Januabt 2, 1920] 



SCIENCE 



9 



that K and K' are in uniform movement of 

 translation." 



The second principle on which the special 

 relativity theory rests is that of the constancy 

 of the velocity of light in a vacumn. Light 

 in a vacuum has a definite and constant 

 velocity, independent of the velocity of its 

 source. Physicists owe their confidence in 

 this proposition to the Maxwell-Lorentz theory 

 of electro-dynamics. 



The two principles which I have mentioned 

 have received strong experimental confirma- 

 tion, but do not seem to be logically com- 

 patible. The special relativity theory achieved 

 their logical reconciliation by making a 

 change in kinematics, that is to say, in the 

 doctrine of the physical laws of space and 

 time. It became evident that a statement of 

 the coincidence of two events could have a 

 meaning only in connection with a system of 

 coordinates, that the mass of bodies and the 

 rate of movement of clocks must depend on 

 their state of motion with regard to the 

 coordinates. 



THE OLDER PHTSICS 



But the older physics, including the laws of 

 motion of Galileo and Newton, clashed with 

 the relativistic kinematics that I have indi- 

 cated. The latter gave origin to certain gen- 

 eralized mathematical conditions with which 

 the laws of nature would have to conform if 

 the two fundamental principles were com- 

 patible. Physics had to be modified. The 

 most notable change was a new law of motion 

 for (very rapidly) moving mass-points, and 

 this soon came to be verified in the case of 

 electrically-laden particles. The most im- 

 portant result of the special relativity system 

 concerned the inert mass of a material 

 system. It became evident that the inertia 

 of such a system roust depend on its energy- 

 content, so that we were driven to the con- 

 ception that inert mass was nothing else than 

 latent energy. The doctrine of the conserva- 

 tion of mass lost its independence and became 

 merged in the doctrine of conservation of 

 energy. 



The special relativity theory which was 

 simply a systematic extension of the electro- 



dynamics of Maxwell and Lorentz, had conse- 

 quences which reached beyond itself. Must 

 the independence of physical laws with regard 

 to a system of coordinates be limited to sys- 

 tems of coordinates in uniform movement of 

 translation with regard to one another? What 

 has nature to do with the coordinate systems 

 that we propose and with their motions? Al- 

 though it may be necessary for our descrip- 

 tions of nature to employ systems of coordi- 

 nates that we have selected arbitrarily, the 

 choice should not be limited in any way so far 

 as their state of motion is concerned. (Gen- 

 eral theory of relativity.) The application of 

 this general theory of relativity was found to 

 be in conflict with a well-known experiment, 

 according to which it appeared that the 

 weight and the inertia of a body depended oh 

 the same constants (identity of inert and 

 heavy masses). Consider the case of a system 

 of coordinates which is conceived as being in 

 stable rotation relative to a system of inertia 

 in the INTewtonian sense. The forces which, 

 relatively to this system, are centrifugal must, 

 in the iNewtonian sense, be attributed to in- 

 ertia. But these centrifugal forces are, like 

 gravitation, proportional to the mass of the 

 bodies. It is not, then, possible to regard the 

 system of coordinates as at rest, and the 

 centrifugal forces of gravitational? The in- 

 terpretation seemed obvious, but classical 

 mechanics forbade it. 



This slight sketch indicates how a general- 

 ized theory of relativity must include the laws 

 of gravitation, and actual pursuit of the con- 

 ception has justified the hope. But the way 

 was harder than was expected, because it con- 

 tradicted Euclidian geometry. In other words, 

 the laws according to which material bodies 

 are arranged in space do not exactly agree 

 with the laws of space prescribed by the 

 Euclidian geometry of solids. This is what is 

 meant by the phrase " a warp in space." The 

 fundamental concepts " straight," " plane ," 

 etc., accordingly lose their exact meaning in 

 physic's. 



In the generalized theory of relativity, the 

 doctrine of space and time, kinematics, is no 

 longer one of the absolute foimdations of gen- 

 eral physics. The geometrical states of bodies 



