Friday, February 28, 2020

Quantum Gravity in the Lab? The Hype Is On.

Quanta Magazine has an article by Phillip Ball titled “Wormholes Reveal a Way to Manipulate Black Hole Information in the Lab”. It’s about using quantum simulations to study the behavior of black holes in Anti De-Sitter space, that is a space with a negative cosmological constant. A quantum simulation is a collection of particles with specifically designed interactions that can mimic the behavior of another system. To briefly remind you, we do not live in Anti De-Sitter space. For all we know, the cosmological constant in our universe is positive. And no, the two cases are not remotely similar.

It’s an interesting topic in principle, but unfortunately the article by Ball is full of statements that gloss over this not very subtle fact that we do not live in Anti De-Sitter space. We can read there for example:
“In principle, researchers could construct systems entirely equivalent to wormhole-connected black holes by entangling quantum circuits in the right way and teleporting qubits between them.”
The correct statement would be:
“Researchers could construct systems whose governing equations are in certain limits equivalent to those governing black holes in a universe we do not inhabit.”
Further, needless to say, a collection of ions in the laboratory is not “entirely equivalent” to a black hole. For starters that is because the ions are made of other particles which are yet again made of other particles, none of which has any correspondence in the black hole analogy. Also, in case you’ve forgotten, we do not live in Anti De-Sitter space.

Why do physicists even study black holes in Anti-De Sitter space? To make a long story short: Because they can. They can, both because they have an idea how the math works and because they can get paid for it.

Now, there is nothing wrong with using methods obtained by the AdS/CFT correspondence to calculate the behavior of many particle systems. Indeed, I think that’s a neat idea. However, it is patently false to raise the impression that this tells us anything about quantum gravity, where by “quantum gravity” I mean the theory that resolves the inconsistency between the Standard Model of particle physics and General Relativity in our universe. Ie, a theory that actually describes nature. We have no reason whatsoever to think that the AdS/CFT correspondence tells us something about quantum gravity in our universe.

As I explained in this earlier post, it is highly implausible that the results from AdS carry over to flat space or to space with a positive cosmological constant because the limit is not continuous. You can of course simply take the limit ignoring its convergence properties, but then the theory you get has no obvious relation to General Relativity.

Let us have a look at the paper behind the article. We can read there in the introduction:
“In the quest to understand the quantum nature of spacetime and gravity, a key difficulty is the lack of contact with experiment. Since gravity is so weak, directly probing quantum gravity means going to experimentally infeasible energy scales.”
This is wrong and it demonstrates that the authors are not familiar with the phenomenology of quantum gravity. Large deviations from the semi-classical limit can occur at small energy scales. The reason is, rather trivially, that large masses in quantum superpositions should have gravitational fields in quantum superpositions. No large energies necessary for that.

If you could, for example, put a billiard ball into a superposition of location you should be able to measure what happens to its gravitational field. This is unfeasible, but not because it involves high energies. It’s infeasible because decoherence kicks in too quickly to measure anything.

Here is the rest of the first paragraph of the paper. I have in bold face added corrections that any reviewer should have insisted on:
“However, a consequence of the holographic principle [3, 4] and its concrete realization in the AdS/CFT correspondence [5–7] (see also [8]) is that non-gravitational systems with sufficient entanglement may exhibit phenomena characteristic of quantum gravity in a space with a negative cosmological constant. This suggests that we may be able to use table-top physics experiments to indirectly probe quantum gravity in universes that we do not inhabit. Indeed, the technology for the control of complex quantum many-body systems is advancing rapidly, and we appear to be at the dawn of a new era in physics—the study of quantum gravity in the lab, except that, by the methods described in this paper, we cannot actually test quantum gravity in our universe. For this, other experiments are needed, which we will however not even mention.

The purpose of this paper is to discuss one way in which quantum gravity can make contact with experiment, if you, like us, insist on studying quantum gravity in fictional universes that for all we know do not exist.”

I pointed out that these black holes that string theorists deal with have nothing to do with real black holes in an article I wrote for Quanta Magazine last year. It was also the last article I wrote for them.


  1. Thank you for this Sabine. I had just read the Quanta article and my gut reaction was that I didn't believe it: you've justified my feeling.

