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(abridged)If the space-time is presupposed, the coordinate representation ofthe solutions $\psi(\vec x, t)$ of the Schroedinger equation of a quantumsystem containing one massive scalar particle has a {\it preferred status}. Itis then possible to perform a multipolar expansion of the density matrix$\rho(\vec x, t) = |\psi(\vec x, t)|^2$ (and more generally of the Wignerfunction) around a space-time trajectory ${\vec x}_c(t)$ to be properlyselected. A special set of solutions $\psi_{EMWF}(\vec x, t)$, named {\itEhrenfest monopole wave functions}(EMWF), is characterized by the conditionsthat: (i) the quantum expectation value of the position operator coincides atany time with the searched classical trajectory, $< \psi_{EMWF} | {\hat {\vecx}} | \psi_{EMWF}> = {\vec x}_c(t)$: this is possible only when the dipolevanishes; (ii) Ehrenfest's theorem holds for the expectation values of theposition and momentum operator: its application to EMWF leads then to a {\itclosed Newton equation of motion for the classical trajectory, where theeffective force is the Newton force plus non-Newtonian terms (of order$\hbar^2$ or higher) depending on the higher multipoles of the probabilitydistribution $\rho$.} These results can be extended to N particle systems andto relativistic quantum mechanics. There is substantial agreement with Bohr'sviewpoint: the macroscopic description of the preparation, certain intermediatesteps and the detection of the final outcome of experiments involving massiveparticles are dominated by these 'classical {\it effective} trajectories'. Inthe framework of decoherence, one gets a transition from an {\it improperquantum} mixture to a {\it classical statistical} one, when both the particleand the pointer wave functions appearing in the reduced density matrix areEMWF.

On the Transition from the Quantum to the Classical Regime for Massive Scalar Particles: A Spatiotemporal Approach