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Tom Banks anti-landscape paper

  1. Aug 29, 2012 #1
    "The Top 10500 Reasons Not to Believe in the Landscape".

    Tom Banks has written a lot of papers developing his unique perspective on how quantum gravity and string theory should work in the real world (specifically, in a universe with a small positive cosmological constant), and this is one of the better papers. Even the joke in the title works better than usual. :-)

    Part of the larger context here is, how to understand quantum gravity in de Sitter space. There is something of a standard view, which combines eternal inflation, the string landscape, and the anthropic principle, along with the idea of de Sitter space as only metastable.

    Banks attacks many of the standard assumptions. He thinks solutions of string theory in de Sitter spaces are island theories with different parameters, not different vacua in the same theory. He rejects the common idea of "uplifting" anti-de-Sitter solutions to de Sitter space. He thinks eternal inflation is based on a misapplication of Coleman-DeLuccia models of tunneling between vacua.

    Actually I find rather a lot to disagree with, in what he says. The fact that eternal de Sitter spaces should be dominated by "Boltzmann brains" (observers created by vacuum fluctuations) and even "Boltzmann universes", I regard as a point against the idea of asymptotic stability of de Sitter space. The fact that the model of dS/CFT within Vasiliev gravity is a modification of a Vasiliev AdS/CFT duality, is evidence against his proposition that there's not much connection between AdS and a dS uplift. And the quasi-positivistic propensity for regarding the universe beyond our cosmological horizon as unreal is bad methodology (as well as nonsensical, it implies that galaxies cease to exist just because they have dropped out of our sight), a typically quantum distortion of epistemology into ontology.

    (Just to be more precise on that last point: Because progress in quantum mechanics was achieved by focusing on observable quantities - and because progress on realist subquantum theories has been slow - there is a significant sector of opinion in physics which regards unobservable things as nonexistent. But that is a big methodological mistake. The focus on observable quantities is meant to keep your speculations grounded in evidence, it doesn't mean that you must regard as unreal something that you can't observe. In the case of quantum gravity in de Sitter space, I think this bad methodology is actively leading people astray, when they regard the world beyond our horizon as unreal.)

    Despite these criticisms, I like this paper. It's rich in practical technical knowledge and perspectives on string theory, such as the steps from string theory proper to effective field theory. And the breadth of Banks's criticism helps just to see what the prevailing paradigm is and how it works.

    If the choice is between Banks and "Susskind et al", where Susskind et al means the synthesis of eternal inflation, string landscape, and anthropic principle, I'm inclined to say, neither. The truth ought to be less messy than the anthropic picture, but I think Banks is getting too many things wrong. There may be some important technical ideas buried in there, however.
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  3. Sep 1, 2012 #2


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    I think the trick to get dS/CFT from higher spin AdS/CFT doesn't work for regular AdS/CFT - someone asked Hartmann this in one of his talks, and I think that was what he replied.
  4. Sep 4, 2012 #3

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    I've never quite understood what's going on with Bank's papers on this subject. This seems like the sort of thing we ought to just be able to sort out. Can these vacua tunnel into each other or not? Most people I know, some of them even quite reasonable :), seem to think that they can, but clearly Banks thinks that they cannot. However, this makes me think that maybe the issue is partially of terminology? Perhaps one cannot grow the whole asymptotic spacetime, yet a chunk of it still makes sense?
  5. Sep 4, 2012 #4
    As a concrete counterpoint to Banks, we could use: "Populating the Whole Landscape". "Every de Sitter vacuum can transition to every other de Sitter vacuum despite any obstacle, despite intervening anti-de Sitter sinks, despite not being connected by an instanton. Eternal inflation populates the whole landscape." Can we reconstruct what the logic of a Banksian rejoinder to that paper would be?
  6. Sep 5, 2012 #5
    See foonote 2 in Anninos et al. To go from negative to positive cosmological constant, you go from N to -N in certain expressions, which you can obtain by replacing O(N) with Sp(N) as the gauge group on the boundary. What Hartmann said at the Strings 2011 Gong Show was that, to do the same thing for stringy AdS/CFT, you'd be going to imaginary values of N, not negative ones.

    The best studied case of AdS/CFT is the duality between N=4 YM and the IIB string in AdS5 x S5, so logically we should be interested in the IIB string in dS5 x S5. There's been some scattered work on that topic, so I might post about it in the dS/CFT thread.
  7. Sep 5, 2012 #6

    Physics Monkey

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    This is an interesting paper. They seem to emphasize considerations of finite volume which I was alluding to above.

    I'm honestly not sure what Banks would say, but it is quite strange that we have here two papers claiming in completely absolute terms two completely opposite things. I continue to suspect that the Banks is talking about something a bit different e.g. where the whole spacetime, boundary conditions and all, changes. But I could be misreading him.
  8. Sep 10, 2012 #7


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    Is Banks saying something like the only non-perturbative definitions of string theory we have are AdS/CFT. Each CFT is a different theory, so there are many string theories, not one, and string theory is more a language like QFT, rather than a single TOE.

    BTW, I came across the interesting comment from Polchinksi that AdS/CFT probably implies BFSS.
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