Modern Theories of Quantum Gravity

Quanta magazine today has a column by Robbert Dijkgraaf that comes with the abstract:

Reductionism breaks the world into elementary building blocks. Emergence finds the simple laws that arise out of complexity. These two complementary ways of viewing the universe come together in modern theories of quantum gravity.

It struck me that at this point I don’t know what a “modern theory of quantum gravity” is. Much of the article is a clear explanation of the usual story of the renormalization group and effective field theory, but towards the end, when quantum gravity comes up, I have trouble following. String theory has gone from being an exciting new idea to being part of historical tradition:

Traditional approaches to quantum gravity, such as perturbative string theory, try to find a fully consistent microscopic description of all particles and forces. Such a “final theory” necessarily includes a theory of gravitons, the elementary particles of the gravitational field.

That “reductionist” tradition is opposed to a new “emergent” holographic theory, and we’re told that

The present point of view thinks of space-time not as a starting point, but as an end point, as a natural structure that emerges out of the complexity of quantum information, much like the thermodynamics that rules our glass of water. Perhaps, in retrospect, it was not an accident that the two physical laws that Einstein liked best, thermodynamics and general relativity, have a common origin as emergent phenomena.

In some ways, this surprising marriage of emergence and reductionism allows one to enjoy the best of both worlds. For physicists, beauty is found at both ends of the spectrum.

Dijkgraaf seems to be saying that a viable emergent theory of four-dimensional quantum gravity based on the complexity of quantum information has been found, but I seem to have missed this. Can someone point me to a paper describing it?

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20 Responses to Modern Theories of Quantum Gravity

  1. NoviceAsAlways says:

    I read the Quanta article before getting to your blog post, so I really appreciate your point of ‘please point out the paper that validates this claim’. As a novice, could you provide a more detailed description of where the authors leaps of logic run off the rails? Even a just basic ‘this claim leads to this claim leads to this’ basic rundown would be really helpful.

  2. Giulia says:

    Peter,

    he is obviously referring to Verlinde’s entropic gravity. As all readers in this blog know, Verlinde repeated in a few lines what Jacobson did before him: whatever the microscopic degrees of freedom of the vacuum might be, they generate Einstein gravity if they behave like black hole horizons. Equivalently, one can say:

    If horizons have microscopic degrees of freedom, the vacuum must have as well.

    If one phrases it in this way, there is no big news in it. And one can rightly say that space is a thermodynamic entity. A deep theory of quantum gravity is not needed to make this statement; one just has to believe in black hole entropy.

  3. Mitchell Porter says:

    “a viable emergent theory of four-dimensional quantum gravity”

    I think it’s called M-theory in four-dimensional anti-de-Sitter space. It emerges from a three-dimensional field theory called ABJM, and you can e.g. obtain the entropy of a black hole in the 4d theory, by an entanglement entropy calculation in the 3d theory. http://arxiv.org/abs/1705.01896

  4. Peter Woit says:

    I still don’t know what Dijkgraaf is referring to:

    Mitchell Porter,
    I’ve never heard anyone claim ABJM as a viable physical theory of quantum gravity. In particular there’s the problem that we don’t live in AdS.

    Giulia,
    It’s not obvious to me he’s talking about Verlinde’s entropic gravity. Last I heard, people at the IAS were skeptical there was any real idea there. Maybe things have changed though.

    NoviceAsAlways,
    The problem here is that I don’t know exactly Dijkgraaf’s “modern theory of quantum gravity” is, so I can’t identify any possible leaps of logic. I’m left wondering whether it’s a “postmodern theory” of quantum gravity.

  5. Chris Herzog says:

    I do not know for certain, but he may be referring to the first question that the It from Qubit Simons Foundation Collaboration Grant is trying to address: “Does spacetime emerge from entanglement?”

    https://www.simonsfoundation.org/mathematics-physical-sciences/it-from-qubit/

    It is probably premature claim success, but it’s certainly an intriguing idea that many people take seriously — that quantum gravity could emerge from quantum information.

