"We should certainly consign such a report to the realm of fables. But let us not scoff too soon! The ideas, which appear so extraordinary to us in this case, are exactly the same as those which lie at the base of the most complete branch of natural science, and to which the most famous of physicists owes the glory attached to his name.
"For in cosmic space such commands are continually resounding, proceeding from each of the heavenly bodies—from the sun, planets, moons, and comets. Every single body in space hearkens to the orders which the other bodies give it, always striving to carry them out punctiliously. Our earth would dash through space in a straight line, if she were not controlled and guided by the voice of command, issuing from moment to moment, from the sun, in which the instructions of the remaining cosmic bodies are less audibly mingled.
"These commands are certainly given just as silently as they are obeyed; and Newton has denominated this play of interchange between commanding and obeying by another name. He talks quite briefly of a mutual attractive force, which exists between cosmic bodies. But the fact remains the same. For this mutual influence consists in one body dealing out orders, and the other obeying them."
[ON THE THEORY OF RELATIVITY]
By Henry L. Brose, M.A.
INTRODUCTORY
PHYSICS, being a science of observation which seeks to arrange natural phenomena into a consistent scheme by using the methods and language of mathematics, has to inquire whether the assumptions implied in any branch of mathematics used for this purpose are legitimate in its sphere, or whether they are merely the outcome of convention, or have been built up from abstract notions containing foreign elements. The use of a unit length as an unalterable measure, or of a time-division, has been accepted in traditional mechanics without inconsistency manifesting itself in general until the field of electrodynamics became accessible to investigators and rendered a re-examination of the foundations of our modes of measurement necessary. It is upon these that the whole science of mathematical physics rests. The road of advance of all science is in like manner conditioned by the inter-play of observations and notions, each assisting the other in giving us a clearer view of Nature regarded purely as a physical reality. The discovery of additional phenomena presages a still greater unification, revealing new relations and exposing new differences; the ultimate aim of physics would seem to consist in reaching perfect separation and distinctness of detail simultaneously with perfect co-ordination of the whole. "The all-embracing harmony of the world is the true source of beauty and is the real truth," as Poincaré has expressed it. The noblest task of co-ordinating all knowledge falls to the lot of philosophy.
A principle which has proved fruitful in one sphere of physics suggests that its range may be extended into others; nowhere has this led to more successful results than in the increasing generalization which has characterized the advance of the principle of relativity. This advance is marked by three stages, quite distinct, indeed, in the nucleus of their growth, yet each succeeding stage including the results of the earlier.
Relativity first makes its appearance as a governing principle in Newtonian or Galilean mechanics; difficulties arising out of the study of the phenomena of radiations led to a new enunciation of the principle upon another basis by Einstein in 1905, an enunciation which comprised the phenomena of both mechanics and radiation: this will be referred to as the "special" principle of relativity to distinguish it from the "general" principle of relativity enunciated by Einstein in 1915, and which applied to all physical phenomena and every kind of motion. The latter theory also led to a new theory of gravitation.
I. The Mechanical Theory of Relativity