## Tuesday, October 27, 2020

### Does a unitary black hole evolution imply the absence of global symmetries?

I won't write a full-blown blog post because the benefits don't surpass the costs in this world increasingly dominated by morons but I will mention that a nice paper
Global symmetry, Euclidean gravity, and the black hole information problem
was written by Daniel Harlow (yes, that one) and Edgar Shaghoulian a week ago. It was dedicated to Leonard Susskind for his 80th birthday. Congratulations, too.

Quantum gravity has various general features, at least believed features, and they include the unitary evaporation of the black holes; and the absence of global symmetries. These features are supported by various arguments that are more or less rooted in AdS/CFT; more or less rooted in string theory; more or less rigorous; more or less universal. With a broad enough definition of quantum gravity, exceptions may exist in spacetimes of a low enough dimension.

These authors claim that the unitary resolution of the black hole information loss paradox implies that global symmetries don't exist. The former is almost a "self-evident consistency fact resulting from quantum mechanics itself" while the second is a "nontrivial dynamical swampland-like principle where one apparently needs the details of string theory". Nevertheless, the latter principle could follow from the former. They presented various arguments at various levels. A perhaps more speculative corollary conjectures that holography is equivalent to the existence of a Euclideanized treatment of quantum gravity.