ORIGIN OF THE EARTH—PAGE 171 
spiral nebulae, apparently without limit. In 1925 Hubble and Huma- 
son found from the redness of their light that the more distant spirals 
are receding from us more rapidly than the closer ones, and that the 
speed of their retreat is in direct proportion to their distance from us. 
At first sight this appears to leave our galaxy (with our sun and earth) 
in a central and somewhat unpopular position, with the rest of the 
universe running away. But a little thought shows that our view of 
the universe is the same as the view from any one of the other galaxies; 
each would see the rest receding from him with velocities proportional 
to their distances from him. 
x Y z Us A 8B c D 
haat I go o> o> oo eae 
x z U: 8 ¢c o 
ee) mee: <—- er Pi) e > — 
Ficure 3.—Upper, our view of some spiral nebulae. The arrows indicate veloc- 
ities. Note that spiral B, which is twice as far from us as spiral A, is receding 
twice as fast. OC, three times as far, is receding three times as fast, and so on. 
Lower, the view of an observer on spiral B, considering himself to be at rest. 
It is the principle of relativity that he has just as much right as we do to con- 
sider himself at rest. He gets the same view as we do; all the spirals are 
receding from B with velocity proportional] to distance. 
Tracing the motions back in time (there is no evidence that the 
spirals are accelerating or decelerating) shows that all the spiral 
nebulae would have been near our galaxy between 2 and 3 billion 
years ago. The coincidence of this with the age of the earth and the 
age of the meteorites is too marked to need further comment—the 
whole universe seems to have started with a bang about 3 billion years 
ago! 
‘ THE BEGINNING OF TIME 
This curious evidence that the spiral nebulae were all close to—if 
not entangled with—our galaxy 8 billion years ago, means that the 
formation of the solar system at that time probably took place under 
conditions somewhat different from those of today. To be sure of the 
reasoning, we must examine the conditions of 3 billion years ago more 
carefully; it was this reexamination which led, in 1945, to the most 
bizarre suggestion of all in this field already rich in speculation. It 
was put forward by the English biologist, J. B. S. Haldane, and is 
based on a new theory—or philosophy—of relativity proposed in 1932 
by the English mathematician, E. A. Milne. First we shall speak of 
Milne and his brand of relativity. 
To make the reasoning clear we must start with Einstein’s earlier 
relativity theory which links space and time in such a way that if 
one observer is moving at constant velocity past another his 
measurements of distances and time intervals will differ from those 
