THE VELOCITY OF LIGHT 57 



which are not themselves in direct causal connection. 

 The assertion made by the relativity theory is more 

 restricted, viz. — 



Neither matter, nor energy, nor anything capable of 

 being used as a signal can travel faster than 299,796 

 kilometres per second, provided that the velocity is 

 referred to one of the frames of space and time con- 

 sidered in this chapter.* 



The velocity of light in matter can under certain 

 circumstances (in the phenomenon of anomalous dis- 

 persion) exceed this value. But the higher velocity is 

 only attained after the light has been passing through 

 the matter for some moments so as to set the molecules 

 in sympathetic vibration. An unheralded light-flash 

 travels more slowly. The speed, exceeding 299,796 

 kilometres a second, is, so to speak, achieved 

 by prearrangement, and has no application in sig- 

 nalling. 



We are bound to insist on this limitation of the speed 

 of signalling. It has the effect that it is only possible to 

 signal into the Absolute Future. The consequences of 

 being able to transmit messages concerning events 

 Here-Now into the neutral wedge are too bizarre to 

 contemplate. Either the part of the neutral wedge that 

 can be reached by the signals must be restricted in a 

 way which violates the principle of relativity; or it will 

 be possible to arrange for a confederate to receive the 

 messages which we shall send him to-morrow, and to 

 retransmit them to us so that we receive them to-dav^' 

 The limit to the velocity of signals is our bulwark 



* Some proviso of this kind is clearly necessary. We often employ 

 for special purposes a frame of reference rotating with the earth; in this 

 frame the stars describe circles once a day, and are therefore ascribed 

 enormous velocities. 



