No blogging here the past few weeks, partly because I was away on vacation for a little while, but more because there hasn’t been anything I’ve seen worth writing about. Yesterday’s pulsar timing array and IceCube announcements unfortunately didn’t tell us anything about fundamental physics. In the past, such observational results pretty reliably led to absurd claims about evidence for string theory that I could complain about, but that phenomenon seems to be dying down. In this case, the only story that had such claims was one from Quanta Magazine, which explained that “the observations so far from NANOGrav and the other teams are consistent with what we’d expect to see from cosmic strings.”
I noticed that the people at the Institute of Art and Ideas have put together a program for Monday that includes a debate on the topic of “Fantasy, Faith and Physics.” The framing of the debate contrasts the conventional view of science with an alternative possibility: “should we accept that some beliefs, especially in the foundations of physics, are akin to religious beliefs dressed in mathematical language to give our theories meaning?” This kind of misses the point about the current problems in fundamental physics, since I doubt any of the panelists are going to defend such an alternative.
Very odd is what leads into this debate, an interview with Michio Kaku about his new book. Why promote such an atrociously bad book (see here and here) and broadcast Kaku’s absurd claims about this subject?
Maybe this debate will somehow lead to a substantive discussion of the main underlying problem, the nearly fifty-year dominance of a failed set (GUTs/SUSY/strings) of ideas about unification. A very powerful and influential part of the physics community, which will be represented in the debate by Juan Maldacena, continues to insist on the centrality of this set of ideas. To get a clear look at his arguments, see a recent IAI interview In defence of string theory and his colleague Edward Witten’s recent colloquium talk What Every Physicist Should Know About String Theory. The argument Maldacena and Witten are making is essentially the same one from the mid-eighties: string theory is the only possible consistent way to go beyond quantum field theory and get a consistent theory of quantum gravity. In my book and many other places, I’ve explained the many problems with this. Put simply, the problem is that there is no such thing as a well-defined string theory which successfully gives the SM and GR in four dimension. The claims about consistency are either about models that don’t reproduce the real world, or about still-unrealized hopes and dreams (which Penrose characterizes as “Fantasy”) rather than anything well-defined.
For a very clear statement of his point of view from Witten, see the question and answer section of the recent colloquium talk, starting around 1:20, where he starts by emphasizing the rigidity of the framework of relativistic quantum field theory. He then states:
My point of view is that string theory is the only significant idea that has emerged for any modification of the standard framework that makes any sense.
This is pretty much exactly the same argument he was making nearly forty years ago. I didn’t find it convincing then, since it seemed to me there was no reason to be so sure that a deeper understanding of relativistic QFT could not possibly lead to a consistent quantum theory with low energy limit GR. Witten had a good argument in 1984 that a possibly consistent generalization of relativistic QFT was worth studying, but the problem is that decades and tens of thousands of papers later, as far as unification goes, this study has been a failure, taking the field down paths (extra dimensions, SUSY) which lead to complex theories that don’t look at all like the real world.
If you look at where things have ended up and the current research directions Maldacena and Witten are pushing, the odd thing is that they seem to have given up on unification, and for years now have been emphasizing the study of black holes in toy models with little to no connection to string theory. The most disturbing thing I heard in the Witten talk was at 1:24:16
If you had sufficient computing power, maybe with a quantum computer with a million qubits, I think you could simulate the dynamics of a quantum black hole…
Here Witten seems to be pointing to exactly the argument recently made by Juan Maldacena (see here), which has a specific claim about what you could do with a million qubits. This particular calculation would not in any way address the problems of the string theory program and is getting into Michio Kaku/wormhole publicity stunt territory.
Update: There’s an interview with Witten here, associated with his receipt of the Hamburg Prize for Theoretical Physics. About string theory, he explains that, despite 50 years of effort
We don’t understand it very well… In fact, I’d say we only understand a small part. So we’ve been struggling with that ever since the 70s and 80s trying to understand the intellectual framework that it should have been placed in.
He remains convinced though that alternative ideas are not the way to go:
… I find it implausible that physicists would discover a theory that is such a rich source of fruitful ideas about things that are definitely important in other fields by accident. And if we were not on the right track, I would say, it was a big accident. So my personal view is that it would be a cosmic conspiracy if string theory isn’t on the right track.
There is a growing number of critics who complain that string theory is very interesting, but hasn’t really delivered. Because we still have no idea whether it’s correct and we couldn’t make any experiments, which tells us if this is the case. According to you: To what extent is that criticism justified?
Well, not much, honestly. Where critics of string theory have had interesting ideas, they’ve tended to be absorbed as part of string theory. That’s happened several times. Twistor theory, black hole thermodynamics and noncommutative geometry are three examples of interesting ideas. They were by some regarded as alternatives or competitors of string theory, but actually in practice were absorbed as part of the picture in string theory.
What if those gravitational waves are from the inflationary era? could it be possible? that would be a major discovery, a Universe photo from before the cosmis microwave background.
@acollierastro did a video on Michio Kaku about how he is basically a crackpot now, which I find is most informing and to the point.
