  By modelling quantum systems as emerging from a (classical) sub-quantum
thermodynamics, the quantum mechanical "decay of the wave packet" is shown to
simply result from sub-quantum diffusion with a specific diffusion coefficient
varying in time due to a particle's changing thermal environment. It is thereby
proven that free quantum motion strictly equals ballistic diffusion. The exact
quantum mechanical trajectory distributions and the velocity field of the
Gaussian wave packet are thus derived solely from classical physics. Moreover,
also quantum motion in a linear (e.g., gravitational) potential is shown to
equal said ballistic diffusion. Quantitative statements on the trajectories'
characteristic behaviours are obtained which provide a detailed "micro-causal"
explanation in full accordance with momentum conservation.
