(Ir)reversibility and entropy

Abstract

Time's arrow is part of our sense experience and we experience it every day: broken mirrors do not come back together, human beings don't rejuvenate, rings grow unceasingly in tree trunks, we have memories of past events and not of future events. In sum, time always flows in the same direction ! Nonetheless, the fundamental laws of classical physics don't favor any time direction and conform to a rigorous symmetry between past and future. It is possible, as discussed in the article by T. Damour in this same volume, that irreversibility is inscribed in other physical laws, for example on the side of general relativity or quantum mechanics. Since Boltzmann, statistical physics has advanced another explanation: time's arrow translates a constant flow of less probable events toward more probable events. Before continuing with this interpretation, which constitutes the guiding principle of the whole exposition, I note that the flow of time is not necessarily based on a single explanation.