Differing Criteria for Temporal Symmetry

摘要

Consider the ratchet and its patently irreversible behaviour. There is some important sense in which the ratchet is a mechanical device, and its observed behaviour can be well modelled by Newtonian theory: one need only posit a wheel moving under the action of a torque whose magnitude varies drastically with the direction of motion. Do facts like these reveal anything philosophically interesting about the character of classical mechanics? In an earlier discussion in this journal, I suggested that they did, by casting serious doubt on the traditional notion that every mechanical process is time-reversible. Steven Savitt has recently questioned this conclusion, however, by claiming that my discussion employed the wrong notion of time-reversibility.