Sabine Hossenfelder already has this covered, but I wanted to add a few comments about this week’s hype, a new article in Quanta magazine by Philip Ball entitled Wormholes Reveal a Way to Manipulate Black Hole Information in the Lab (based on this paper). It’s the latest in a long tradition of bogus claims that studying relatively simple quantum systems is equivalent to studying string theory/quantum gravity. For an example from ten years ago, see here. The nonsensical idea back then (which got a lot of attention) was that somehow studying four qubits would “test string theory”.
A first comment would be that this is just profoundly depressing, because Ball is one of the best and most sensible science writers around (see my review of his excellent recent book on quantum mechanics) and Quanta magazine is about the the best semi-popular science publication there is. If this article were appearing in any one of the well-known examples of publications that traffic in misleading sensationalism, it wouldn’t be surprising and would best be just ignored.
Hossenfelder has pointed out one problem with the whole idea (we don’t live in AdS space), but a more basic problem is the obvious one pointed out by one of the first commenters at Quanta:
In the end, if an experiment is performed based on standard quantum mechanics, and verifies standard quantum mechanics as expected, then it is irrelevant that this aspect of standard quantum mechanics might be analogous to a vaguely-formulated and incomplete speculative idea about spacetime emergence — nor can it provide any experimental support whatsoever for that idea.
I understand that, for science journalists hearing that a large group of well-known physicists from Google, Stanford, Caltech, Princeton, Maryland and Amsterdam has figured out how to study quantum gravity in the lab (by teleporting things from one place to another via traversable wormholes!!), it’s almost impossible to resist the idea that this is something worth writing about. Please try.
Update: Philip Ball responds here.
Update: More from Philip Ball (and, if it appears, a response from me) at the Quanta article comment section,
comments from one of the paper’s author’s also comments here.
Update: Commenter Anonyrat points out that the Atlantic is republishing this piece, as A Tiny, Lab-Size Wormhole Could Shatter Our Sense of Reality: How scientists plan to set up two black holes and a wormhole on an ordinary tabletop.
Update: In the future, I hope to as much as possible outsource coverage of this kind of thing to the Quantum Bullshit Detector. Today, see for instance this.
Thanks for the link. It won’t be long until the headlines say we can test string theory on a quantum computer.
I think that’s just too old hat now. The time evolution has been:
1990s: We can test string theory/quantum gravity at the LHC!
Early 2000s: We can test string theory/quantum gravity at RHIC! We’re creating black holes at RHIC!
Late 2000s: We can test string theory/quantum gravity in condensed matter experiments! We’re creating black holes in our condensed matter lab!
2010s-now trend: The landscape has explained it all, who cares anymore about boring string theory? We now can test quantum gravity in atomic physics experiments! And we’re creating multiple universes when we do it! And we’re creating not just black holes but wormholes! And the wormholes are traversable and we’re teleporting through them!!!!!!
Looks to me like we’re just months away from the physics hype singularity…
Quanta, the best new pop-sci outlet…
I hardly knew ye.
Reading the Quanta article, it looks like their experimental proposal is to do quantum teleportation (of a particularly complex kind) and call it a traversable wormhole by invoking the non-existent ER-EPR correspondence.
This seems to me to be an ingenious way to address the objection that AdS-CFT is untestable in the real world.
The original article is very difficult to parse, so I haven’t read it yet, but I’d love to hear from somebody who believes that this isn’t what they’re proposing.
I quote the Quanta article:
One question: were things like this the inevitable outcome of using the arXiv and dispensing with refereed journals?
I couldn’t have said it better than SH or Peter Shor. I also don’t see what this accomplishes except to verify quantum teleportation(which has already been experimentally verified). Is this another sort of QM=GR idea? A true traversable wormhole should allow matter/info to arrive at a distant point in space faster than light could get there traveling from its origin. This is clearly not a traversable wormhole by that definition, and the same physicists involved in this enterprise would likely agree that true traversable wormholes are not likely to ever exist for consistency reasons.
That seems accurate to me.
When I first read the ER=EPR proposal, it seemed motivated by very thin heuristics that just barely scratched the surface of quantum theory. E.g., entanglement can’t be used for superluminal signaling, and messages can’t pass through wormholes; therefore, the phenomena are analogous. This amounts to saying that a wormhole exists between any two correlated objects in the Spekkens toy model.
