  Bohmian mechanics, a hydrodynamic formulation of quantum mechanics, relies on
the concept of trajectory, which evolves in time in compliance with dynamical
information conveyed by the wave function. Here this appealing idea is
considered to analyze both classical and quantum interference, thus providing
an alternative and more intuitive framework to understand the time-evolution of
waves, either in terms of the flow of energy (for mechanical waves, sound
waves, electromagnetic waves, for instance) or, analogously, the flow of
probability (quantum waves), respectively. Furthermore, this procedure also
supplies a more robust explanation of interference phenomena, which currently
is only based on the superposition principle. That is, while this principle
only describes how different waves combine and what effects these combinations
may lead to, flow lines provide a more precise explanation on how the energy or
probability propagate in space before, during and after the combination of such
waves, without dealing with them separately (i.e., the combination or
superposition is taken as a whole). In this sense, concepts such as
constructive and destructive interference, typically associated with the
superposition principle, physically correspond to more or less dense swarms of
(energy or probability) flow lines, respectively. A direct consequence of this
description is that, when considering the distribution of electromagnetic
energy flow lines behind two slits, each one covered by a differently oriented
polarizer, it is naturally found that external observers' information on the
slit crossed by single photons (understood as energy parcels) is totally
irrelevant for the existence of interference fringes, in striking contrast with
what is commonly stated and taught.
