In recent years a hot topic in some theoretical physics circles has been the 2013 “ER=EPR” conjecture first discussed by Maldacena and Susskind here. Every so often I try and read something explaining what this is about, but all such efforts have left me unenlightened. I’m left thinking it best to wait for this to be better understood and for someone to then produce a readable exposition.

Instead of that happening, it seems that the field is moving ever forward in a post-modern direction I can’t follow. Tonight the arXiv has something new from Susskind about this, where he argues that one should go beyond “ER=EPR”, to “GR=QM”. While the 2013 paper had very few equations, this one has none at all, and is actually written in the form not of a scientific paper, but of a letter to fellow “Qubitzers”. On some sort of spectrum of precision of statements, with Bourbaki near one end, this paper is way at the other end.

Susskind starts out:

It is said that general relativity and quantum mechanics are separate subjects that don’t fit together comfortably. There is a tension, even a contradiction between them—or so one often hears. I take exception to this view. I think that exactly the opposite is true. It may be too strong to say that gravity and quantum mechanics are exactly the same thing, but those of us who are paying attention, may already sense that the two are inseparable, and that neither makes sense without the other.

I just finished writing a book about quantum mechanics, and it all seemed to me to make perfect sense without invoking gravity, but as explained above I guess I’m one of those who is not (successfully) paying attention. Another route to understanding would be to focus on the new experimental implications of the ideas. In the abstract Susskind claims that his ideas imply that we’ll observe quantum gravity using quantum computers in a lab “sometime in the next decade or so”. When that happens maybe this will all become clearer.

Update: Sabine Hossenfelder has a commentary on the paper here.

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31 Responses to GR=QM?

  1. Vik says:

    Dr. Woit,

    I’m very interested in your QM book, but I prefer bound books. Will the book be on sale?

    Thank you.

  2. Peter Woit says:

    Yes, I see that Springer now has a publication date of December 4.

  3. Vik says:

    “Susskind claims that his ideas imply that we’ll observe quantum gravity using quantum computers in a lab ‘sometime in the next decade or so’.”

    Actually, you misquoted him. In his abstract, he says, “…about the possibility of seeing quantum gravity in a lab equipped with quantum computers. I expect this will become feasible sometime in the next decade or two.”

    So, he uses the word “possibility” and “feasible”, not “observe” as you wrote.

  4. Richard Gaylord says:

    “I’m left thinking it best to wait for this to be better understood and someone produce a readable exposition.”. poorly written.

  5. Anonyrat says:

    Seems like a reductio ad absurdum proof that AdS/CFT is wrong.

  6. Anonyrat says:

    Or a side effect of the legalization of marijuana.

  7. Cobi says:

    “On some sort of spectrum of precision of statements, with Bourbaki near one end, this paper is way at the other end.”
    As you mention yourself, it is not a paper but a “letter to colleagues” or, in more conventional categories, an essay.
    To take an essay as evidence that the field is moving into a “post modern” direction is somewhat dishonest.

  8. ttt says:

    So, he uses the words “possibility of seeing” , not “observe” as you wrote.

    a difference without a distinction

  9. Peter Woit says:

    It’s not a popular essay aimed at non-experts, thus making precise statements inappropriate. It’s a letter to other experts, no reason it shouldn’t contain precise statements. If you think there are any in it, perhaps you can explain what they are?

  10. atreat says:

    In the original ER=EPR, Susskind and Maldecena described their highly speculative conjecture like this: “In fact, we are going to take the *radical position* that in a theory
    of quantum gravity they are inseparably linked, even for systems consisting of no more
    than a pair of entangled particles.”

    I am not aware of a single experimental result that helped validate this “radical position” and yet Susskind in this latest paper suggests to others to take this “radical position” as given and then adds on another *even more radical* speculation. I think this is perhaps what Peter is referring to when he describes this research program as post modern.

    Given this paper, I’m curious what this says about Susskind’s String Theory work for all those years. If GR=QM it would seem Susskind now believes that String Theory is superfluous.

