The main purpose of the paper is an essentially probabilistic analysis of relativistic quantum mechanics. It is based on the assumption that whenever probability distributions arise, there exists a stochastic process that is either responsible for temporal evolution of a given measure or preserves the measure in the stationary case. Our departure point is the so-called Schr{o}dinger problem of probabilistic evolution, which provides for a unique Markov stochastic interpolation between any given pair of boundary probability densities for a process covering a fixed, finite duration of time, provided we have decided a priori what kind of primordial dynamical semigroup transition mechanism is involved. In the nonrelativistic theory, including quantum mechanics, Feyman-Kac-like kernels are the building blocks for suitable transition probability densities of the process. In the standard free case (Feynman-Kac potential equal to zero) the familiar Wiener noise is recovered. In the framework of the Schr{o}dinger problem, the free noise can also be extended to any infinitely divisible probability law, as covered by the L{e}vy-Khintchine formula. Since the relativistic Hamiltonians $| abla |$
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