I am reading the first chapter of Akhiezer, Berestetskii QED (1981). They state that Dirac was wrong to assume that the evolution of the wave function is described by
$\psi(t) = e^{-iHt} \psi(t_0)$
They say that in relativity it is also not legit to try to write a single particle formalism; that we only can describe an ensemble of the particles. Why is that?
Actually, why do we assume time reversal invariance in QM at the first place?