What Does the " Arrow of Time " Mean?

International audienceOne hundred and fifty years after the work of Ludwig Boltzmann on the interpretation of the irreversibility of physical phenomena, we are still not sure what we mean when we talk of " time " or the " arrow of time. " One source of this difficulty is our tendency to confuse time and becoming: that is, the course of time and the arrow of time, two concepts that the formalisms of physics do distinguish clearly. The course of time is represented by a line on which it is customary to place a small arrow that, ironically, must not be confused with the " arrow of time. " On the one hand, this small arrow indicates that the course of time is oriented. On the other hand, the arrow of time indicates the possibility for physical systems to experience, over the course of time, changes or transformations that prevent them from returning to their initial state forever