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mitchell porter
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"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.
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.