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Rh stood for the British House of Commons and was elected for the St. George's Hanover Square division of London Jan. 1916. He was created G.C.M.G. in 1911 and G.C.B. in 1916. He published My Reminiscences (1917), as well as Five Free Trade Essays (1875), and other economic papers. He died suddenly in London, Sept. 12 1918.

REID, WHITELAW (1837-1912), American journalist and diplomatist (see 23.52), died in London Dec. 15 1912. His last public address was delivered before the students of the University College of Wales, Aberystwyth, on " Thomas Jefferson." In 1912 appeared The Scot in America and the Ulster Scot and posthumously, in 1913, American and English Studies.

See Royal Cortissoz, The Life of Whitelaw Reid (1921). REINACH, JOSEPH (1856-1921), French author and politician (see 23.55), was not reelected to the Chamber of Deputies in 1914. During the World War his series of articles, " Les Commentaires de Polybe," in the Figaro, were remarkable for their clear vision. He died April 18 1921. RÉJANE, GABRIELLE [CHARLOTTE REJU] (1857-1920), French actress (see 23.58), died in Paris June 14 1920. During the World War she visited England and appeared at the Court theatre, London, in a patriotic play, Alsace, and at the Coliseum in The Bet, when she played the part of a Frenchwoman visiting the English battle-zone. She was made Chevalier of the Legion of Honour for her war services. RELATIVITY. The progress of physical science during the decade 1910-20 was specially remarkable for the definite emergence into general public discussion of the principle of Relativity, as expounded by Prof. Albert Einstein, professor of Physics in the Kaiser Wilhelm Institut, Berlin. Its meaning and its history as part of present-day physical theory are discussed below.

Introduction. The primary aim of the investigator in pure science is the discovery of natural laws. As a secondary and hardly less important aim, he tries to invent a mechanism which shall account for the laws already known. The secondary aim is forced upon him partly by the constitution of the human mind; our intellects, unsatisfied with a mere accumulation of facts, impel us ever to search for the causes underlying the facts: Vere scire est per causas scire. But to the working scientist the discovery of a mechanism has an additional and more practical value. When he has found a mechanism which will account for certain laws, he can proceed to examine the complete set of laws which the mechanism demands. If his mechanism corresponds with sufficient closeness to reality he may in this way be led to the discovery of new natural laws. On the other hand, the new laws deduced from the supposed mechanism may be false. If the falsity of the new laws is not at once revealed science may for a time be led into wrong paths. When more accurate experi- menting or observation discloses that the laws are not true, a recasting of ideas becomes necessary, and the branch of science concerned may experience a time of revolution followed by a period of rapid growth.

An obvious illustration of these general statements is provided by the history of astronomy. The laws of the motions of the planets, as observed from the earth, were tolerably well known to the Greeks. They had also evolved an explanatory mechanism, starting from the metaphysical premise that the paths of the planets must necessarily be circles. The earth was the centre of the universe and round this revolved spheres to which the planets were attached. To explain the retrograde motion of the outer planets, these were supposed attached to secondary spheres revolving about points on the primary spheres which in turn revolved about the earth. This mechanism of cycles and epicycles held the field as an explanation of planetary motion for eighteen centuries. Finally the observations of Tycho Brahe provided a test which revealed the falsity of the whole structure. The position of Mars was found to differ from that required by the mechanism of epicycles by an amount as great as eight minutes of arc. " Out of these eight minutes," said Kepler, " we will construct a new theory that will explain the motions of all the planets."

The history of the succeeding century of astronomy need not be recapitulated here (see 2.811). The earth yielded its place as the centre of the universe, and the structure of cycles and epicycles crumbled away. The laws of planetary motion were determined with a precision which for the time appeared to be final. The mechanism underlying these laws was supposed to be a " force " of gravitation. This force was supposed to act between every pair of particles in the universe, its intensity varying directly as the product of the masses of the particles and inversely as the square of the distance separating them the famous law of Newton.

In science, history repeats itself. Recent years have provided a further instance of the general processes we have been con- sidering. Under the Newtonian mechanism every planet would describe a perfect ellipse about the sun as focus, and these elliptic orbits would repeat themselves' indefinitely except in so far as they were disturbed by the gravitational forces arising from the other planets. But, after allowing for these disturbing influences, Leverrier found that the orbit of the planet Mercury was .rotating in its own plane at the rate of 43 seconds a century. Various attempts have been made to reconcile this observed motion with the Newtonian mechanism. The gravitational forces arising from the known planets were demonstrably unable to produce the motion in question, but it was possible that Mercury's orbit was being disturbed by matter so far unknown to us. Investigations were made as to the disturbance to be expected from various hypothetical gravitating masses a plajiet, or a ring of planets, between Mercury and the sun, a ring of planets outside the orbit of Mercury, a belt of matter extended in a flattened disc in a plane through the sun's centre, an oblateness, greater than that suggested by the shape of the sun's surface, in the arrangement of the internal layers of the sun's mass. In every case the mass required to produce the observed disturbance in the motion of Mercury would have also produced disturbances not observed in the motions of the other planets. The solution of the problem came only with the theory of relativity. Just as Tycho's eight minutes of arc, in the hands of Kepler and Newton, revolutionized mediaeval conceptions of the mechanism of the universe, so Leverrier's 43 seconds of arc, in the hands of Einstein, has revolutionized our igth-century conceptions, not only of purely astronomical mechanism, but also of the nature of time and space and of the fundamental ideas of science. The history of this revolution is in effect the history of the theory of relativity. It falls naturally into two chapters, the first narrating the building of an earlier physical theory of relativity, and the second dealing with its extension to gravitation.

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 tliereon."

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 uniformly forwards in a right line without any circular motion."

As a concrete application of this 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 30 km. a second, so that there can be no question of the earth being permanently at rest, but we are unable to determine whether