The Mind of Newton

THE MIND OF NEWTON A. S. EVE T HE idea is common, though false, that the star of Newton has set now that the star of Einstein is in the ascendant. Yet to-day no one pays greater homage to Newton than Einstein himself, nor is there probably any other living man who more fully grasps what ' may be termed the Mind of Newton. In his Principia Newton for the first time laid down those great principles of moving bodies which have ever since been used by'man in all his inventions and discoveries. His system of dynamics still pervades all branches of mechanical science, whether in astronomy, engineering, . or atomic physics. His system worked, and still holds for the motions of a planet round the sun, provided an attractive force exists between the one mass and the other, equal to their product and lessening as the square of their distance apart. The mechanism of this force however is not apparent, or "observable",,. and Newton declined to assert any hypothesis as to its cause and nature, above and beyond ,. the above statement. We are accustomed to forces producing motion, as when a horse pulls a cart by traces, or a man shuts a door by pushing it with his hand. Here we have visible, tangible and apparently satisfactory causes. But between sun and ea,th, or earth and moon, what is the invisible agency of which Newton spoke?~:,~. It was not long before many acute minds, including Newton's, saw the difficulty of "action at a distance", and' the truly metaphysical character'of the force which the Principia demanded. Hence the studious efforts of 191 THE UNIVERSITY OF TORONTO QUARTERLY I Einstein to resolve the difficulty and to substitute a geometry of space which would cause the planet Jupiter to go round the Sun in a "natural" path, without the aid of the imaginary force which Newton evoked. But, it will be asked, how does ,the sun modify space so many .h{;ndreds of millions of miles away so that the geometry of space at Jupiter guides it as surely as rails move a tramcar round a curve? And it is supposed that the question is foolish, for this geometry of space is the sun, although there must needs be a rather abrupt change in geometry when we en ter that region which we, seeing its brilliance, and feeling its radiant heat, are wont more particularly to call The Sun! What then was Newton's scheme? He knew and accepted the evidence of Galileo Galilei that bodies. moving over a smooth horizontar plane . swerved aside only when some mechanical force was applied. He knew that "motio" or momentum was added to a body when a mechanical force acted for some duration on the body, and this experience was summarized with the conception that the rate at which the momentum was changed was a measure of the force applied. He added his third great law, for which he gives some credit to Wren, "Vallis and Huygens, "the greatest geometers of our times", namely, that "to every action there is always opposed an equal reaction". He not only applied this to cases such as "Press a stone with your finger, the finger is also pressed by the stone", to a horse dragging a stone by a rope, but particularly to those cases when "one body impinges on another". He hung up by strings pairs of round balls of various sizes and materials and making one strike the other found that the momentum (mass times the velocity) lost by one ball was exactly that gained by the other ball. From such humble beginnings, founded however on THE MIND OF NEWTON actual experiments, putting as it were direct questions to Nature- on such did he build the foundations of his great edifice, which still stands. It \vill be remembered that Newton's system relates to bodies, while Einstein's scheme finds a suitable geometry of the field or region between or near the bodies. It is Einstein's ambition to make his field equations include electrical as well as gravitational events. , Jt is the modern practice to decry the use in Natural Philosophy of...