21. Mechanics of a material particle and of radiant energy

419

54. The field of continuous distributions of electricity 58. Mechanics of a material system, and gravitation

484

493

Proceedings of the American Academy of Arts and Sciences.

VOL. XLVIII. No. 12. NOVEMBER, 1912.

ON THE EXISTENCE AND PROPERTIES OF THE ETHER.

By D. L. WEBSTER.

ON THE EXISTENCE AND PROPERTIES OF THE ETHER.

By D. L. WEBSTER.

Presented by G. W. Pierce. Received September 12, 1912.

IN the science of mechanics of ordinary matter we are accustomed to regard velocity as essentially relative but acceleration as absolute; and to say that, if a body is not acted upon in any way by other bodies, its acceleration is zero, but that, if it is acted upon by any other body, the accelerations of the two are opposite, and inversely proportional to their masses. But how can we test this law? and how can we measure the acceleration? If we measure the velocity relative to the earth, or to the sun, or to any star, at any two times separated by a very short interval, how can we be sure that the system of reference has not been accelerated during the time that has elapsed? And if it has, on what system is its acceleration measured?

This difficulty is made still more puzzling if we consider two mechanical systems, such as the solar system, exactly similar in every way, but one of which is removed to a practically infinite distance from all other matter while the other is subject to the attraction of a tremendous mass, so large and far removed that its gravitational field is practically uniform, and at absolute zero temperature so that no radiation would be received from it. These systems would be accelerated relatively to each other, but which of them would be accelerated? No observer on either of them could tell by any mechanical

means.

An answer to these questions appears to be given by the electromagnetic equations, which assume the especially simple form with which we are familiar when expressed in terms of the length, mass, and time units of any one of a certain set of systems, any one of which appears to be moving relative to any other with a constant velocity, less than the velocity of light. These systems may all be assumed to be unaccelerated, and assuming the impossibility of any system's moving relative to one of these with a velocity greater than that of light, we say that all other systems are accelerated.