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Mudassir Ali

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Originally Answered: Ron maimon what do you think about the following paper and this general idea that gravity might be an emergent force Page on Arxiv?

Wen is a great physicist, and he had an idea. I don’t want to criticize it too much, because it is a new idea, and you need to respect this. In this case, his observations are similar to those of Zaanen from the late 1990s, who noticed that elasticity theory resembles GR mathematically, but there, atomic scale defects add a new twist. Wen is pointing out that you can make a theory of spin 2 particles using some crazy lattice Lagrangian on a bunch of scalars. I didn’t check it, it’s probably true that the excitations are spin-2 at long distances, it is possible to make such a thing, and Wen usually knows what he is doing.

But there is no way in heck to produce a quantum gravity from any sort of lattice field, at best you get a peturbation theory of long-wavelength gravitons. The paradoxes of black holes show that you can’t describe gravity with field theory, because the field theory description must break down near the horizon, where the ‘t Hooft quantum field entropy diverges. The black hole entropy is finite and proportional to the area.

To reconcile these ideas, the main lesson of string theory is that you can’t stay in field theory, you need a holographic description, and such a description is always formulated on asymptotic states, on boundaries, either at flat infinity, as in S-matrix theory, on on black hole horizons, as in AdS/CFT. So Wen’s model, while probably producing a spin-2 excitation, doesn’t reproduce gravitational physics in a physical way, in a way consistent with black hole entropy and holographic principle.