EVOLUTION AND ENTROPY 309 



As in the case of measuring cosmic distances, it may be 

 profitable to make a brief summary of the method employed 

 to reach the verdict of an expanding universe. This method 

 makes use of an analogy between light and sound. When, 

 for instance, a fast moving train approaches a by-stander near 

 the track there is a rise in the whistle's pitch and, as the train 

 recedes, a noticeable lowering of pitch. The physical reason 

 given for this phenomenon is the addition and subtraction of 

 frequency or wave-length because of the moving sound-source. 

 As the train approaches, its own motion is added to that of 

 the sound thus making for a shorter wave-length and higher 

 pitch of the whistle; as the train recedes, there is a net 

 lengthening of the sound wave and hence a lower frequency 

 or lower pitch. 



Something similar is believed to happen in the case of light 

 waves reaching the earth from distant galaxies. The wave- 

 length is shifted toward the red or longer wave lengths of the 

 visible spectrum, indicating in the italicized words of George 

 Gamow " that " the entire space of the universe populated by 

 billions of galaxies, is in a state of rapid expansion, with all 

 of its members flying from each other at high speeds." 



The expansion of the universe was proposed as a principle of 

 cosmogony by Abbe Georges Lemaitre who postulated a " pri- 

 meval atom " in which all the elementary particles of matter 

 were densely packed together. Lemaitre regards this Ur-atom 

 as an isotope of a neutron.^^ Gamow, who is in sympathy with 

 this type of theory, has written: 



The nearest guess is that the overall density of the universe at the 

 time was comparable to that of a nuclear fluid tiny droplets of 

 which form the nuclei of various atoms. This would make the 

 original pre-expansion density of the universe a hundred thousand 

 billion times greater than the density of water; each cubic centi- 

 meter contained at that time a hundred million tons of matter." 



19 



The Creation of the Universe (New York, 1952) , p. 23. 

 The Primeval Atom (New York, 1950) p. 142. 

 Op. cit., p. 19. 



