Lorentz covariant physical Brownian motion: Classical and quantum

In this work, we re-examine the Goldstein-Kaç (also called Poisson-Kaç) velocity switching model from two points of view. On the one hand, we prove that the forward and backward Chapman–Kolmogorov equations of the stochastic process are Lorentz covariant when the trajectories are parameterized by their proper time. On the other hand, to recast the model as a quantum random evolution, we restate the Goldstein-Kaç model as a Hamiltonian system, which can then be quantized using the standard correspondence rules. It turns out that the density matrix for the random quantum evolution satisfies a Chapman–Kolmogorov equation similar to that of the classical case, and therefore, it is also Lorentz covariant. To finish, we verify that the quantum model is also consistent with special relativity and that transitions outside the light cone, that is, transitions between states with disjoint supports in space–time, cannot occur.