Time, direction of

You can pour a tumblerful of water into the sea, but you can never get that same tumblerful of water out again. James Clerk Maxwell gave this as an example of an irreversible process. There are many other homely examples: coffee and milk will mix if stirred, but white coffee does not unmix if stirred in reverse. An ice cube in a glass of hot water will melt, but we never see water at room temperature spontaneously separate into ice and hot water. Physical theories like thermodynamics or hydrodynamics, which codify this type of irreversible phenomenon, do not allow the same kind of behaviour in the forward and backward direction of time. There is thus a striking asymmetry in the two temporal directions. This is usually referred to as the ‘direction of time’ (or ‘time asymmetry’ or ‘anisotropy’ or the ‘arrow of time’).

The source of this asymmetry has been sought in various theories of physics, both classical and quantum. Some explanations appeal to some sort of boundary condition, typically an initial condition, which the explanation admits to be, not a law of the theory, but a matter of happenstance. Other explanations advocate some additional general principle about, for example, temporally asymmetric notions of causality or randomness.