256 PHYSICAL SCIENCE 



Since the velocity z^ of a moving body is usually 

 small compared with the velocity c of light, this 

 result gives — 



M = ;;^l I + — ^\ = m ■\- 2 



That is, the effective inertial mass of a moving 

 body is its mass at rest plus its kinetic energy 



- mv^ divided by the square of the velocity of 



liofht. It seems that mass is of the nature of 

 energy or energy of the nature of mass : matter 

 and energy may be identified. 



From this result it seems reasonable to 

 suppose that a region filled with any form of 

 energy, even for instance light or radiant heat, 

 would possess inertia equal to the energy contents 

 divided by the square of the velocity of light. 

 It does not immediately follow that the energy 

 would be subject to gravitation. It may possibly 

 be that the equivalence between mass and weight, 

 proved experimentally by Galileo and Newton, 

 applies to m in the equation, to the mass at rest, 

 and not to M which contains also the kinetic 

 energy. The problem of gravitation needs further 

 consideration. 



The principle of relativity was first applied 

 to the phenomena of gravitation by Einstein, in 

 191 1. He pointed out that it was impossible 

 by any experiment to distinguish a gravitational 

 force from the force experienced by an observer 

 who is accelerated, that is, whose motion is 

 changing. For instance, when a lift starts to 

 rise, the occupants feel all the effects of a sudden 



