Table of contents

The main stages in the discovery by Einstein of the equations of the general theory of relativity in November 1915 are recounted. The evolution of his ideas from the first formulation of the equivalence principle in 1907 is followed. The part played by Hubert in the finding of the final equations of the theory is discussed.

A review is given of experimental verifications of the general theory of relativity (GTR) in which basic assumptions and consequences of the theory are compared with experiment. It may now be considered that the principle of equivalence and some of the other propositions forming the foundations of GTR, and also the effects predicted by it for weak fields (φ/c²≪1, where φ is the Newtonian gravitational potential) have, on the whole, been reliably confirmed. However, in the case of strong fields, the theory has not as yet been directly confirmed. In particular, the existence of black holes has not yet been demonstrated, to say nothing of a quantitative agreement between GTR formulas and measurements of the metric near a black hole. Several suggestions are put forward in relation to further verifications of GTR.

Some characteristic methods that Einstein used to analyze the problems of the electrodynamics of moving bodies are discussed. Einstein rejected the method of "accumulation of hypotheses", reevaluated the results of electrodynamic experiments, generalized them in the form of the relativity principle, interpreted this as a law of nature, and, significantly, made it the point of departure of a new theory—the special theory of relativity. It is shown that Einstein gave great importance to generalizing the results of experiments and the part played by theory, in which nature is regarded as an integral whole; for Einstein, all the variable parameters used in a theory are interconnected and interdependent; this also applies to space and time, which lose their substantiality and, thus, absoluteness. It is pointed out that it was only the realization that the metric expresses the laws of physical interconnection that made possible the formulation of the general theory of relativity. Einstein's negative position with respect to the conventionalism of Poincare and Reichenbach, in particular the interpretation of geometry as a conventionally chosen method of describing experience independent of physical interconnections, is discussed. In connection with the criticism of conventionalism, consideration is given to descriptions of one and the same experiment that have different forms, and it is shown that some of these descriptions merely represent formal transformations that do not reveal new connections in nature whereas others give the same results only at a definite stage of understanding, their subsequent development however revealing that they correspond to different levels of penetration into the essence of the phenomena.

Einstein's papers on quantum theory over the period 1905-1925 are reviewed. These papers played a fundamental role in the development of quantum concepts. This review traces the relationship between these papers, on the one hand, and Wien's and Planck's theoretical work on thermal radiation and Einstein's own work on statistical thermodynamics, on the other. Einstein's 1905 development of the concept of light quanta from Wien's radiation law, the application of this concept to elementary interactions of radiation with matter, Einstein's 1906–1907 analysis of the Planck radiation law, and Einstein's development of the foundations of a quantum theory of specific heat are all discussed. Particular attention is paid to Einstein's 1909 development of the concept of a particle-wave duality for radiation from a study of the energy and momentum fluctuations in a radiation field. Einstein's 1916 papers on the quantum theory of radiation are analyzed, and it is emphasized that Einstein was the first to suggest a probabilistic interpretation of the particle-wave duality, in addition to introducing coefficients to characterize the probabilities for optical transitions and offering his well-known derivation of the Planck radiation law. There is a special discussion of Einstein's expression, in his 1925 paper on the quantum theory of monatomic ideal gases, of high regard for de Broglie's concept of a particle-wave duality for material particles.

The history of quantum electronics is considered in relation to Einstein's theory of radiation. It is shown that the papers of Albert Einstein on the quantum theory of radiation, published between 1905 and 1925, established the physical basis of quantum electronics. A discussion of quantum electronics devices and applications demonstrates, that irrespective of the wavelength (radio frequencies, microwaves, infrared, visible, ultraviolet, etc.), Einstein's theory of radiation is the cornerstone of quantum electronics as a whole.

This article contains information on Einstein's experimental work. It pays main attention to three of his studies (1915–1916) aimed at experimental demonstration of Ampere molecular currents. It treats the content of these studies, their prehistory, and the peculiar revaluation of their results after the discovery of the spin of the electron. It traces the effect of these studies on Einstein's attitude toward the Stern-Gerlach experiments and the studies of Uhlenbeck and Goudsmit, and also points out their genetic tie with the well-known work of Einstein and Ehrenfest.