  11. Peter Woit says:


    Good question about what the implications are for string theory.

    The “post-modern” remark is less about these conjectures being “radical” than them being incomprehensible. I quite seriously have no idea whether Susskind has a new important idea that I’m not understanding because I’m not familiar with all the background and assumptions of those that he’s writing for, or whether he has just stopped making sense.

  12. sdf says:

    How is a commentary without any precise statements acceptable as a submission to arXiv?

    Apropos, would this have been regarded as acceptable as a submission to arXiv if the author’s name was not Leonard Susskind or another well-known physicist?

  13. Daniel Hogg says:

    As someone who has worked in a quantum computing lab (on the experimental side), I find myself in much the same boat as Peter, being unable to confirm whether I’m not understanding Susskind’s ideas, or if the ideas themselves make no sense. What I do know is that for all of the hype in the media, quantum computing is still quite primitive. Decoherence is a major obstacle, the ‘computers’ (if you can call them that) that we create are limited to a very small number of qubits. The papers that do produce results claiming tens of thousands of qubits seem to have fatal flaws such as a lack of distributed multipartite entanglement. So an attempt to piggyback quantum gravity onto the current ‘revolution’ in quantum computing needs to respect that the field is still quite limited in what we can measure/accomplish in the laboratory setting.

    From the experimental side of things, what seems to be missing is an effect that is being proposed that could be measured under laboratory conditions. People in the field of quantum optical computation do not currently think about gravity because there aren’t any observable interactions that we can test for (that we know of, yet). If Susskind wants to propose something that experimentalists can look for, great. As written, it comes across as too speculative.

  14. Blake Stacey says:

    The “ER = EPR” conjecture has the advantage that, if true, it would instantly imply the answer to one of the Millennium Prize problems, at least in the case of large N.

  15. Jeff M says:

    Well, I’m with sdf, how is this a legitimate submission to arXiv? Is it possible Susskind will submit it to a journal? If he does, and it’s accepted, that would strike me as a very clear sign that HEP is done. On another note, Sabine is hysterical.

  16. Mitchell Porter says:

    EPR->ER means that where there is entanglement, there should be wormholes. There are recent papers arguing in some detail that quantum teleportation is teleportation “through the wormhole”.

    Susskind apparently wants people who work in quantum information to get used to the idea that quantum logic gates, etc, involve actual wormhole geometries. And the experimental frontier that he hopes to see breached, is (I think!) the creation of micro black holes *simply through the manipulation of entanglement*.

  17. Peter Woit says:

    Blake Stacey,
    It’s news to me that there’s a precise formulation of ER=EPR that one could imagine proving and that would imply (I assume this is the claim) P not equal to NP. What’s a reference for this?

    Mitchell Porter,
    I don’t understand this at all. Qubits, entanglement and quantum logic gates I thought could be described precisely by QM with no need for wormholes. Is Susskind claiming that we’ll see some violation of QM that indicates quantum gravity, or will what we see precisely follow QM, with Susskind’s quantum gravity effects some sort of interpretation that those of us not paying attention can just ignore (and that has no known connection to actual gravity?).

  18. David Urbanik says:

    The “ER=EPR” solves P vs. NP comment seems like an obvious joke. If “ER=EPR”, then one can cancel the E and the R to find that P = 1, so we can only have P = NP if N=1, hence showing P is not NP for “large N”.

  19. Peter Woit says:

    Richard Gaylord,
    Thanks. Improved.

  20. srp says:

    Peter asks: “Is Susskind claiming that we’ll see some violation of QM that indicates quantum gravity, or will what we see precisely follow QM, with Susskind’s quantum gravity effects some sort of interpretation that those of us not paying attention can just ignore (and that has no known connection to actual gravity?).”

    From the paper: “A skeptic may argue that all of this can be explained without ever invoking bulk gravity or wormholes; plain old quantum mechanics and some condensed matter physics or quantum circuitry is enough. This is absolutely true, but I think it misses the point: Theories with gravity are always holographic and require a lower dimensional non-gravitational description. This does not mean the bulk world is not real.”

