Galilei Group and Nonrelativistic Quantum Mechanics

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Abstract
This paper is devoted to the study of the Galilei group and its representations. The Galilei group presents a certain number of essential differences with respect to the Poincaré group. As Bargmann showed, its physical representations, here explicitly constructed, are not true representations but only up‐to‐a‐factor ones. Consequently, in nonrelativistic quantum mechanics, the mass has a very special role, and in fact, gives rise to a superselection rule which prevents the existence of unstable particles. The internal energy of a nonrelativistic system is known to be an arbitrary parameter; this is shown to come also from Galilean invariance, because of a nontrivial concept of equivalence between physical representations. On the contrary, the behavior of an elementary system with respect to rotations, is very similar to the relativistic case. We show here, in particular, how the number of polarization states reduces to two for the zero‐mass case (though in fact there are no physical zero‐mass systems in nonrelativistic mechanics). Finally, we study the two‐particle system, where the orbital angular momenta quite naturally introduce themselves through the decomposition of the tensor product of two physical representations.