Observable Supertranslations

Abstract: We show that large gauge transformations in asymptotically flat spacetime can be implemented by sandwiching a shell containing the ingoing hard particles between two finite-width shells of soft gauge excitations. Integration of the graviton Dirac bracket implies that our observable soft degrees of freedom obey the algebra imposed by Strominger on unobservable boundary degrees of freedom. Thus, we provide both a derivation and an observable realization of this algebra. The conservation laws associated with asymptotic symmetries are seen to arise physically from free propagation of infrared modes. This explains in physical terms our recent result that soft charges fail to constrain the hard scattering problem, and so cannot be relevant to the black hole information paradox.