  21. GR=QM is impossible because from G(Newton’s constant) and c(velocity of light) one cannot get h(Planck’s constant).

  22. Blake Stacey says:

    Peter Woit:

    Sorry, I was just making a silly joke about canceling the E and the R.

  23. Blake Stacey says:

    … because I work in quantum information, and I tried valiantly to get some sense out of the “ER = EPR” claim, and after all that, the best I think it deserves is silly jokes.

  24. Peter Woit says:

    Thanks for pointing that out.
    Rereading the paper with that clarified, I think I’m finally beginning to see what Susskind is up to. By “QM” he means QM, but by “GR” he means not GR, but “hypothetical dual to QM system that we know almost nothing about” (which for some very specific QM systems in a special limit is related to GR in one dimension higher).

    Those of us who aren’t in the mood for completely ill-defined flights of fancy can safely continue to not pay attention. As for where this is going, it looks to me like the most optimistic scenario is that some better understanding of this kind of duality will lead to some useful approximate calculational method for some particular very complicated QM systems (by relating something in them to something in a GR calculation in one-dim higher). Then we’ll see lots of new university press releases and stories in the press about “wormholes created in the lab”, with claims that this is “evidence for string theory”.

    More interesting of course would be if this duality could be read the other way, explaining physical 4d quantum gravity in terms of a well-defined dual QFT. I don’t see any indication of that happening, and Susskind’s claims don’t seem to have anything to do with it.

  25. Lee Smolin says:

    Dear Friedwardt Winterberg:

    An excellent point. But you can get h from G and c if there is also assumed a fundamental length or energy scale, and vise versa. So one way to think about this could be that a QFT (h,c) whose symmetries preserve a fundamental scale (E_p), has G and hence gravity coded in it.


  26. To stretch an analogy, this is the kind of thought experiment that Einstein described that lead him to the principle of equivalence and thence to GR. Except he didn’t release a paper describing his thoughts about what it would be like to ride a beam of light or an elevator – he spent time learning new mathematics and wrote up a serious theory with predictions that were verifiable, and then verified.

  27. Blake Stacey says:

    Peter Woit (August 13, 2017 at 4:45 pm):

    Your “most optimistic scenario” pretty much agrees with the most optimistic evaluation I could make. A geometric way of calculating entanglement properties in many-body systems, let’s say—that could be useful. But I strongly suspect that the further one goes down that path, the less it will look like gravity in any way, and the hope of “building spacetime out of entanglement” (to put it the way a flashy keynote at PI would) will remain as distant as ever.

  28. wolfgang says:


    >> this kind of duality will lead to some useful approximate calculational method

    as I understand it, an important point he makes is that the distinction between the quantum-computer CFT side and the AdS bulk part is arbitrary.
    If the quantum computer(s) are sufficiently large, they may be able to communicate with observers inside the bulk.

    Of course one could turn this around (I dont think he does this yet) and argue that we are just the bulk description of somebody else’s quantum computer.

  29. Mauro Claudio says:

    Peter Woit, have you missed this ?

    Amazing statement: “teleportation … after a suitable time”.
    A teleportation whit a delay!
    Teleportation Through the Wormhole
    Leonard Susskind, Ying Zhao

    ER=EPR allows us to think of quantum teleportation as communication of quantum information through space-time wormholes connecting entangled systems. The conditions for teleportation render the wormhole traversable so that a quantum system entering one end of the ERB will, after a suitable time, appear at the other end. Teleportation requires the transfer of classical information outside the horizon, but the classical bit-string carries no information about the teleported system; the teleported system passes through the ERB leaving no trace outside the horizon. In general the teleported system will retain a memory of what it encountered in the wormhole. This phenomenon could be observable in a laboratory equipped with quantum computers.

  30. Peter Woit says:

    Mauro Claudio,

    Thanks! I somehow missed that one. Surprising that this summer there weren’t a bunch of articles in the press about how quantum computers would open up wormholes suitable for teleportation.

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