It’s not clear to me that the Brown et al. proposal couldn’t be done, qualitatively, with stabilizer states, Spekkens toy bits or correlated Gaussian modes. (The “thermofield double state” doesn’t sound any more exotic, really, than using EPR pairs as purifications of thermal states in Gaussian quantum mechanics. The point seems to be that it’s entangled and its marginals are Gibbs states.) I could be missing something important, but right now, I’m not seeing how it’s really nonclassical, in the sense that nothing like it can be emulated in a local hidden-variable theory.
I wonder why people keep talking about “traversable wormholes” when there are numerous theorems in classical and semi-classical GR that put into serious doubt their physical feasibility. At the classical level, you need negative energy to stabilize them. This was proved by Hawking under very general conditions. Second, it was also proved by Hawking that when you put a quantum field in it, the whole thing instantly explodes and destroys all matter configuration that was holding it in place. One can still speculate that the Casimir effect may provide negative energy. Good luck with collecting the amount needed; besides, it will explode anyway. Well, you can speculate that quantum gravity may act as a cutoff for that energy divergence of the quantum field, but that is, as per today, just words, not serious physics. All currently tested serious physics says they can’t exist. Period.
I really don’t understand how these things can pass the peer review.
Sources (two papers by the leading expert on wormholes and closed timelike curves, the Nobel prize Kip Thorne):
The second update points to a Twitter thread by a colleague of the authors who was thanked in the acknowledgments, not an author.
The Atlantic online provides this headline for a re-hosting of the Quanta article:
“A Tiny, Lab-Size Wormhole Could Shatter Our Sense of Reality
How scientists plan to set up two black holes and a wormhole on an ordinary tabletop”
Slightly off-topic, but I found it through one of the twitter threads: the APS March meeting has been cancelled due to concerns about Covid19
Since physics labs around the country can create and manipulate wormholes, could everyone stay home and create wormholes to a lab in Denver, then run the conference via wormhole? Has anyone studied the problem of Covid19 transmission via physicist’s wormholes? I’m not sure I would want to be entangled with someone who has the virus…
Let me try to put it in simpler terms.
Suppose you take a balloon, and measure the distance between points on it. Then you inflate it more, and you see how much these distances have expanded. This does not tell you that the universe is expanding. It’s an analogy.
The same thing is true for the experiment proposed in the paper. The experiment should succeed because quantum information theory works, not because of anything about general relativity and wormholes.
So, I did my homework and just read all the papers involved. They actually try to address the two points I mentioned about “ordinary” wormholes, so kudos to them in that regard. Problem is, the solutions are highly unsatisfactory. Regarding negative energy, they consider two asymptotic anti de Sitter universes, and in this way can obtain a negative energy carrying quantum field in an analogous way to the Casimir effect, where the boundaries of the anti de Sitter universes connected by the wormhole act as the conducting metal plates in the latter. The issue with this is, of course, that we don’t live in anti de Sitter. So that isn’t going to work for addressing the first point against the existence of wormholes. The second point is that, with stable ordinary wormholes, you can create closed causal curves and this produces energy blow ups in quantum fields. They solve this by considering a wormhole connecting two different universes and which collapses quickly enough (it’s just a perturbed, by the negative energy quantum field, Kruskal wormhole.) Problem is, you need to start with a very particular initial state (two white holes in two different universes, or two different universes connected by a wormhole that collapses into two black holes.) This critique is also valid for the usual wormholes, since those geometries also assume an already created wormhole and no convincing method for their creation is proposed. In these new wormholes, they use the AdS/CFT duality and the ER=EPR hypotheses to argue that they are equivalent to a quantum teleportation experiment; thus, by creating a quantum teleportation you may be actually creating the wormholes. Of course, AdS/CFT duality and ER=EPR are highly speculative and mathematically unproven hypotheses, besides the already mentioned fact that we don’t live in an anti de Sitter universe. So, the most likely result from a quantum teleportation experiment will be just to prove again the known phenomena of quantum teleportation and nothing more.
This paper may be of interest to you (and other readers who care about substance not just sarcastic commentary): https://arxiv.org/abs/1807.04726
It’s using Casimir energy, but it’s building wormholes in flat space using Standard Model matter content.
Thanks! Especially impressive that, according to the Atlantic, you can build that thing on an ordinary tabletop.
Hey Mark, thanks! That was a very neat paper. I’m glad they are trying to address the actual problems. I think this is a good contribution to our understanding of wormholes, in line of Thorne’s classic work. The most valuable thing, for me, is the exploration of quantum matter configurations (in the real world) that could be used to get the negative energy.
The only objection I have is that, again, a very special initial state for the geometry is assumed (even more weird than a mere standard eternal wormhole.) But they actually recognize this (in 9.3 Open questions) rather than adding baseless speculation, so kudos to them.
