Today Quanta has One Lab’s Quest to Build Space-Time Out of Quantum Particles. No, this kind of experiment is not going to “Build Space-Time”, now or ever. This kind of obfuscation about quantum gravity advances neither fundamental physics nor the public understanding of it, quite the opposite. The article does make clear what the motivation is: deal with the problem that

String theory, still the leading candidate to replace the Standard Model, has often been accused of being untestable.

by claiming that it somehow can be tested in a lab.

To me, these types of articles about testing quantum gravity give off the impression that modern theoretical research into quantum gravity, AdS/CFT, and whatever else is coming out of the string theory community has more to do with to engineering toy constructions in the lab and pushing the limits of condensed matter and atomic physics than anything fundamental about the universe and high energy physics.

We’ll see lots more of this because there’s so much money now being thrown at quantum simulations in the quantum technology packet. Doesn’t matter if it makes any sense. This entire funding process is so broken.

About the motivation for this paper …

The people building quantum computers today are very much doing cutting-edge research. But right now, quantum computers are too small to solve any real problems, so people are looking around for neat demonstrations they can do.

This “simulation” of quantum gravity is almost certainly worthless in terms of what it actually tells us about quantum gravity, but if you just have 18 qubits, you can’t do any other calculations that would be worthwhile, either. And this “simulation” certainly seems to have been useful in terms of publicity value.

We’ll see whether quantum computer experimentalists keep on doing these “simulations” once they have a few thousand qubits and can do useful calculations.

anonymous commentator,

Given that David Gross and other leading string theorists think that it’s “BS” to attempt to quantize gravity by trying to find a consistent quantum theory of the connection degrees of freedom that characterize geometry, one wonders what they think about claims that the way to quantize gravity is to study systems of trapped atoms in an atomic physics lab.

I normally avoid attributing to scientists motives based on money rather than honest belief about the best way forward, but it certainly seems that the replacement of conventional HEP physics funding with large amounts of funding for “Quantum Information Science” is having an effect.

I don’t believe that funding is the reason that string theorists are now approaching their research from a quantum information viewpoint — at least, it’s not the only reason. This may also be due to the AMPS (Almheiri, Marolf, Polchinski, and Sully) paper, where quantum information theory showed that Susskind’s theory of black hole complementarity was probably incorrect. This was the first real result in years in the black hole information field, and researchers there started looking for more ways to apply quantum information to their research.

And indeed, they are finding more and more ways to apply quantum information to this field, although I suspect that not all of these ways are justified.

I’ll defend the intersection of “quantum gravity” research and stat mech/cond mat, but mainly insofar as it aids the latter solve important, intractable problems.

One of my favorites (quite old now) is the Knizhnik, Polyakov, and Zamolodchikov (KPZ) formula for computing scaling dimensions of operators in 2D classical stat mech models at criticality (CFTs), defined on random manifolds. The “annealed sum” over all possible geometries shifts scaling dimensions in a simple way. Why is it useful? Well it turns out to be easier to calculate certain critical exponents of _logarithmic_ CFTs that describe geometric critical phenomena, e.g. 2D classical percolation, in the random case–using matrix methods. Then you can use KPZ _backwards_ to get exponents on the flat plane. This was pioneered by Duplantier et al. Annoyingly, log-CFTs (which are for the most part, still poorly understood) are the most important for applications to many interesting quantum phenomena in condensed matter, such as the plateau transition of the quantum Hall effect.

From a condensed matter POV, though, summing over ALL geometries is rarely directly useful like this. There are, however, some important problems where a sum over _weakly_ random geometries can be important. This seems to be the case for 2D massless Dirac carriers in 2D quantum materials (cuprates, graphene, surface states of topological phases), where at least in some contexts it is natural for disorder to modulate the “speed of light” for the carriers. We are hoping there might be a way forward using some recent developments in 2D quantum gravity. In fact, this is one motivation for an Aspen workshop we are going to hold on “Random geometry” in Fall 2022, covid willing…

Matthew Foster,

There’s no question that issues like the ones you mention involve very non-trivial theory with a long history, and in that theory, connections to quantum gravity (especially in lower dimensions) are important.

The Quanta article though is about something completely different, experimental atomic physics results with zero connection to quantum gravity. There’s now a very long history of attempts to confuse everyone about this, very sad to see that Quanta is doing this, something that deserved mention here.

I’ve wasted too much time in the past explaining the obvious points that the people doing this are trying to obscure (to these extent these experiments test anything, it’s whether some approximation scheme works in certain calculations in standard atomic QM, nothing to do with quantizing gravity).