Mave,
Not possible, see for instance
https://twitter.com/WKCosmo/status/1674096955046219776
Percy,
I should have linked to that. See
https://www.youtube.com/watch?v=wBBnfu8N_J0
Roman Jackiw passed away in June. You have had several relatively favorable mentions of his work in the blog. Do you have any memories or summaries of his work and life?
David Yager,
As far as I can remember, I never met Jackiw. I was very familiar with his work on instantons, and over the years I’ve gotten a lot out of some of the papers in his selected works volume
https://www.amazon.com/Diverse-Theoretical-Mathematical-Physics-Advanced/dp/9810216963
Jackiw was a very clear writer and thinker about a range of topics in quantum field theory.
I would like to mention that the new results are also compatible with what we’d expect to see from quantum gravity (namely: nothing).
A bit off topic but since there is the actuality is not so hot, what about Witten’s paper
“A Note On The Canonical Formalism for Gravity” (the second version of the paper has just came out on arxiv https://arxiv.org/abs/2212.08270). He is talking about the Hamiltoninan constraint and Wheeler de Witt equation , which is usually the kind of topics that people in loop quantum gravity are working with.
Is there any relation with what Witten is doing here 5which does not seem to be string related) and lqg ?
With reference to Mave’s question and Peter’s reply, there do seem to be some investigations about this sort of thing. I knew nothing about this before today, but tried a Google search for:
can the nanohertz gravitational waves be from the inflationary era?
This has turned up:
https://arxiv.org/abs/2202.01131
https://arxiv.org/abs/2208.03330
https://arxiv.org/abs/2306.16219
@martibal
Witten’s paper seems to be about holography, which is mentioned as the motivation in the introduction. It explicitly uses the metric variables (and not the Ashtekar-Barbero ones) and makes references to the work’s ramifications for higher dimensions and string theory, so I would say it is a story about AdS/CFT (and is string related) and has no direct relation to LQG.
Perhaps it’s possible to reconstruct this work using the LQG formalism which could lead to a notion of doing holography with LQG, but I doubt Witten is the person who would pursue this.
Twitter is unusable for those of us without an account (and who refuse to get one and support fascists). And it is also likely that it will go down permanently at some point with how unstable it’s been and how they’ve fired a bunch of their own programmers. Can someone please quote?
anon,
If you’re referring to the Will Kinney twitter link above, the problem is that it’s a multi-tweet thread with embedded graphics. I’m not competent to process it in a way that will appear in a comment here.
My understanding is that Twitter has now backed down and you can access tweets without having a Twitter account.
“Twistor theory, black hole thermodynamics and noncommutative geometry are three examples of interesting ideas. They were by some regarded as alternatives or competitors of string theory, but actually in practice were absorbed as part of the picture in string theory.”
For noncommutative geometry and black holes thermodynamics at least, this is completely wrong. They may have an non empty intersection with string theory, but they have not at all been absorbed by it, neither in a theoretical nor in a practical way. Witten certainly knows that, so why is he saying such nonsense ?
Regarding twistor, we can guess what the opinion of Peter is 🙂
Did Penrose ever mentioned that twistors were aimed at be absorbed by string theory ?
1) @martibal
“Why is Witten saying such nonsense about noncommutative geometry (ncg) and black hole (BH) thermodynamics absorbed in practice by string theory”?
1Playing the devil advocate one may say this statement is making sense if literally understood as the fact that string theorists have definitely found more than one practical way to build on heuristic insight from BH thermodynamics & a few mathematical tools from ncg (for ex to quote Witten: “Connes cocyles …has had a number of interesting recent applications to quantum field theory and gravity cf https://arxiv.org/abs/2112.12828) in order to address many computations in quantum gravity toy models.
Now, what is IMO inexcusable is when Witten claims:
“Where critics of string theory have had interesting ideas, they’ve tended to be absorbed as part of string theory”.
I recommend to read John Baez review of “Foundations of Mathematics and Physics One Century After Hilbert: New Perspectives” edited by Joseph Kouneiher
(https://www.ams.org/journals/notices/201910/rnoti-p1690.pdf) to have a balanced and informed view on some interesting ideas that have not been absorbed (yet?) by string theorists.
2) It could be interesting to add here that in a 2021 interview, Connes said that Witten under the influence of Atiyah had recasted Vaughan Jones discovery of a new knot invariant, “dress[ing it] up in terms of functional integrals and things of this type while the real input… was from [Vaughan] own work on subfactors”
(quote from Allyn Jackson interview https://celebratio.org/Connes_A/article/842/).
This shows I guess how difficult it is to build a global vision of spacetime geometry in a quantum formalism following the heuristics Connes has built starting on classifying factors 50 years ago, meeting then Sullivan at IHES and interacting later with article physicists and cosmologists. May be we have to abandon solid theoretical research into natural geometric structures (= Riemanian paradigm) for a Solid Theoretical Research On Natural (=spectrally informed by cosmology and particle physics) Geometry program, emphasizing our doubts and quit preaching. (following Veneziano example praised by Connes at the very end of https://youtu.be/qVqqftQ92kA?t=6807)