Information Content of Gravitational Radiation and the Vacuum

TL;DR

This paper extends bounds on information flow at null infinity to include gravitons, showing they vanish in flat spacetime regions without gravitational radiation and are consistent with the equivalence principle.

Contribution

It completes the derivation of information bounds by incorporating gravitons and analyzes their implications in classical gravity without matter.

Findings

Bounds vanish in Minkowski space regions without gravitational radiation

Bounds are compatible with information in regions with Bondi news and gravitational memory

Vacuum degeneracy does not lead to observable consequences if the equivalence principle holds

Abstract

Known entropy bounds, and the Generalized Second Law, were recently shown to imply bounds on the information arriving at future null infinity. We complete this derivation by including the contribution from gravitons. We test the bounds in classical settings with gravity and no matter. In Minkowski space, the bounds vanish on any subregion of the future boundary, independently of coordinate choices. More generally, the bounds vanish in regions where no gravitational radiation arrives. In regions that do contain Bondi news, the bounds are compatible with the presence of information, including the information stored in gravitational memory. All of our results are consistent with the equivalence principle, which states that empty Riemann-flat spacetime regions contain no classical information. We also discuss the possibility that Minkowski space has an infinite vacuum degeneracy labeled by a choice of Bondi coordinates (a classical parameter, if physical). We argue that this degeneracy cannot have any observational consequences if the equivalence principle holds. Our bounds are consistent with this conclusion.