In relativistic quantum physics the possible one-particle states Ψ of a fixed species span a space on which an irreducible unitary representation of the Poincaré group acts. This mathematical structure imparts them their physical properties mass and spin. The mass m is a discrete value out of a conceivable nonnegative continuum and determines the energy \(P^0 = \sqrt {m^2 + {\mathbf {P}}^2}\) as a function of the spatial momentum P. For m > 0 the total spin s of the particle allows for the \(2 s + 1 \in \mathbb N\) possible values s, s − 1, …, −s of angular momentum in some arbitrarily chosen direction. For m = 0 the particle’s angular momentum in direction of P, its helicity h, is some fixed integer or half-integer, \(2h\in \mathbb Z\) .