CERN on Wednesday is hosting a colloquium talk by Joseph Lykken, who supposedly will discuss Prospects for experimental quantum gravity. There’s by now a long tradition of string theorists dealing with criticism that their research program is inherently immune from experimental test by making bogus claims about experimental testability. Lykken has been at it for at least twenty years (see here), and this sort of misleading claim about testability is the latest in a long campaign.
If you read the abstract, it looks like what Lykken is actually talking about is numerical simulations of an SYK model with of order 100 Majorana fermions on a quantum computer. Ignoring the quantum computer hype (unclear how long it will really be before such simulations are feasible), keep in mind that the SYK model is a quantum mechanical toy model, not a model of quantum gravity in a physical dimension. The only thing a quantum computer could test would be the validity of certain approximations schemes in such a toy model. For comments by David E. Kaplan about similar testability claims, see the interview discussed in the previous posting, which includes:
That there are actual people who are deciding string theory’s important, wanting to do string theory, and they’re even protecting the field. And some of those people are talking about how entropy now of a black hole can be described as a geometric thing, an entanglement, and that Hawking’s paradox about evaporating black holes is really wormholes, virtual wormholes coming from the inside to the outside, and all kinds of language. And you could test information theory of black holes using atomic physics experiments. And it’s literally bullshit.
There are people—prominent people—in physics who say, “I’m applying for this money from the DOE, but I know it’s bullshit.” And then there are experimental atomic physicists who don’t know and are shocked to learn that “What? String theorists don’t have a Hamiltonian? They don’t actually have a [laugh] description? What am I testing?”
Update: Lykken was giving Colloquium talks on Experimental String Theory nearly a quarter century ago. He was also one of the main sources for the embarrassing NYT 2000 article Physicists Finally Find a Way to Test String Theory.
“For the first 25 years, the thinking has been that superstring theory is so difficult to see experimentally that you have to figure it out by its own mathematical consistency and beauty,” Dr. Lykken said. “Now that’s completely changed. If this new picture is true, it makes everything we’ve been talking about testable.”
Hopefully science journalists have learned something and we won’t see a forthcoming NYT article on how “Physicists Finally Find a Way to Test Quantum Gravity”.
Update: Lykken’s slides are here. His proposal for an experimental test of quantum gravity is explicitly acknowledged as the same as one made by Lenny Susskind here in 2017. At the time that made no sense to me and I wrote about it here. It still makes no sense.
Peter, Lykken is deputy director of Fermilab and heads the Quantum Institute there. Just from the abstract it’s unclear if this is Lykken the physicist or Lykken the politician presenting.
A few years ago, I saw a talk he gave at Aspen definitely as Lykken the politician. His unabashed enthusiasm for quantum computing as the vanguard of HEP was, frankly, difficult to watch and he received significant backlash from the audience that this quantum push depleted resources from other important directions of the field. Since then, it seems that quantum computing, machine learning, etc., have only grown in funding support from the DOE and NSF HEP divisions.
SYK, with its 100 Majorana fermions, is as relevant to quantum gravity as homeopathy is to medicine. Indeed, the 100 Majorana fermions can’t feel their mutual Newtonian gravity but by a profound miracle they are going to describe quantum gravity.
Trick or Treat: Yet another claim that the CMB contains evidence for the multiverse:
If my ‘trick-or-treat’ bag had a candy for each and every one of these claims,…
I’m just here to support the “It’s bullshit” statement. And I too have encountered physicists who full well know that what they are saying is bullshit, but do it because they can get money for it. At the moment this is everything quantum simulation. Most of them delude themselves into thinking that somehow something will come out of it. (And those who don’t manage to delude themselves leave — strong selection bias.)
Quantum computers are the new particle accelerators, basically. All the nonsense that we saw 20 years about about bullshit to test at the LHC is now bullshit to test on a quantum computer. Except no quantum computer is anywhere in sight, so they can keep on doing this for much longer.
It’s extremely worrisome that this is still possible and no attempts at fixing the problem have been made.
@Andrew: I believe he is no longer director.