It has always seemed to me that the “It from Qubit” program was more likely to lead to a geometric description of some kinds of correlated multipartite states than to a theory of quantum gravity. In other words, the hype gets the arrow pointing in the wrong direction.

Peter,

I’ll only say that SYK has a very similar flavor, in that

(1) on one hand, it’s a very simple zero-dimensional “quantum mechanics” problem of randomly interacting Majorana fermions, or equivalently, an SO(n) bilinear “magnet” with random, fully broken SO(n) symmetry.

On the other hand, it

(2) exhibits emergent conformal invariance in 1+0-D, which maps to Schwarzian/Liouville quantum mechanics. The physics of the model is encoded in the sum over diffeomorphisms of the circle. While there is no true geometry here, it is a toy version of Liouville CFT. Moreover, at long times for a finite number of N Majoranas, there is a random matrix description that has a holographic dual in terms of JT gravity.

So realizing SYK in the lab would in principle allow one to experimentally test all of these connections. It’s not “true” quantum gravity, but it’s a first step. The problems I mentioned in my post are likely much harder, because they do involve 2D Liouville or other geometric (logarithmic) CFTs, and these are still poorly understood. But there has been some more recent cross-fertilization with cond mat and stat mech that might shed some further light, for example the interpretation of 2D Liouville as a log-correlated random energy model. This is a relatively simple toy model for glassy physics, which exhibits a “freezing transition” below a critical temperature. The freezing transition is related to violations of the so-called Seiberg bound in Liouville, and might have something to do with “macroscopic” operators that punch holes into the “worldsheet” of the theory.

Most of the text of the Quanta article appears pretty honest about what is being attempted, and what has been accomplished. I take it your main objection is to the headline, and the string theory preamble?

It must be the season… Here is yet another claim re string theory’s supposed testability: https://phys.org/news/2021-09-s-matrix-bootstrap-theory-quantum-gravity.html?utm_source=nwletter&utm_medium=email&utm_campaign=daily-nwletter

Peter : one technical problem is that after such articles they don’t provide the email address of author or a place for readers to comment or provide feedback.

I wish they did so.

Shantanu,

Quanta does have a comment section on the article, but their comment sections are unmoderated and thus useless (overrun by cranks, people who have no idea what they are talking about). Phys.org mostly just republishes press releases. Possibly this story has its origins in a press release somewhere. Physical Review Letters encourages people to issue press releases when a paper is published, it looks like this is an example.

By the way, looking at the PRL article, this is about the superstring in 10d, no mention of the main problem of string theory (how do you get 4d?). From the results of the authors I don’t see anything like an argument that “string theory is the unique theory of quantum gravity”, even in 10d. Again, lots of hype for a result that doesn’t do much of what is advertised. This has been the story of string theory in the media forever.

Matthew Foster,

The headline and string theory framing are completely outrageous. The rest of the article is fine if you strip out all mention of quantum gravity. It’s this that’s just completely misleading.

As for SYK, again you’re bringing in a complicated and interesting theoretical topic, but one that has nothing to do with this article.

This reminds me of this paragraph from Sophie Rilson’s paper that Peter Woit linked in his previous blog post:

‘Quantum field theorist Matt Strassler and Woit had an, at times, furious debate about the relationship between string theory as a tool and as a theory of quantum gravity (see comments on (Strassler, 2013a)). Woit accused Strassler of misleading the public by claiming that progress coming from the use of string theory as a tool is indicative of progress in string theory as a theory of quantum gravity (which Woit calls string unification). For Woit, the applications of string theory are tests of an “approximation scheme” as opposed to tests of a theory (Woit, comment on Strassler, 2013a).’

Ian McCormack,

That was a bit different. There the argument was over something relatively well-defined: AdS/CFT relating 4d and 5d and whether it’s a good approximation to QCD. At the time there was a lot of hype about “tests of string theory” that really were tests of this approximation.

In this case, instead of a specific 4d/5d AdS/CFT duality conjecture as a tool, people are talking about all sorts of very different things in toy models in low dimensions, with basically zero relation to string theory or gravity in 4d (note that “gravity” in 1d, 2d, 3d is something of a different nature, since there are no physical gravitational degrees of freedom in those dimensions).

Hi Peter,

No vacuum gravitational waves in 2+1-D, yes. But dynamics are nontrivial with source matter fields. That doesn’t qualify as physical DOF? Einstein tensor is nonzero there.

Matthew Foster,

I don’t think source matter fields change the basic situation: the real problems of quantum gravity aren’t there. In 3+1 d the physical state space is infinite dimensional, with an infinity of gravitational degrees of freedom interacting non-linearly with renormalizability problems. In lower dimensions the degrees of freedom are pure gauge (except for maybe a finite number), which is very different.