Weight and Fate of Light. 555 



Similarly the Michelson-Morley experiment, even if performed on the 

 surface of a mass as big as the sum would be likely still to give 

 a negative result : hence it is useless to try to repeat the experiment on 

 some mountain elevation or in the interior of the earth. 



So, by (3), it is possible for the speed of light to be zero 

 in a region where 7 = 0, that is in the neighbourhood of 

 a mass so great that 



2M _c^ 

 R ~G ; 



and in that case light cannot altogether escape from the 

 body. It would have a better chance of circling round and 

 round the body, with e = 90°, because rddjdt is controlled by 

 \/y instead of y; but strictly speaking it could not do this 

 either, but would be stopped in its tracks. 



Einstein's method thus makes the speed of light a 

 minimum where ordinary gravitational considerations would 

 make it a maximum. Instead of increasing in speed as it 

 approaches a massive body, light lessens in speed, as if it 

 were repelled, not attracted. But the striking discordance 

 between the two systems is that, whereas the speed of light 

 fully subject to gravity would depend on the distance it had 

 travelled against a retarding force, Einstein makes it 

 assume a velocity characteristic of the place where it is at 

 each moment, without reference to past history. 



2V 



So taking 7 as equal to 1 1-, and the velocity of light 



c 

 a 1 



ong a radius as yc, the velocity of light anywhere inside 

 a stellar system such as has been considered above, pp. 549 

 and 551, at a distance r from its centre, is 



v = c— ^(R 2 -£r 2 )G. 



An aggregate mass whose 



p U 2 = c 2 /4:7rG = l-l x 10 27 c.g.s. 



would therefore reduce light at its centre to relative rest. 



The question has often been asked, What becomes of all 

 the radiation poured into space by innumerable suns through 

 incalculable ages ? Is it possible that some of it is trapped, 

 without absorption, by reservoirs of matter lurking in the 

 depths of space, and held until they burst into new stars ? 



And a farther more important question begins to obtrude 

 itself : — What happens to light when, in free though modified 

 ether, it is stopped relatively to a gravitational mass ? Does 

 it retain its energy, mainly in rotational form, tie itself into 

 electrons, and add to the mass of the body? 



