We had a really interesting discussion of the quantum physics of de Sitter spacetime yesterday here in Aspen, starting with a review of the behaviour of scalar fields in such a background, led by Don Marolf, and then, after lunch, an open-ended discussion led by Steve Shenker. This is all quite difficult, and is of course quite relevant, since a piece of de Sitter is relevant to discussions of inflation, which seems (from cosmological observations) to have been a dominant phase of the very early universe. As the most symmetric space with positive cosmological constant, de Sitter may also be relevant to the universe today, since dark energy (first recognized after 1998’s observations of the universe’s accelerating expansion) may well accounted for by a positive cosmological constant.

So we need to understand this type of spacetime really well… and it seems that we don’t. Now there’ve been a lot of people looking at all this and doing really excellent work, and they understand various issues really well – I am not one of them, as I’ve not worked on this in any detail as yet. Do look at the papers of Marolf, and of Shenker, and collaborators, and references therein, and catch up with what’s been going on in your own way. For what it is worth, the sense that I get is that we’re trying to solve very difficult issues of how to interpret various quantum features of the spacetime and getting a lot of puzzles by trying to make it look a lot like things we’ve done before.

Now, we may solve all these puzzles…. but my current take on this all is that we’re missing something really key. I think that we’ve not completed the job of formulating quantum theory. This is more than just saying blandly that we don’t understand quantum gravity fully. I think that we’ve learned a lot about quantum gravity from string theory, but I think we’ve been fortunate – and perhaps a bit spoiled – with the progress we’ve made in spaces with zero cosmological constant and negative cosmological constant, which gave us great insights as to how to deal with perturbative gravitons, and huge chunks of the non-perturbative story, leading to things like AdS/CFT and gauge/gravity dualities in general (with the rich set of applications and techniques that I’ve mentioned here a lot)). But I am not so sure that de Sitter (and hence cosmological and related matters) is going to go through without us radically re-thinking what we even mean by some of the things we routinely do in quantum theory, and probably enlarging the toolbox somewhat. When I say radical, I’m thinking that we may need totally new rules, perhaps as fresh to us now as the ones that were needed back in the early parts of last century when we were trying to make sense of interpreting and predicting the results of the experiments back then, leading to modern quantum mechanics and ultimately the relativistic quantum field theory that we use routinely today. Those rules were developed in a way that allowed us to treat spacetime as a well-behaved bystander, and the later work we’ve done in string theory allowed us to continue to make great progress, even allowing the spacetime to be more than a bystander… but still well-behaved, with nice asymptotic regions that look safe and familiar, and perhaps above all – stable. The point is that de Sitter is not a nice bystander at all, so why are we sure that our most basic rules of quantum theory are complete enough for the job of understanding it? (I mentioned this near the end of the discussion. The good news was that I was not burned at the stake…)

I’m not saying I have any answers yet. I’ve been mulling over this and related issues for a number of weeks now, and I have not yet found a good setting to try out the one cool – but highly speculative – idea I have so far. The only thing I’m fairly sure of is that we’ve to make a whole bunch of conceptual leaps to complete the story, not just tinker with the things we already have understood (although, of course, you have to start somewhere, so kudos to those at the coalface doing hard work with the tools we have in work that will surely help show the way…)

I think this means that there are exciting times ahead!


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12 Responses to Heretic…?

  1. Blake Stacey says:

    It does sort of seem that the situations in which quantum gravity poses only practical, technical problems rather than deeply conceptual or philosophical ones are the situations in which a gravity theory has a non-gravitational dual.

  2. Clifford says:

    Well, yes and no. It took a lot of conceptual and philosophical as well as technical hard graft ovr decades to get us to the point where we understood that a quantum gravity system could have a non-gravitational dual. Holography is far from obvious, for example. That is all highly non-trivial, and already quite profound. So while that may all have been a special case of something larger, as I am wondering… It does not make it trivial, or merely technical.



  3. BMc says:

    Please do keep us up to date with your thoughts on this matter. I´m certain that what you are driving at is right, though I have no clue as to how to proceed. Maybe the fact that dS/CFT didn´t work is a clue?

