MATTER, SPACE, AND TIME 245 



rediscover the persistence of matter and the 

 conservation of energy. As Professor Eddington 

 disturbingly suggests, every law of nature which 

 seems to us rational may be a concealed con- 

 vention which we have ourselves unconsciously 

 inserted. Hence an unavoidable conclusion which 

 yet seems to us irrational may be the sign of 

 transcendent importance — the sign of a real law 

 of nature at last. If so, we seem almost brought 

 back to Tertullian's credo qttid impossibile. 



The new outlook on physics was first suggested 

 to Planck by the facts of radiation. If the aether 

 be continuous, all radiant energy must pass from 

 matter to aether, just as the energy of floating 

 bodies set in vibration passes into the surround- 

 ing water. If we are to hold any mechanical 

 view of the aether, we must therefore consider 

 that it too possesses a structure, though probably 

 much finer than that of matter. Even if we take 

 it as of the same order of fineness as that of 

 matter, mathematical calculation shows that most 

 of the energy radiated by matter should be 

 concentrated in the short wave-lengths of the 

 ultra-violet light. But observation shows that 

 in a continuous spectrum, such as that of the 

 sun, the maximum heat effect, which measures 

 the total energy, is in the infra-red, that is, in 

 those waves too long to affect our eyes instead 

 of in those too short. 



To meet this difficulty Planck, in 1901, 

 supposed that radiation was emitted and absorbed 

 by matter not continuously but in small indi- 

 visible units. To calculate the rate of emission 

 of energy then becomes an exercise in the theory 

 of probability — how many units are likely to be 



