1 84 THE QUANTUM THEORY 



forms of radiation. It applies to light absorbed by an 

 atom as well as to light emitted, the absorption being 

 discontinuous also. Evidently h is a kind of atom — 

 something which coheres as one unit in the processes of 

 radiation; it is not an atom of matter but an atom or, 

 as we usually call it, a quantum of the more elusive 

 entity action. Whereas there are 92 different kinds of 

 material atoms there is only one quantum of action — 

 the same whatever the material it is associated with. 

 I say the same without reservation. You might perhaps 

 think that there must be some qualitative difference 

 between the quantum of red light and the quantum of 

 blue light, although both contain the same number of 

 erg-seconds; but the apparent difference is only relative 

 to a frame of space and time and does not concern the 

 absolute lump of action. By approaching the light- 

 source at high speed we change the red light to blue 

 light in accordance with Doppler's principle; the energy 

 of the waves is also changed by being referred to a 

 new frame of reference. A sodium flame and a hydro- 

 gen flame are throwing out at us the same lumps of 

 action, only these lumps are rather differently orientated 

 with respect to the Now lines which we have drawn 

 across the four-dimensional world. If we change our 

 motion so as to alter the direction of the Now lines, 

 we can see the lumps of sodium origin under the same 

 orientation in which we formerly saw the lumps of 

 hydrogen origin and recognise that they are actually the 

 same. 



We noticed in chapter iv that the shuffling of energy 

 can become complete, so that a definite state is reached 

 known as thermodynamical equilibrium; and we re- 

 marked that this is only possible if indivisible units are 

 being shuffled. If the cards can be torn into smaller and 



