In 1925 it became apparent that Bohr's theory of electronic orbits failed to explain the finer structure of spectra, and a new "wave mechanics," due chiefly to Heisenberg, de Broglie and Schrodinger, took its place. The electron then took on the like ness of both a particle and a wavelet, with either its position or its velocity mathematically indeterminate. We seem to be getting down to fundamental concepts, which cannot be represented to the mind in the guise of physical models, but must be left in terms of mathematical equations.
To explain the propagation of light, it had been necessary to imagine the existence of an aether to carry the waves. Yet the measured velocity of light proved to be the same in all directions, and therefore independent of the relative motion of the aether. The first comprehensive explanation of this difficulty was given by A. Einstein in 1905. Einstein pointed out that absolute space and time are metaphysical figments of the imagination. Time and space as known to us are determined by an observer, himself moving in certain ways, and may be different if measured by another observer. Thus time and space are relative and not abso lute. Moreover they must be so connected that the velocity of light is always constant however measured—the first constant of the new physics. While space and time individually are relative, yet, as pointed out by Minkowski, a certain combination of them is absolute—the same for all observers—one second being equiva lent to 18o,000 miles, the distance described by light in that time. According to Einstein, in this space-time continuum, bodies move in natural paths, straight in empty space but curved near matter owing to something analogous to curvature in space-time. Thus Newton's idea of gravitational force may be unnecessary. (See SPACE-TIME. ) Mathematical investigation showed that this theory of relativity leads to consequences approximately the same as Newton's theory—so nearly the same, indeed, that it has needed all our experimental resources to decide in favour of Einstein. The most famous of the crucial experiments is based on the very minute deflection of a ray of light as it passes near the sun. This is twice as great on Einstein's theory as on Newton's.
The principle of relativity also involves the equivalence of mass and energy already suggested by other lines of research. In
recent years the possible conversion of mass into energy has been used by astronomers to explain the old problem of the continued output of radiant heat from the sun and other stars. That output is now believed to be due to subatomic changes at the enormous temperature of many million degrees which must exist inside dense stars radiating heat and light. Some of it may be supplied by the conversion of one element into another, but a far greater store of energy would be liberated by the mutual cancellation of protons and electrons, whereby mass, passing completely into radiation, would be annihilated.
Thus in two directions—the quantum theory and relativity— recent science seems to be breaking away from the fundamental conceptions by which it has been guided since the days of Galileo and Newton. It is too soon as yet to look for a complete and con sistent scheme of physics based on these new ideas. They are at present in that fascinating early stage of new knowledge when consistency is hardly a virtue, when indeed it is more useful to follow bravely each fresh line of development than to make premature attempts to co-ordinate the whole.
This historical sketch may help to make clear the method of science, and to throw some light on its value and meaning. Modern science began with the Renaissance. Mediaeval Scholasticism was a complete synthesis of knowledge, deduced rationally from premises regarded as certain—firstly, the doctrines of the Chris tian Faith as set forth by the Fathers and safeguarded by the Roman Church ; secondly, human knowledge as found chiefly in Aristotle and applied by Christian Doctors, especially St. Thomas Aquinas. There was still room for interpretation and exposition, but no need for change, no idea of development. Thus the new ex perimental method brought about a revolution. The idea of a com plete synthesis of knowledge was dropped. The new science was empirical and not rational; it observed and experimented on some quite limited problem; accepted the results whether or no they were in accordance with preconceived ideas; started again on an other problem, and sometimes found that the two solutions agreed. It did not deduce knowledge a priori, but put it together slowly and laboriously like fitting the pieces into a puzzle.