Asymptotically flat space--times and elementary particles

It is suggested that elementary particles should be defined as irreducible unitary representations of the group of asymptotically flat space-times in general relativity. The implications of such a definition are described in physical terms. In particular, it is pointed out that this definition (unlike the corresponding "flat space-time" definition) explains the observed discreteness of elementary-particle spins. The possibility of using the definition to explain "internal" symmetries is discussed.