    I do have a more general question about such investigations: even if the ADS problem did not apply, is this actually an investigation into black holes or, in effect, just the construction of an analogue computer designed to solve the equations that you believe apply to the black hole? I always have the feeling that unless you are experimenting on the system of interest - rather on some other system that you believe to be governed by the same equations - you are not gathering empirical evidence to test your theory. To pick up a point from one of your previous posts, if such an experiment demonstrates that Hawking radiation exists does it still not give any proof that it is emitted from a black hole?

    1. Hi Mike,

      One should not confuse these studies with analogue gravity, which works differently. Both rely on dualities, that is a mathematical identification of one system with another. But the laboratory system is a different one in both cases. In the hope that you excuse the self-reference, I have an illustration in Figure 1 in this paper.

      To use AdS/CFT you need an AdS space. To use analogue gravity, you don't.

      "is this actually an investigation into black holes or, in effect, just the construction of an analogue computer designed to solve the equations that you believe apply to the black hole"

      Needless to say, it is not an investigation of an actual black hole. Yes, as you say, it is basically a custom-designed computation. Also, you must keep in mind that this analogy only works in certain limiting cases.

      You can therefore also not actually test the AdS/CFT duality with this method, because for that you would have to have experimental access to both the AdS and the CFT side of the duality.

      (In my paper with the Figure I point out that you *can* test AdS/CFT if you combine it with analogue gravity.)

  2. The concluding line of "Quantum Gravity in the Lab"(arXiv:1911.06314v1) says: "More generally,the purpose is to shed light on deep and theoretically challenging questions about nature, including the necessary conditions to have a semi-classical bulk and the effects of quantum and stringy corrections to the semi-classical gravity picture. Thus, we believe the experiments described here are worth the effort to realize the long-term potential for experimental insight into quantum gravity." Surely, the authors are correct--"the experiments described here are worth the effort." Not only is the paper interesting in itself, the references are vital to understanding as "...a detailed analysis of many of the results presented here will provided in a future paper 'Teleportation By Size'..." (page 6). The brief paragraph regarding trapped ions is (if anything) thought-provoking (page 16). The entire paper is thought-provoking. By the way, how does one define research in Theoretical Physics ? If the research expounded in this paper does not qualify, what does ? Who am I to say what a theoretical physicist should, or should not, be allowed to study ?

    1. This kind of comment is of the form, "Quit criticizing errors or misrepresentations in this paper lest it detract or discourage people from reading all the good and positive things I find in this paper!" Weak sauce.

    2. Your assessment of my comment rightly points out a philosophy I have long advocated: "do not throw the baby out with the bathwater." My preference is not to discard everything simply because portions of it may need fixing (or, portions may even prove ultimately to be incorrect). In the paper at issue, I chose to focus upon what I opined to be positive aspects. I do not necessarily agree with everything those authors wrote, but that is not a reason to ignore the entire paper. This was also a position advocated by Max Planck, while editor of the German physics journal Annalen. Planck "rarely rejected a paper outright (for fear that it may hold something of value) and he overlooked errors if the author had good training in physics." (see page 310, Intellectual Mastery of Nature, volume two, Jungnickel and McCormmach). To further clarify my position, I do not at all advocate ignoring "errors," but I do advocate learning from those errors. If that be "weak sauce" then so be it, I will not alter my position. Progress in science should always strive for a positive way forward.

  3. Copyedit: 2nd paragraph, "full with" -> "filled with" or "full of".

    Last paragraph: "black holes in article" -> "black holes in an article".

  4. It’s not about what a theoretical physicist should be allowed to study, it’s about being honest and clear about the connection between theoretical models and reality. Investigations into the physics of fictional worlds can be fascinating, but we need to be clear that these worlds aren’t our world.

    1. Quoting the Quant article: "If the AdS/CFT correspondence is right, then these experiments would be more than a laboratory analogue of a black hole." Note their qualification "if."
      The arXiv paper at one point invokes the butterfly effect: "The reason is the butterfly effect: at large times, a small perturbation (putting in the particle) can destroy any correlations between the two sides that would have existed without the perturbation..." (page 12). The arXiv paper also says: " In terms of capabilities listed above, preparation of an infinite-temperature thermofield double state has already been achieved using Rydberg atoms..." (page 14). The Quanta article concludes: “Here we have theoretical physicists from quantum gravity talking to experimental atomic physicists... historically they have been about as far apart as any two groups in a physics department. So this is a new thing, and it’s great.” Nothing wrong with that.

