Once it had become clear that electrons are a constituent part of every atom and determine its basic optical, magnetic and other properties, and that they carry the current in conductors and produce the chemical bonding of atoms and molecules, the idea naturally occurred of constructing a unified electromagnetic world picture. The components of this picture, it was assumed, would be charged particles and the electromagnetic field.
It must be recognized that the attempt to construct an electromagnetic world picture constituted a logical development and a definite step forward. This picture relied, of course, on the relativistic mechanics of particle motion (rather than Newton's classical laws), since it was with electrons that the laws of motion of fast particles were first verified, while the electromagnetic field itself intrinsically required a relativistic treatment. This is especially true of electromagnetic waves, which move with the limiting velocity of light. It was even suggested to represent charges as singularities within the field itself, or perhaps as some kind of condensa tions of the field. Let us recall that Lenin clearly perceived the following: that the development of the mechanics of fast particles (essentially, the theory of relativity and the field hypotheses on the structure of the electron) constituted a marked improvement over the former classical-mechanical picture of the world.
The fundamental idea that all the proper energy of the electron resides in the energy of its electromagnetic field was first expressed by Thomson and Abragam. However, the subsequent discovery of the neutron, the neutrino, and of other charged and neutral particles, as well as the inter pretation of gravitation in geometrical terms, conclusively showed that it is quite impossible to reduce matter to the electromagnetic field alone. The hypothesis of an electromagnetic world picture therefore quietly gave way, in contrast to the reluctant and tumultuous retreat of mechanism.
Let us dwell in brief on a third serious attempt to construct a unified world picture, which was associated with the founding of Einstein's general theory of relativity (1915).
In a quite unexpected way, Einstein associated gravitation with the curvature of space-time, produced by any kind of matter. According to his theory,
the components of the metrical tensor g,,,,, which describes the curvature of space in Riemannian geometry, are identical with the ten components of the gravitational potential, which is a generalization of the Newtonian potential with its single component.
Einstein's theory of gravitation, which he conceived as a "general theory of relativity" extending to any kind of motion the results of the ordinary "special" theory of relativity, which applies only to rectilinear uniform motion, was a magnificient conception of great profundity, but it introduced in practice only small corrections into the ordinary Newtonian theory.
At the present time, these corrections have been revealed in only three effects: 1) in the small shift of the perihelion of Mercury's orbit, amounting to 43" per century; 2) in the deflection of a light ray in a gravitational field, for instance of the ray coming from a star past the Sun in the space curved by the Sun, amounting to 1.75" (near the edge of the Sun's disk); 3) in the red shift of spectral lines in a gravitational field, which has been detected both for the Sun and for dense stars.
At the beginning of 1960 Pound and Rebka of Harvard University and Cranshaw and his colleagues at Harwell first detected an analog of Einstein ian effects in the laboratory rather than under astronomical conditions. They succeeded in measuring a small line shift in the gamma-ray spectrum emitted by atoms of the isotope at various heights differing by only a few meters. This was done by making use of a remarkable property of gamma rays, discovered in 1958 by the German physicist Mossbauer. He found that the gamma spectrum emitted by radioactive nuclei of iron atoms located at the lattice points of a crystal exhibits very sharp lines, due to the fact that the recoil momentum is taken up by the lattice.
All these effects prove ultimately that space-time is curved, i. e. , that it deviates from a flat, four-dimentional, pseudo-Euclidean continuum. This also vindicates the insights of Lobachevskii, Bolyai, and Riemann, who first established the possibility of a non-Euclidean geometry correspond ing not to a flat but to a curved three-dimensional space.