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 present 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 again to a
minimum. If instead of Newton s law we insert a somewhat different law
of attraction into the calculation, we find that, according to this new
law, the motion would still take place in such a manner that the
distance sun-planet exhibits periodic variations; but in this case the
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