118 THE PATH OF SCIENCE 



More and more, as these galaxies are being observed, it be- 

 comes clear that there are enormous numbers of galaxies and 

 that we have to think of the universe not as composed of 

 millions of stars but as composed of millions of galaxies, each 

 composed of hundreds of millions of stars. 



When a star is observed with a spectroscope, the absorption 

 lines corresponding to certain elements are not found at 

 exactly the same wave length as those lines show in the 

 laboratory. The explanation of this was given as long ago 

 as 1842 by Christian Doppler, who showed that if a lumi- 

 nous body is moving in the line of sight, the frequency of 

 the light emitted will be changed by its velocity. If a star is 

 coming toward us, we shall receive more light waves of a 

 given ray in a given time than if the star were standing still. 

 The frequency, therefore, of the light will be increased, and 

 a spectral line will be moved toward the blue. If the star is 

 moving away from us, the spectral line will move toward the 

 red. When the light of the most distant nebulae was ob- 

 served, it was found that the lines were strongly displaced 

 toward the red and that this displacement increased in pro- 

 portion to the faintness of the nebula and therefore pre- 

 sumably in proportion to its distance. The effect is so great 

 that the picture obtained is that of an exploding universe, 

 one in which the outer nebulae are retreating in all directions 

 as if the whole universe were expanding. The mathematical 

 astronomers have analyzed the suggestion that the universe 

 may be considered to be expanding, using as their basis Ein- 

 stein's general theory of relativity, in which the four-dimen- 

 sional universe involving the three dimensions of space and 

 time may be considered a closed system and the expansion of 

 this closed system can be reconciled with the principles of the 

 general field theory. 



