The building blocks of asymptotically flat spacetimes

The classification of all possible induced representations arising from theories admitting a Poincar\'e symmetry has molded our very conception of particles in flat space. In this note, we argue that if one takes the same viewpoint on the asymptotic symmetry group instead of the global one, the familiar relativistic classification is enhanced to provide a new notion of excitations in asymptotically flat spacetimes. These excitations blur the line between the quantum description of a particle and a spacetime geometry, and will appear in any theory with the same symmetry. By interacting together, they produce both the geometry of and propagating particles in spacetime.