Peter Shor wrote:
I think the problem is that real progress on high-energy particle physics and quantum gravity has been very slow for the last few decades, but people in these fields want to be doing impressive things, so many have turned to puffing up what they’re doing, using all sorts of tricks to make it seem more exciting than it is.
In fields where there’s more real progress and less grandiose ambitions, the arXiv seems to work pretty well.
I’m following this blog since I read “Not even wrong”, but this is one of the weirdest things I’ve ever seen here.
Do I understand what’s happening?
– You do an experiment on X, something you understand quite well;
– if you assume certain things (some of which are dubious, to put it mildly) the behavior of X should have in some way a resemblance with the behavior of something hypothetical, called Y, that you’re not testing with the experiment;
– The result of the experiment on X tells you something about Y.
If this interpretation is correct, this idea is an interesting subject for a paper on Philosophy of Science. But it’s not science like I know it.
Yes, that’s right. This is “quantum gravity in the lab” where you are not testing anything to do with quantum gravity. Another way to see the problem: the idea seems to be to use some model to predict the experimental result. What are the possible outcomes?
1. The prediction works. You’ve learned that this is a useful model for describing this kind of physical setup. You’ve learned nothing about whether it’s useful for describing quantum gravity.
2. The prediction doesn’t work. You’ve learned that this is not a useful model for describing this kind of physical setup. Again, you’ve learned nothing about whether it’s useful for describing quantum gravity.
To me it always feels like these analogue models for (quantum) gravity are a bit like what paper models are for architecture. You can certainly learn from them, if a wall here or there looks nice in a given context. And you might be able to use them to see if taking this wall or that wall out still gives you a stable structure.
But often it seems to me, the conclusion people draw from them is that real houses should also best be built from paper.
There’s another important assertion in the article, which can seemingly hold regardless of what is implied about “reality”: that Ads/CFT provides a simpler description of the system being modeled. I take that to mean, roughly, that it makes the math easier. Similar benefits (e.g. for some tough problems in QCD) have been promoted elsewhere. Any merit to this specific aspect of the claim?
That’s the hope, but the reality is that this hasn’t worked for realistic systems. You can see the problem in the case of QCD, where AdS/CFT gives you a calculational method for N=4 susy Yang-Mills, but not Yang-Mills. You end up doing computations in a different theory than QCD, then hoping that some rough features of that computation apply to QCD. As far as I can tell, all advertised applications of AdS/CFT have similar problems. The underlying problem is that people believe there is some underlying gauge-gravity duality principle, that gravity duals always exist, but other than in certain very special examples don’t know how to find the gravity dual. The “applications” of this duality to realistic problems become just general claims that certain kinds of phenomena on the realistic side will correspond to other phenomena on the gravity side, without any detailed prediction for exactly how this will work.
AdS/QCD is rich in quantitative predictions for confined QCD states, see here.
Am I correct in thinking that this surfeit of AdS/CFT talk is tantamount to looking for a nail in the dark under a lamppost because the light is better there, and anyway all you’ve got is a hammer, so the solution to your problem had better require a nail? Maybe two.
AdS/CFT is conformally invariant, spectrum looks nothing like QCD. AdS/QCD is a phenomenological model, engineered to try and reproduce the strong-coupling long-distance behavior of QCD. But if you want to do that, there’s something much simpler: it has been known for ever that the strong coupling expansion of QCD is an expansion in surfaces and you can think of it as a string theory if you want, and also get “predictions” for the strong-coupling/long-distance behavior.
The hard part of the QCD problem is not this, it’s connecting this behavior to the short distance behavior (asymptotic freedom), through a region where your calculational method doesn’t work. AdS/QCD does not at all solve this problem.
A bit off topic, but I thought it was inspired…
Today Time Magazine announced its 100 Women of the Year going back to 1920 and to my utter astonishment, the Woman of the Year in 1921 was Emma Noether…
Kudos to Time!
The problem solved by AdS/QCD is the working definition of QCD in the confined regime, producing decent analytic predictions for hadron spectra otherwise accessible at best via lattice numerics. This is effectively the conceptual explanation of the old Skyrme model for baryons, improved by including the full tower of mesons beyond the pion, which turn out to be unified as KK-modes of the holographic theory. Sutcliffe 10 explains nicely how the secret holographic nature of the Skyrmion model is really the famous theorem of Atiyah-Manton 89.
It’s all rather remarkable, but remains underappreciated. A good account happens to be in Chapter III of the collection “The Multifaceted Skyrmion“.
