Raphael Bousso

University of California, Berkeley
Information in Free Fall

Unlike all other forces, gravity has resisted a quantum mechanical description. For nearly a century, this difficulty has stood in the way of a complete unification of our description of Nature. More recently, fascinating relations between quantum information and the geometry of spacetime have been discovered. They tie the two realms together, but they also sharpen the conflict. At the horizon of a black hole, we are forced to choose between the central principles of quantum mechanics and of gravitation. Quantum mechanics demands that information is never lost; but this requires the horizon to be a special place in space, a notion that conflicts with a core principle of gravitation. Physics thrives on crisis, and the proposed research aims to take advantage of this sharp paradox. Two possibilities will be investigated. One is to give up standard quantum mechanics for the observer who enters the black hole. A second approach is to modify the conditions under which space emerges empty, so that the horizon is a special place simply because it is not empty space. Either modification would be dramatic. It will be challenging to find implementations that do not conflict with well-tested physics.

There is a conflict between the central principles of quantum mechanics and of general relativity: unitarity and the equivalence principle. This conflict first emerged through Hawking's argument that information is lost in black holes, which rested on the equivalence principle. More recently, it was shown that if information is preserved, the horizon is a violent place, in apparent contradiction with the equivalence principle. However, both arguments invoke a third assumption: that local effective field theory is valid to good approximation outside of the event horizon. It is possible, therefore, that both unitarity and the equivalence principle can be upheld, at the expense of locality.
The proposed research aims to sharpen the conflict between unitarity and general relativity by eliminating this possibility in favor of a weaker assumption. I will also investigate possible resolutions of the paradox. One is the possiblity that quantum mechanics is valid for the S-matrix but must be modified for the infalling observer - and thus, presumably, in cosmology. Another is that the equivalence principle does not actually apply at the horizon of a black hole. This would require a radical revision of the vacuum at the Planck scale. This, too, would likely have implications in cosmology, particularly in eternal inflation.

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