  4. Blake Stacey says:

    Yes, holography is non-obvious and quite interesting! Viscosity bounds, Yangians and integrability of N = 4 super-Yang-Mills, holographic RG, extensions of the Zamolodchikov c-theorem, fluid/gravity correspondence, Kerr/CFT, etc., etc. — I love it, and I’m trying to learn more about it. My fedora’s off to the people who’ve made this much progress so far!

    (And does it seem to you that these applications never get talked about in the glossy magazines, or even on physics blogs? Or, at least, that the coverage is remarkably under-proportioned with respect to the interest physicists have in them, as suggested by dedicated special issues in New J. Phys and so forth?)

    What I was trying to get at up at the top of the comment thread was just this sneaky feeling I have, now and then, when I read someone going on about how hard quantum gravity is. “What could it mean to have a superposition of multiple causal structures of spacetime?” they ask. Well, maybe such questions are easier to address when we have a dual description at hand which is non-gravitational. The existence of a dual boundary theory is, in a handwavy way, a guarantee that the bulk behaviour never gets so weird that we can’t do bookkeeping. It’s when that guarantee goes away that our problems become worse. Getting to the idea of a gauge/gravity duality required big conceptual advances, but maybe, in the scenarios where that idea applies, we don’t need so many more.

    Like I said, it’s just a vague feeling on my non-expert part, made of an unstable isotope of handwavium.

    My own goals for learning about this field are significantly more modest. I’d like to know, for example, more about how taking the non-relativistic limit in the fluid/gravity correspondence seems to give rise to a pressure term in the Navier-Stokes equation which comes out, and whether this incompressibility condition can be avoided, because then we might be able to say something about Kardar-Parisi-Zhang-type surface-growth problems. There’s also a matrix generalization of KPZ which Kardar and Zee cooked up (arXiv:cond-mat/9507112) for which the large-N limit yields planar diagrams, and I’d like to know if that fits into the general stringy-dual picture. . . . Stuff like that. Significantly less grand than cosmology!

  5. Clifford says:


    Yes, I think we more or less agree. And by the way, this is on blog that talks about the applications… I can’t speak for others… They may well exist.



  6. Blake Stacey says:

    I’m certainly happy that Asymptotia has been here to explore and exposit these applications! They show up elsewhere, too, on occasion — Sarah Kavassalis has brought them up over on the PLoS blogs — but the coverage looks quite out of balance with, say, the journal articles or the cross-lists between cond-mat and hep-th.


  7. Clifford says:

    Well maybe, or maybe not. Some coverage is good, but going overboard could be a risk. I think there’s a huge amount of understanding to get right before making daily news headlines out of it…



  8. Blake Stacey says:

    That could be a risk, yes (would that all headline writers in the science press were so cautious…but let’s not spoil the genial atmosphere here by going down that path!).

  9. Clifford says:

    BMc… yes…. dS/FT, or whatever it was supposed to be called, is confusing. I think everyone agrees that’s a major clue that we need something else…. The issue is how far we need to go to find the something else…

    Fun times….


  10. Ele Munjeli says:

    You know, when I first met you, I thought you must be a bohemian of some sort, a writer or artist perhaps. It always cheers me when you rebel against the establishment. We take for granted the best scientific ideas, even scientists, because they are so well engineered it’s hard to stretch thinking beyond it.

    I always think Isaac Newton’s bad ideas are so much harder to comprehend than the ones we all assume as natural laws. But studying bad ideas from great thinkers recognizes the seduction of good ones. Don’t take anything for granted, amigo.

  11. Clifford says:

    Hey Ele,



  12. Elliot says:

    I think I have stated previously that I believe that an information theoretic approach may be the way to make “sense” of quantum behavior. Nothing lately has changed that belief. I am confident things like the uncertainty principle can be derived directly from first principles using information theory. I also believe that there is a direct correlation between gravity and information theory as well. Much more direct than the black hole arguments of Bekenstien etc.

    Maybe something like this will emerge but even if I am way off base (which is entirely possible) I agree that we are “missing something” and some fairly radical new way of thinking about this will be required to put things on a solid conceptual footing.