On the real representations of the Poincare group

The formulation of quantum mechanics with a complex Hilbert space is equivalent to a formulation with a real Hilbert space and particular density matrix and observables. We study the real representations of the Poincare group, motivated by the fact that the localization of complex unitary representations of the Poincare group is incompatible with causality, Poincare covariance and energy positivity. We review the map from the complex to the real irreducible representations---finite-dimensional or unitary---of a Lie group on a Hilbert space. Then we show that all the finite-dimensional real representations of the identity component of the Lorentz group are also representations of the parity, in contrast with many complex representations. We show that any localizable unitary representation of the Poincare group, compatible with Poincare covariance, verifies: 1) it is a direct sum of irreducible representations which are massive or massless with discrete helicity. 2) it respects causality; 3) if it is complex it contains necessarily both positive and negative energy subrepresentations 4) it is an irreducible representation of the Poincare group (including parity) if and only if it is: a)real and b)massive with spin 1/2 or massless with helicity 1/2. Finally, the energy positivity problem is discussed in a many-particles context.