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77
Relativity: The Special and General Theory
Albert Einstein: Relativity
Appendix
Appendix
III
The Experimental
Confirmation
of the General
Theory
of
Relativity
From a systematic theoretical point of view, we may imagine the process of evolution of an
empirical science to be a continuous process of induction. Theories are evolved and are expressed
in short compass as statements of a large number of individual observations in the form of
empirical laws, from which the general laws can be ascertained by comparison. Regarded in this
way, the development of a science bears some resemblance to the compilation of a classified
catalogue. It is, as it were, a purely empirical enterprise.
But this point of view by no means embraces the whole of the actual process ; for it slurs over the
important part played by intuition and deductive thought in the development of an exact science. As
soon as a science has emerged from its initial stages, theoretical advances are no longer achieved
merely by a process of arrangement. Guided by empirical data, the investigator rather develops a
system of thought which, in general, is built up logically from a small number of fundamental
assumptions, the so?called axioms. We call such a system of thought a theory. The theory finds
the justification for its existence in the fact that it correlates a large number of single observations,
and it is just here that the " truth " of the theory lies.
Corresponding to the same complex of empirical data, there may be several theories, which differ
from one another to a considerable extent. But as regards the deductions from the theories which
are capable of being tested, the agreement between the theories may be so complete that it
becomes difficult to find any deductions in which the two theories differ from each other. As an
example, a case of general interest is available in the province of biology, in the Darwinian theory
of the development of species by selection in the struggle for existence, and in the theory of
development which is based on the hypothesis of the hereditary transmission of acquired
characters.
We have another instance of far?reaching agreement between the deductions from two theories in
Newtonian mechanics on the one hand, and the general theory of relativity on the other. This
agreement goes so far, that up to the preseat we have been able to find only a few deductions from
the general theory of relativity which are capable of investigation, and to which the physics of
pre?relativity days does not also lead, and this despite the profound difference in the fundamental
assumptions of the two theories. In what follows, we shall again consider these important
deductions, and we shall also discuss the empirical evidence appertaining to them which has
hitherto been obtained.
(a) Motion of the Perihelion of Mercury
According to Newtonian mechanics and Newton's law of gravitation, a planet which is revolving
round the sun would describe an ellipse round the latter, or, more correctly, round the common
centre of gravity of the sun and the planet. In such a system, the sun, or the common centre of
gravity, lies in one of the foci of the orbital ellipse in such a manner that, in the course of a
planet?year, the distance sun?planet grows from a minimum to a maximum, and then decreases