  In the theory of special relativity, energy can be found in two forms:
kinetic energy and rest mass. Potential energy of a body is actually stored
under the form of rest mass, interaction energy too, temperature is not.
Information acquired about a dynamical system can be potentially used to
extract useful work from it. Hence the "mass-energy-information equivalence
principle" that has been recently proposed. In this paper, it is first recalled
that for a thermodynamic system made of non interacting entities at constant
temperature, the internal energy is also constant. So that, the energy involved
in a variation of entropy ($T\Delta S$) differs from a change in potential
energy stored or released and cannot be associated to a corresponding variation
of mass of the system, even if it is expressed in term of quantity of
information. This debate gives us the opportunity to deepen the notion of
entropy seen as a quantity of information, to highlight the difference between
logical irreversibility (a state dependent property) and thermodynamical
irreversibility (a path dependent property) and to return to the nature of the
link between energy and information that is dynamical.