  5. One side in this debate is misinformed...
    Whom to believe?

    "Teach the Controversy" !

  6. Conceptually, I understand how AdS could affect properties of general relativity due to the large-scale “shape” of space. I don’t understand how the large-scale shape of space would affect the properties of quantum gravity.

  7. Hype is attributable to the Greek word hyper. Happily enough you don't have to be a psychologist to be able to recognize the symptoms.

  8. Hi Sabine,

    It seems the Conformal means that it preserves the angles, so it could be that the CFT could have a positive curvatures. Note that AdS5 is multiplied by S5, so the whole structure do have a positive component to it.

  9. "Let us have a look at the paper behind the article."

    It doesn't seem that it has been accepted anywhere. Has it even been submitted? Or is it intended to be arXiv-only? If not, then perhaps you will get to referee it!

  10. Our universe is not conformal, this is a symmetry that must be broken, maybe it yields a theory with a positive cosmological constant? The equivalence is in terms of operators but, maybe, it could be that the equivalence could be realized on the flat saddle surface between S5 and AdS5, but it doesn't have to fall in the negative curved space.

  11. Quantum Gravity combined with ADS in the article signals. for me, another excursion in to thought experiments. As exercise in exploring theory with the mathematical tools available it does seem worthwhile.

    I wonder which model of quantum gravity would be addressed.

  12. Why ant-de Sitter spaces? It comes down to string theory. The bosonic string has a negative vacuum energy and it sits best in a spacetime with a negative vacuum energy. The AdS spacetime fits the bill. There are those who will say “pooh-pooh” on string theory, for we do not live in an anti-de Sitter spacetime. If we did there would be red and blue shifted light sources or galaxies. Also AdS spacetimes admit closed timelike curves, or in effect time travel. However, de Sitter and anti-de Sitter spacetime in a spacetime of one dimension lower are bounded by a light cone. The de Sitter spacetime is a single hyperboloid sheet that is outside the cone and there are then two AdS spacetime inside the two conical openings. These interestingly meet at I^∞ and share the same boundary information there. In the CFT duality with AdS this boundary contains all the quantum information of the AdS and the dS.

    Why work with a black hole in AdS? The reason is the holographic information on the black hole horizon and the boundary ∂AdS_n are equivalent. The black hole in an equilibrium with the AdS has the same quantum state configuration as the boundary. The AdS is then a perfect “box” that can hold a black hole. As a result talking about the AdS is equivalent to talking about the black hole. I have been involved with work on others that brings the dS into this picture. We can transform between these.

    I think the statement about extreme energy in quantum gravity is about physics that generates excitons of the field, which are quantum units of black holes. Indeed it is possible for superposition physics with gravitation to be detected, which does not require extreme energy.

    I have not read this paper yet, though I read the Q-Mag article by Phillip Ball the other day. The title was a bit off to me, but wormholes share a feature with AdS spacetimes in that they violate the Hawking-Penrose energy conditions and permit closed timelike curves. I would need to read this article closely, and it is rather lengthy. There are analogues of AdS spacetimes in condensed matter systems and so this concept of building a lab analogue may not be entirely crazy.

  13. " To briefly remind you, we do not live in Anti De-Sitter space. "

    It's the CFT part that bother me.

    CFTs are a very specific type of QFT. It seems like this is an important point that is often glossed over. QCD is not a CFT. Most of condensed matter physics is not conformal. Also, from my understanding, supersymmetry is also needed.

    It's not even obvious to me that you can describe an atomic physics experiment with a CFT. I'll admit though that this is possibly just due to my own ignorance.

    1. CFT is QFT that is conformally invariant. This means under a conformal transformation the theory is same, which has analogues with Huygen’s principle in optics. This is then a measure preserving transformation. The Jacobian matrix of the conformal field conserves the area or volume of the field theoretic information. This is analogous to Huygen’s principle that light waves are transformed by optical filters to extremize a quantity, least time, that is conserved and related to action in classical mechanics. From this with CFT in 2 dimensions this leads to Virasoro algebraic systems, which underpins the bosonic string theory, and there are conformal blocks and OPEs.

