SCIENCE 



[N. S. Vol. LI. No. 1305 



tions of this great association to see to it 

 that full advantage is taken of the opportunity 

 offered by the present evolutionary stage of 

 research and public esteem. We must choose 

 between inertia and some display of aggressive 

 energy. 



John M. Coulter 

 Univebsitt of Chicago 



TIME, SPACE, AND GRAVITATION^ 



After the lamentable breach in the former 

 international relations existing among men of 

 science, it is with joy and gratefulness that I 

 accept this opportunity of communication with 

 English astronomers and physicists. It was in 

 accordance with the high and proud tradition 

 of English science that English scientific men 

 shoidd have given their time and labor, and 

 that English institutions should have provided 

 the material means, to test a theory that had 

 been completed and published in the country 

 of their enemies in the midst of war. Al- 

 though investigation of the influence of the 

 solar gravitational field on rays of light is a 

 pui'ely objective matter, I am none the less 

 very glad to express my personal thanks to my 

 English colleagues in this branch of science; 

 for without their aid I should not have ob- 

 tained proof of the most vital deduction from 

 my theory. 



There are several kinds of theory in physics. 

 Most of them are constructive. These attempt 

 to build a picture of complex phenomena out 

 of some relatively simple proposition. The 

 kinetic theory of gases, for instance, attempts 

 to refer to molecular movement the mechan- 

 ical thermal, and diffusional properties of 

 gases. When we say that we understand a 

 group of natural phenomena, we mean that we 

 have found a constructive theory which em- 

 braces them. 



THEORIES OF PRINCIPLE 



But in addition to this most weighty group 

 of theories, there is another group consisting 

 of what I call theories of principle. These 

 employ the analytic, not the synthetic method. 

 Their starting-point and foundation are not 



iFrom the Londoa Times. 



hypothetical constituents, but empirically ob- 

 served general properties of phenomena, prin- 

 ciples from which mathematical formula are 

 deduced of such a kind that they apply to 

 every case which presents itself. Thermody- 

 namics, for instance, starting from the fact 

 that perpetual motion never occurs in ordi- 

 nary experience, attempts to deduce from this, 

 by analytic processes, a theory which will 

 apply in every case. The merit of construc- 

 tive theories is their comprehensiveness, adapt- 

 ability, and clarity, that of the theories of 

 principle, their logical perfection, and the 

 security of their foimdation. 



The theory of relativity is a theory of prin- 

 ciple. To understand it, the principles on 

 which it rests must be grasped. But before 

 stating these it is necessary to point out that 

 the theory of relativity is like a house with 

 two separate stories, the special relativity 

 theory and the general theory of relativity. 



Since the time of the ancient Greeks it has 

 been well known that in describing the motion 

 of a body we must refer to another body. 

 The motion of a railway train is described 

 with reference to the groimd, of a planet with 

 reference to the total assemblage of visible 

 fixed stars. In physics the bodies to which 

 motions are spatially referred are termed sys- 

 tems of coordinates. The laws of mechanics 

 of Galileo and Newton can be formulated only 

 by using a system of coordinates. 



The state of motion of a system of co- 

 ordinates can not be chosen arbitrarily if the 

 laws of mechanics are to hold good (it must 

 be free from twisting and from acceleration). 

 The system of coordinates employed in 

 mechanics is called an inertia-system. The 

 state of motion of an inertia-system, so far as 

 mechanics are concerned, is not restricted by 

 nature to one condition. The condition in 

 the following proposition suffices: a system of 

 coordinates moving in the same direction and 

 at the same rate as a system of inertia is itself 

 a system of inertia. The special relativity 

 theory is therefore the application of the fol- 

 lowing proposition to any natural process: 

 " Every law of nature which holds good with 

 respect to a coordinate system K must also 

 hold good for any other system K' provided 



