66 THE RUNNING-DOWN OF THE UNIVERSE 



the laws of Nature a clause requiring that it must go 

 this way round and not the opposite. 



There is a similar reversibility of motion in fields of 

 electric and magnetic force. Another illustration can be 

 given from atomic physics. The quantum laws admit 

 of the emission of certain kinds and quantities of light 

 from an atom; these laws also admit of absorption of 

 the same kinds and quantities, i.e. the undoing of the 

 emission. I apologise for an apparent poverty of illus- 

 tration; it must be remembered that many properties of 

 a body, e.g. temperature, refer to its constitution as a 

 large number of separate atoms, and therefore the laws 

 controlling temperature cannot be regarded as control- 

 ling the behaviour of a single individual. 



The common property possessed by laws governing 

 the individual can be stated more clearly by a reference 

 to time. A certain sequence of states running from past 

 to future is the doing of an event; the same sequence 

 running from future to past is the undoing of it — be- 

 cause in the latter case we turn round the sequence so as 

 to view it in the accustomed manner from past to future. 

 So if the laws of Nature are indifferent as to the doing 

 and undoing of an event, they must be indifferent as 

 to a direction of time from past to future. That is their 

 common feature, and it is seen at once when (as 

 usual) the laws are formulated mathematically. There 

 is no more distinction between past and future than 

 between right and left. In algebraic symbolism, left is 

 — x, right is +*; past is — f, future is +J. This holds 

 for all laws of Nature governing the behaviour of non- 

 composite individuals — the "primary laws", as we shall 

 call them. There is only one law of Nature — the second 

 law of thermodynamics — which recognises a distinction 

 between past and future more profound than the 