“the SYK model is a quantum mechanical toy model, not a model of quantum gravity in a physical dimension”
I am curious why the “physical dimension” part matters so much. One common technique we use to solve complicated problems in physical dimensions is to start with a simpler but analogous problem in lower dimensions and try to work our way up. I am not claiming that this always works (certainly there are phenomena we see in low dimensions that don’t show up when we go to higher dimensions, or vice versa) nor am I claiming that it will definitely work in the case of quantum gravity, but it does seem like a reasonable thing to try in the face of an (apparently) intractable problem.
Remember that the whole reason quantum gravity is a problem is the lack of renormalizability, and this problem comes about because of the behavior at short distances of physical degrees of freedom describing space-time. In 1+1 and 2+1 dimensions there are no physical degrees of freedom, just gauge (and topological). So, in these dimensions the problem you are trying to solve is not there. In more than 3+1 dimensions the non-renormalizability problem is there, even worse. The original claim of the string theorists was that string theory solved this problem (in 9+1 dimensions), but that’s another story…
The SYK model isn’t even a QFT. It’s a 0+1 dimensional model. I’ve nothing against people doing calculations in toy models, but you can’t then claim to have solved problems of actual quantum gravity based on toy models of a very different nature. And claiming that when you do such calculations you are doing “experimental quantum gravity”, creating wormholes in a lab, etc., etc. is just complete bullshit.
Lack of renormalizability is not the only problem for quantum gravity. The whole point of the black hole information problem is that it involves arbitrarily large distances (the horizon scale of a black hole that can be as big as you like, with arbitrarily low curvature), and nearly 50 years of attempts to solve the problem using quantum field theory has not led to a definitive resolution.
The construction of toy models in low dimensions makes the problem particularly acute – there is NO dynamics in gravity itself and no short-distance problem in constructing the quantum theory; at the same time, there is no underlying statistical physics explanation of black hole thermodynamics (because of the paucity of dynamical degrees of freedom in gravity itself), so quantizing gravity as a field theory cannot be the whole story.
The interest in the SYK model stems in part from the fact that it exhibits a collective mode whose action is equivalent to that of 2d topological (JT) gravity. So you have an underlying quantum statistical mechanics, and can derive from it an effective thermodynamics which happens to be the same as that of AdS2 black holes. It is less clear, however, that there is any sense in which one has local bulk physics in two dimensions (there are reasons to think not).
Quantum systems that accurately model black hole thermodynamics are relatively rare; solvable ones are rarer still. So it seems worthwhile to learn what one can from this one, while indeed being careful not to get carried away with what aspects of gravitational physics can be captured by such a model. Thus far, picturesque evocations of quantum wormholes remain largely just that – picturesque evocations.
Coming from the other direction, if one wanted to try to strip away as much as possible from quantum gravity in trying to address the black hole information problem, one has to retain at least the qubits responsible for the black hole entropy, and the gravitational collective modes that capture the thermodynamics. I’m not saying that what is left is the SYK model, I’m simply saying that what is left after such a winnowing of content shares many of its features. If one wants to answer questions about what is going on locally at a black hole horizon, and how specifically information is stored and re-radiated from a black hole, I suspect the baby has been thrown out with the bath water here.
Thanks for the clear explanation of the significance of the SYK model as a toy model for black hole thermodynamics. It’s a good point that in principle this might indicate the nature of the resolution of the black hole information paradox (although it would be a resolution that says nothing about the fundamental problem of finding a consistent dynamics of the short-distance degrees of freedom).
It would be really helpful if all people working on this stuck to providing accurate explanations of what this research is about, and avoided misleading claims about how the study of these models on a computer or in corresponding actual quantum systems was somehow experimentally accessing wormholes and physical quantum gravity effects.
Peter: I always considered Joe Lykken has a phenomenologist as he was always keynote speaker in DPF and another major conferences. Did not know he works on string theory or quantum gravity. (He has only 3 papers in gr-qc). However I see that has papers on string theory and D-branes (mainly in 90s).
Lykken became a string theorist when everyone else did (late 1984), then spent many years promoting the idea of testing string theory at the LHC. The full Wikipedia entry for his research is currently
“In 1996 Lykken proposed “weak scale superstrings,” which posited extra dimensions of space within the reach of particle colliders, such as the Fermilab Tevatron, and the CERN Large Hadron Collider. This, and related ideas will be subject to direct and indirect experimental tests in the coming years.”
Like many others, he seems to now be rebranding himself as a quantum information theorist, see https://arxiv.org/abs/2010.02931
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