  The principle of invariance of the velocity of light is only valid for the
wrong measurements of inertial observers who ignore their own movement and
consider themselves at rest. The Langevin (or clock) paradox arises when it is
assumed that the real velocity (and not only the wrong measured velocity) of
light in free space can be the same for different observers. Nevertheless, it
is generally assumed as true such incoherent assumption that leads to the
paradox. To clarify the issue is of great interest to analyse the physics
underlying the phenomenon of time dilation (which is not only the result of a
mathematical formula) and this has been done in this work. The exact value of
light dragging coefficient is also derived.
