008) article from Lenny on Technicolor (and Supersymmetry):
Thoughts on a Long Voyage
Savas Dimopoulos wrote on TC back in 1994 (https://arxiv.org/abs/hep-ph/9412297) and again in 2000 (https://arxiv.org/abs/hep-th/0105034) – these two articles are *almost* identical.
Thanks. The Dimopoulos comment
“By the summer of 1980 it became clear that these theories suffered from generic problems of flavor violations  that could perhaps be cured only by complicating the theory immensely and losing any hope of calculability.”
agrees with my memory that very soon after Technicolor came on the scene, people lost interest once they started looking at the kind of Extended Technicolor models needed to agree with experiment.