AdS/QCD is a model (basically a jazzed-up quark model). It is not the only conceptual explanation for the Skyrme model. Plus, it probably has little to do with real QCD (which must have both confinement AND asymptotic freedom, in particular, generation of logarithms in correlation functions at short distances).
History is repeating itself (in molasses). Way back in the mid- to late-1970’s, the strong-coupling lattice approximation was used to get a fairly decent picture of hadron masses. Except for the pseudo-scalar mesons (which later were incorporated), the results were claimed to show that QCD describes the world.
Then critics responded that this works for the same reason quark models and string models work. Confine heavy quarks with a linear potential and out comes a model of the hadrons! No surprise there.
So the strong-coupling lattice people, very sensibly, gave up (after a only a year or two) and turned to Monte-Carlo simulations (or eventually starting doing string theory, in some cases)
In my opinion, it is long overdue for AdS/QCD people to give up as well (and why can’t they?).
The basic technical point (excuse the capital letters which follow, but they are important) is this; you need a theory which can get to the ZERO-bare-coupling fixed point. This does NOT mean weak renormalized coupling. True QCD has INFINITESIMAL bare coupling, not strong bare coupling. The renormalized coupling (depending on how you define it) can be large, but NEVER the bare coupling.
If you don’t believe me about infinitesimal bare coupling, it was shown for more conventional QFT’s in the 50’s, using the Kaellen-Lehmann spectral representation; see Itzykson and Zuber, for example. See also how the renormalization group works in this context.
You can play games to get Skyrmions (I remember people did this with the bag model too), but this is still insufficient. Fitting hadron masses is nothing new (and not good enough).
Sorry for the long discussion. I have said similar words on this blog a few times here before. Some string theorists seemed incapable or unwilling to hear it (it’s basic, not advanced renormalization theory), then responded with insults. My verbose discussion here is to try to leave no ambiguity.
That the quark model correctly predicts hadron bound states is a purely computer-experimental observation (established only fairly recently for the first few hadrons arXiv:0906.3599) of which a conceptual understanding remains an open problem, dubbed one of the Millennium Problems.
While it is an old idea that the string model of mesons gives a conceptual handle on the confinement mechanism, its detailed and quantitatively accurate development used to be lacking. Making the string model of hadrons actually work is what holographic QCD is all about.
Since holographic QCD readily explains fundamental characteristics of confined QCD that remain mysterious not just in the quark model but also in popular ad hoc strong coupling model building such as the bag model (not only the confinement and chiral symmetry breaking mechanism itself, but also for instance vector meson dominance and the Cheshire cat property of the bag model are readily explained by holographic QCD) it is attractive to researchers interested in real-world QCD (e.g. Rho et al. 16, doi:10.1142/9710).
The Skyrme model for hadrons is in fact reasonable not in its original form but only after adjoining the tower of vector mesons to the pion. But in that tower-corrected form the Skyrme model works wonders: nuclei all the way up to carbon(!) are well-decribed already by Skyrmions in the pion+rho field (arXiv:1811.02064). This tower-correction of the old Skyrmion model had let nuclear physicist to discover (PhysRevD.69.065020) the hidden 5th dimension, leading to proof of vector meson dominance for nucleons. Conversely, the tower-corrected Skrymion model emerges from holographic QCD, where all mesons are unified as the transversal KK-modes of the 5d flavour gauge theory.
That available results in holographic AdS/QCD currently only (“only”) address the strongly coupled confined QCD phase and not its asymptotically free UV is not intrinsic to the holographic theory but owed to its computational development: holography is studied for strong ‘t Hooft coupling only for the convenience to be able to disregard string-scale and strong string-coupling effects on the bulk side. Inclusion of small-N corrections into AdS/QCD to get the full picture remains to be developed but need not and is not being ignored (e.g. PhysRevD.74.076004).
This “holographic QCD” discussion has gotten off-topic, and Peter Orland is right about the problems with it. You admit the main point, that no one can use this to address what happens at short distances (despite 22 years of effort). This is a fundamental problem not one of “convenience”. Sorry, but enough arguing this point, where the situation is clear (this doesn’t work).
Sorry for being off-topic Peter,
Twistors are a lot more interesting than AdS/QCD approaches, which won’t resolve any nonperturbative QCD problems, especially not millennial ones. Although twistors probably won’t solve these problems either, they still seem to have new applications waiting.
I have both Ward and Wells’ and Huggett and Tod’s books, and should probably get my money’s worth out of them.
Just realized that is more relevant to your other recent post. Oops.