The Arrow of Time

IT is widely believed that all irreversible mechanical processes involve an increase of entropy, and that ‘classical’ (that is, non-statistical) mechanics, of continuous media as well as of particles, can describe physical processes only in so far as they are reversible in time 1 . This means that a film taken of a classical process should be reversible, in the sense that, if put into a projector with the last picture first, it should again yield a possible classical process.