      QFT, and in particular QCD, is not strictly conformal because if charged fermions or quarks are present these fields “remove” the volume of the field theory. This occurs in the physical universe because at some point in the early universe, maybe with inflation or the end reheating epoch, there was a symmetry breaking that resulted in excess baryon and lepton numbers. I have a certain preference for the sphaleron theory with this, but which ever is the case conformal invariance was violated by the breaking of symmetry. This is something not well understood and has bearing on the origin of chiral breaking or parity violation.

      The occurrence of conformal systems in solid state physics happens when the electrons populate states that render them decoupled from optical phonons in the system. Optical phonons are lattice vibrations that have charge separations. This happens when the electrons become polaritons in a type of superconducting state. This means there is a type of conformal field structure here. This is then an AdS_2 ~ CFT_1 theory. This has Virasoro structure seen in the bosonic string.

  14. Salaries paid in imaginary currency would redirect research, and in principle yield useful results in real-space.

  15. What if there is no cosmological constant but a logical term, variable, which gives dS/CFT/AdS correspondence in structures?

  16. It's good that you call out the bullsh*t Sabine. You should do it more. There's a lot of it about.

  17. I think QFT on the string worldsheet is described by a CFT, where this comes in. Also, I think, though I could be wrong, its at a critical point of condensed matter systems that conformality kicks in, because the system, to a large degree, becomes scale invariant. I'm not a CFT or condensed matter expert so take this with a large pinch of salt.

    1. "I think QFT on the string worldsheet is described by a CFT"

      Though the world sheet (in bosonic string theory anyway, don't know about others) is conformal, I think the CFT in the AdS/CFT correspondence lives on the boundary of the AdS.

      "scale invariant"

      However, the conformal group includes rotations (and boosts in Minkowski space). In condensed matter there's usual a preferred basis.

    2. I was replying to this: 'It's the CFT part that bother me.'

      So I was giving the physical idea as to how conformal symmetries can arise in physical systems we actually know about.

      'However, the conformal group includes rotations (and boosts in Minkowski space). In condensed matter there's usual a preferred basis.'

      In 2d they include a lot more. In fact the symmetry group there is infinite-dimensional. If I remember rightly, this is locally, and not globally.

  18. Dr. Hossenfelder,

    I strongly agree with J. Duffield above, and I greatly appreciate your candor and willingness to question things in today's physics. The only way physics and science can move forward and grow is if people stand up and point out what is wrong. So please continue on this path. It seems that others are starting to join you and this only makes for stronger and better physics.

    I read the piece in Quanta, please forgive my candor but my first thought in reading this piece was that science is telling me that entangled black holes that are connected by a worm hole are a new modern day telephone. And, that while this stuff is real ETs and UFOs are not.

    On a more serious side, I am trying to figure out if this was using black hole theory for quantum computing, or quantum computing theory for black holes, or both. In any case this appears to me to be one unknown being used to work on another unknown using something that is not part of the reality of our universe.

    The piece also stated that this theory may ultimately be able to be used to help establish String Theory.... When I started reading this piece it was not too hard to guess that unification and string theory would some how pop up.

    I have so many basic questions regarding this.

    1. Steve,

      All this talk about black holes and wormholes is is basically a fancy way of interpreting the math of many-body systems. As I wrote, it's not uninteresting - there's much to learn about quantum behavior of many-body systems - but it's not going to tell us how to quantize gravity.

  19. In general, a good place to look for answers to major unsolved issues in physics - missing mass, missing energy, missing anti-particles - is were the existing physics tools are weak. The many-body problems are such an area because it is difficult to solve reciprocal relationships.

    1. There could be a number of invalid assumptions in this rant about the importance of AdS our world and the way we live our lives. Applications in condensed matter physics have produced major advancements in the human condition. Producing a bubble of AdS space-time, a new and different universe, may prove to be useful in inspiring revolutionary, useful, and beneficial technologies. We won’t know what we might find unless we look.

    2. These experiments will not "produce a bubble of AdS space-time".

  20. Chancing that it might be of interest, there is a recent article on SYK Model posted to the arXiv: "Pedagogical introduction to SYK model and 2D Dilaton Gravity"(2002.12187v1). After many, many years (over 40) following the journal literature regards quantum gravity, there is no one today who can accurately predict "how to quantize gravity." One positive aspect of that preprint (Quantum Gravity in the Lab) is that I devoted time this afternoon to studying, and learning about, the SYK Model, which made this a fruitful day, indeed.



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