  6. Peter Woit says:

    Chris Herzog,
    Your suggestion is the most plausible one so far. But if so, the “modern theories of quantum gravity” that Dijkgraaf is advertising are actually not what one would normally call a physical theory of quantum gravity, but more of a speculative general hope about some feature such a theory might someday have. It would be better if he made that clear if that’s what he’s writing about.

  7. Mitchell Porter says:

    Peter: in fact, a deformation of ABJM is holographically dual to the Nicolai-Warner critical point of N=8 supergravity, a solution of the theory whose properties did attract some phenomenological interest prior to superstrings. You yourself have expressed a liking for the idea of twisting the N=8 theory, so ABJM may be a lot closer to your own interests than you know.

    However, I mention it here, not as a theory of everything, but as the main example in four dimensions, of what Dijkgraaf is talking about. The AdS5/CFT4 example (N=4 CFT) is even better understood (e.g. see David Berenstein’s papers), but you asked for something with gravity in four dimensions.

    These AdS theories are not immediately viable as phenomenology, because they are AdS and because they involve a special choice of fields, but they prove the viability of Dijkgraaf’s concept – gravity emerging from quantum mechanics.

  8. Anon says:

    This article is a perfect example of John Horgans ironic science. Its a weird kind of literary criticism.

  9. Frog Leg says:

    There was an interesting classification of QG theories posted in the arxiv:
    https://arxiv.org/pdf/1708.07445.pdf
    (h/t S. Hossenfelder, https://mobile.twitter.com/skdh/status/906009289331949569)

  10. Peter Woit says:

    Art,
    I hadn’t really noticed the links to Quanta articles embedded in the Dijkgraaf piece. I’m suspicious that they weren’t put there by Dijkgraaf himself. In particular, I wouldn’t have thought that for the general idea of holography he would send people to Verlinde’s work. But, who knows, I’m quite confused about what people promoting this kind of work specifically have in mind and intend to point to.

  11. Marko says:

    Maybe he is talking about tensor networks, which are being hyped quite a lot recently in the quantum information community (Preskill, Susskind, Carroll, etc, and a bunch of their students). But I think that tensor networks are still quite far far away from being called a theory, they are nowhere near developed enough.

    Of course, none of that has ever kept anyone from hyping about the “modern theories of quantum gravity”…

    🙂
    Marko

  12. Low Math, Meekly Interacting says:

    An odd article for Quanta. Nearly 2/3 of it is devoted just to setting up the main thesis (almost half of it is a synopsis of “Powers of Ten”). So emergent gravity gets crammed into a relatively brief denouement that does little but namecheck the (seemingly) key concepts. Contrast this to, say, Frank Wilczek’s anyon or entanglement articles. Like them or not, those have, at least, some actual content. Maybe Dr. Dijkgraaf had a deadline and was too busy with other things.

  13. Peter Woit says:

    LMMI,
    The funny thing is it’s not unusual in cases like this for journalists to write much more substantive pieces than scientists. I think the scientists decide it’s too hard to actually explain what is going on, so stick to generalities, whereas the journalists (the good ones) at least make an attempt to convey what they’ve been able to understand of the story.

  14. tulpoeid says:

    (Partially repeating Art’s remark,) it seemed to me that there’s no mystery: The paragraph starting with “A complementary approach to combining gravity and quantum” links to everything the author talks about.

    The alienating thing is of course that after listing the most fashionable theories of QG he pulls very hard to call them either reductionist or emergent, for not necessarily good reasons other than writing an article about these two concepts. But he doesn’t seem to make mystery claims about new theories.

  15. Giulio says:

    I think it’s clear Dijkgraaf is referring to Verlinde: the key sentence “quantum space-time, including all the particles and forces in it, emerges from a completely different “holographic” description” links to a Quanta story about Verlinde.

  16. Peter Woit says:

    Giulio,
    All the links in the article are to other articles at Quanta. I suspect an editor at Quanta put them there, not Dijkgraaf.

  17. Peter Woit says:

    Moyses,
    I see no evidence that that proposal for how 4d spacetime emerges from the 10 d superstring is what Dijkgraaf is referring to

  18. Evan Thomas says:

    Peter,

    Looks like I was mistaken, but you should go with my initial impression of your question, one of rhetorical sarcasm. Meaning, there is no viable “modern theory of quantum gravity” 😉

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