Irreversibility and the second law of thermodynamics

The second law of thermodynamics has a curious status. Manymodern physicists regard it as an obsolete relic from a bygone age, while many others, even today, consider it one of the most firmly established and secure achievements of science ever accomplished. From the perspective of the foundations of physics, a particularly interesting question is its relationship with the notion of irreversibility. It has often been argued, in particular by Planck, that the second law expresses, and characterises, the irreversibility of all natural processes. This has led to much debated issues, such as whether the distinction between past and future can be grounded in the second law, or how to reconcile the second law with an underlying microscopic mechanical (and hence reversible) theory. But it is not easy to make sense of these debates since many authors mean different things by the same terms. The purpose of this paper is to provide some clarification by distinguishing three different meanings of the notion of (ir)reversibility and to study how they relate to different versions of the second law. A more extensive discussion is given in [23].