## January 10, 2016

### BMiSsed

There’s a general mantra that we all repeat to ourselves: gauge transformations are *not symmetries*; they are *redundancies* of our description. There is an exception, of course: gauge transformations that don’t go to the identity at infinity *aren’t* redundancies; they are actual symmetries.

Strominger, rather beautifully showed that BMS supertranslations (or, more precisely, a certain diagonal subgroup of $\text{BMS}^+$ (which act as supertranslations on $\mathcal{I}^+$) and $\text{BMS}^-$ (which act as supertranslations on $\mathcal{I}^-$) are symmetries of the gravitational S-matrix. The corresponding conservation laws are equivalent to Weinberg’s Soft-Graviton Theorem. Similarly, in electromagnetism, the $U(1)$ gauge transformations which don’t go to the identity on $\mathcal{I}^\pm$ give rise to the Soft-Photon Theorem.

A while back, there was considerable brouhaha about Hawking’s claim that BMS symmetry had something to do with resolving the blackhole information paradox. Well, finally, a paper from Hawking, Perry and Strominger has arrived.