The arrow of time

A preferred direction in time is defined by four classes of physical processes: entropy generating processes, which define the thermodynamic arrow; information-generating processes, which define the historical arrow; the cosmic expansion, which defines the cosmological arrow; and the decay of neutral kaons, which defines the microscopic arrow. A theory is developed which shows that the thermodynamic, historical, and cosmological arrows can be derived from two closely related cosmological postulates. The first postulate states that no statistical property of the Universe serves to define a preferred position or direction in space. The second postulate states that local thermodynamic equilibrium prevails near the cosmological singularity. Expansion from the singularity is shown to generate macroscopic information as well as entropy; the widely held view that the Universe is running down is shown to rest on a mistaken assumption concerning the relationship between information and entropy.