Henry Maxfield

Recent discoveries suggest that semiclassical gravity is more consistent with unitarity than previously believed. I will argue that it makes predictions for the measurements of asymptotic observers that are in complete accord with the idea that black holes are ordinary quantum systems, with states counted by the Bekenstein-Hawking formula. The argument uses the semiclassical gravitational path integral, incorporating newly discovered `spacetime wormhole' topologies. These new ideas revive an old paradigm, relating the information problem to the physics of baby universes.

Hawking famously observed that the formation and evaporation of black holes appears to violate the unitary evolution of quantum mechanics. Nonetheless, it has been recently discovered that a signature of unitarity, namely the "Page curve" describing the evolution of entropy, can be recovered from semiclassical gravity. This result relies on "replica wormholes" appearing in the gravitational path integral, which are examples of spacetime wormholes studied more than 30 years ago and related to interactions with closed "baby" universes.