• 1 Centre National de la Recherche Scientifique

Etienne Klein’s lecture has well put in situation the treatment of time and that of the irreversibility of physical phenomena by physicists since Galileo until Einstein. Starting from the Einstein’s relativity, the first part of this paper will recall some features of contemporary physics that should be kept in mind when speaking about time : the necessity to resort to the concept of space-time and its structure ; the Langevin’s traveler will be used for justifying the introduction of gravitation in general relativity theory ; the contrast between the law of entropy increase in isolated systems and the possibility of self-organization in open systems will be emphasized. In the second part, the absence of the notion of present in physical theory will be taken up; Whitehead’s philosophy of nature is introduced, the central point of which about time is the non-existence of instants of time. Despite its limitations, the Whitehead’s philosophy of nature opens perspectives for a metaphysics of time that is compatible with both quantum mechanics and relativity. Furthermore, it has the merit of proposing an explanation of the present and of the mechanisms of passage from potentialities to concrete realities.