A canonical realization of the BMS algebra

A canonical realization of the BMS (Bondi-Metzner-Sachs) algebra is given on the phase space of the classical real Klein-Gordon field. By assuming the finiteness of the generators of the Poincare' group, it is shown that a countable set of conserved quantities exists (supertranslations); this set transforms under a particular Lorentz representation, which is uniquely determined by the requirement of having an invariant four-dimensional subspace, which corresponds to the Poincare' translations. This Lorentz representation is infinite-dimensional, nonunitary, reducible and indecomposable. Its representation space is studied in some detail. It determines the structure constants of the infinite-dimensional canonical algebra of the Poincare' generators together with the infinite set of the new conserved quantities. It is shown that this algebra is isomorphic with that of the BMS group.