"It is only an alternation of energy that operates here" is the visible inscription on a man's grave where another man can be seen lying on his back. The man, professor of Physics, is explaining the theory of relativity "One of the most important concepts used in the description of natural phenomena is that of energy ..." In physics, this is always associated with some process or some type of activity: " it can change shape, but nothing is lost… ". During the explanation, he traces, outside of chronological conventions, in an alternation of situations and thoughts, even paradoxical ones, part of that path lived with his man, abruptly interrupted by death.