54 TIME 



The Velocity of Light. A feature of the relativity 

 theory which seems to have aroused special interest 

 among philosophers is the absoluteness of the velocity of 

 light. In general velocity is relative. If I speak of a 

 velocity of 40 kilometres a second I must add "relative 

 to the earth", "relative to Arcturus", or whatever refer- 

 ence body I have in mind. No one will understand any- 

 thing from my statement unless this is added or implied. 

 But it is a curious fact that if I speak of a velocity 

 of 299,796 kilometres a second it is unnecessary to 

 add the explanatory phrase. Relative to what? Rela- 

 tive to any and every star or particle of matter in the 

 universe. 



It is no use trying to overtake a flash of light; 

 however fast you go it is always travelling away from 

 you at 186,000 miles a second. Now from one point 

 of view this is a rather unworthy deception that Nature 

 has practised upon us. Let us take our favourite observer 

 who travels at 161,000 miles a second and send him in 

 pursuit of the flash of light. It is going 25,000 miles 

 a second faster than he is; but that is not what he will 

 report. Owing to the contraction of his standard scale 

 his miles are only half-miles; owing to the slowing down 

 of his clocks his seconds are double-seconds. His 

 measurements would therefore make the speed 100,000 

 miles a second (really half-miles per double-second). 

 He makes a further mistake in synchronising the clocks 

 with which he records the velocity. (You will remember 

 that he uses a different Now line from ours.). This 

 brings the speed up to 186,000 miles a second. From 

 his own point of view the traveller is lagging hopelessly 

 behind the light; he does not realise what a close race 

 he is making of it, because his measuring appliances 

 have been upset. You will note that the evasiveness of 



