  Two well-known conceptual conundrums of quantum mechanics referred to as
instantaneous action-at-a-distance and inseparable wave-particle character are
tackled using the principle of least action. Since any measurement is an
action, it is reasoned that the spin of a particle just as the polarization of
a photon remain indetermined for the observer until at least a quantum of
action flows from the object to the observer. The detection places the quantity
in question in the observer's frame of reference. This reference frame for one
photon will instantaneously apply also to the polarization of the other photon
provided that the two photons have not been perturbed ever since the correlated
pair emerged from a radiative decay. The wave-particle duality of a single
photon or an electron, that the double-slit experiment demonstrates, can also
be understood in terms of actions. The energy density difference between the
source of particles and their sink at the detector is a driving force that will
cause flows of energy densities such as photons or electrons to propagate down
along the paths of least action. Since no space is empty of energy density, the
propagating particle as a flow of density will invariably perturb surrounding
densities. When the driving density difference including the perturbation is
leveling off via two or more paths, the ensuing flows of densities depend on
each other. Therefore the flows through the two slits can never be separated
from each other which will manifest as the inseparable particle-wave character.
Keywords: action-at-a-distance; double slit experiment; energy dispersal;
evolution; quantum paradoxes; the principle of least action
