72 THE RUNNING-DOWN OF THE UNIVERSE 



the whole vessel and remain so ever afterwards. The 

 molecules will not return to one half of the vessel; the 

 spreading cannot be undone — unless other material is 

 introduced into the problem to serve as a scapegoat for 

 the disorganisation and carry off the random element 

 elsewhere. This occurrence can serve as a criterion to 

 distinguish past and future time. If you observe first 

 the molecules spread through the vessel and (as it 

 seems to you) an instant later the molecules all in one 

 half of it — then your consciousness is going backwards, 

 and you had better consult a doctor. 



Now each molecule is wandering round the vessel 

 with no preference for one part rather than the other. 

 On the average it spends half its time in one compart- 

 ment and half in the other. There is a faint possibility 

 that at one moment all the molecules might in this way 

 happen to be visiting the one half of the vessel. You 

 will easily calculate that if n is the number of molecules 

 (roughly a quadrillion) the chance of this happening is 

 ( y 2 ) n . The reason why we ignore this chance may be seen 

 by a rather classical illustration. If I let my fingers 

 wander idly over the keys of a typewriter it might 

 happen that my screed made an intelligible sentence. 

 If an army of monkeys were strumming on typewriters 

 they might write all the books in the British Museum. 

 The chance of their doing so is decidedly more favour- 

 able than the chance of the molecules returning to one 

 half of the vessel. 



When numbers are large, chance is the best warrant 

 for certainty. Happily in the study of molecules and 

 energy and radiation in bulk we have to deal with a 

 vast population, and we reach a certainty which does 

 not always reward the expectations of those who court 

 the fickle goddess. 



