THE PHYSICAL THEORY OF RELATIVITY The earliest successful attempt to formulate the laws governing the general motion of matter is found in Newton's laws. The first law states that : Every body perseveres in its state of rest or of uniform motion in a right line unless it is compelled to change that state by forces impressed thereon.
In this law no distinction is made between rest and uniform motion in a straight line, and the same is true of the remaining laws. Hence follows the remarkable property to which Newton draws explicit attention in his fifth corollary to the laws of motion : The motions of bodies included in a given space are the same among themselves, whether that space is at rest, or moves uni formly forwards in a right line without any circular motion.
As a concrete application of his principle, Newton instances "the experiment of a ship, where all motions happen after the same manner whether the ship is at rest or is carried uniformly forward in a right line." Just as a passenger on a ship in a still sea could not determine, from the behaviour of bodies inside the ship, whether the ship was at rest or moving uniformly forward, so we cannot determine from the behaviour of bodies on our earth whether the earth is at rest or not. We believe the earth to be moving round the sun with a speed of about 3okm.
a second, so that there can be no question of the earth being permanently at rest, but we are unable to determine whether it is at rest at any specified point of its orbit, or, in the probable event of its not being at rest, what its absolute velocity may be. There is no more reason for thinking the sun, than the earth, to be at rest. Newton wrote as follows : "It is possible that in the remote regions of the fixed stars, or perhaps far beyond them, there may be some body absolutely at rest, but impossible to know, from the positions of bodies to one another in our regions, whether any of these do keep the same position to that remote body. It follows that absolute rest cannot be determined from the position of bodies in our regions." The above quotations are all from the first book of the Principia A1atizenaaticd. Previous to them all Newton writes : "I have no regard in this place to a medium, if any such there is, that freely pervades the interstices between the parts of bodies." The two centuries which elapsed of ter the publication of the Principia witnessed a steady growth of the belief in the reality of such an all-pervading medium. It was called the ether, or aether, and by the end of these two centuries (1887) it was almost universally believed that light and all electromagnetic phenomena were evi dence of actions taking place in this ether. Light from the most
distant stars was supposed to be transmitted to us in the form of wave motions in the ether, and we could see the stars only be cause the sea of ether between us and these stars was unbroken. It had been proved that if this sea of ether existed it must be at rest, for the alternative hypothesis that the ether was dragged about by ponderable bodies in their motions had been shown to be incompatible with the observed phenomenon of astronomical aber ration and other facts of nature. On this view it was no longer necessary to go to Newton's "remote regions of the fixed stars, or perhaps far beyond them," to find absolute rest. A standard of absolute rest was provided by the ether which filled our labora tories and pervaded all bodies. Owing to our motion it would appear to be rushing past us, although without encountering any hindrance—"like the wind through a grove of trees," to borrow the simile of Thomas Young. The determination of the absolute velocity of the earth was reduced to the problem of measuring the velocity of an ether current flowing past us and through us.
In this same year (1887) the first experimental determination of this velocity was attempted by the Chicago physicist Michelson. The velocity of light was known to be, in round numbers, 300,000 km. a second, a velocity which was interpreted as representing the rate of progress of wave motion through the ether. If the earth were moving through the ether with a velocity of i,000km. a sec ond, the velocity of light relative to a terrestrial observer ought to be only 299,000km. a second when the light was sent in exactly the direction of the earth's motion through the ether, but would be 3oi,000km. a second if the light was sent in the opposite direction. In more general terms, if the earth were moving through the ether, the velocity of light, as measured by a terres trial observer, would depend on the direc tion of the light, and the extent of this de pendence would give a measure of the earth's velocity. The velocity of light along a single straight course does not per mit of direct experimental determination, but the same property of dependence on direction ought to be true, although to a less extent, of the average to-and-fro velocity of a beam of light sent along any path and then reflected back along the same path.