Null infinity as an open Hamiltonian system

A bstract When a system emits gravitational radiation, the Bondi mass decreases. If energy is Hamiltonian, it can thus only be time-dependent Hamiltonian. In this paper, we show that understood as Hamiltonian on covariant phase space. Our derivation starts from formulation in domains with boundaries are null. We introduce most general boundary conditions generic such null boundary, and compute quasi-local charges for boosts, angular momentum. Initially, these at finite distance, there natural IR regulator. To remove regulator, double foliation together an adapted Newman-Penrose tetrad. Both directions surface orthogonal. study falloff specific foliations take limit to infinity. At infinity, recover usual space two radiative modes full non-perturbative level. Apart technical results, framework gives important physical insights. First of all, explains significance corner term added Wald-Zoupas render quasi-conserved integrable. The simply derivative respect background fields drive time-dependence Secondly, propose new interpretation thermodynamical free edge future law then statement always decreases its way towards thermal equilibrium.