Introduction

Summary

Eddington and the running-down of the universe

The phrase ‘time's arrow’ seems to have entered the discussion of time in Sir Arthur Eddington's Gifford Lectures, which were published in 1928. An ‘arrow’ of time is a physical process or phenomenon that has (or, at least, seems to have) a definite direction in time. The time reverse of such a process does not (or, at least, does not seem to) occur. Eddington thought he had found such an arrow in the increase of entropy in isolated systems. He wrote:

The law that entropy always increases – the second law of thermodynamics – holds, I think, the supreme position among the laws of Nature.

Since he held the universe to be an isolated system, he thought that its entropy, which he called its ‘random element’, must ineluctably increase until it reached thermodynamic equilibrium (until it is ‘completely shuffled’), by which point all life, and even time's arrow itself, must have disappeared. He called this process ‘the running-down of the universe’. This vision of the universe is stark, compelling, and by no means hopelessly dated. P. W. Atkins recently wrote:

We have looked through the window on to the world provided by the Second Law, and have seen the naked purposelessness of nature. The deep structure of change is decay; the spring of change in all its forms is the corruption of the quality of energy as it spreads chaotically, irreversibly, and purposelessly in time. All change, and time's arrow, point in the direction of corruption. […]