phenomena (_e.g._ of light).
Since it has often been contended by opponents of the theory of
relativity that the special theory of relativity is overthrown by the
general theory of relativity, it is perhaps advisable to make the facts
of the case clearer by means of an appropriate comparison. Before the
development of electrodynamics the laws of electrostatics were looked
upon as the laws of electricity. At the present time we know that
electric fields can be derived correctly from electrostatic
considerations only for the case, which is never strictly realised, in
which the electrical masses are quite at rest relatively to each other,
and to the co-ordinate system. Should we be justified in saying that
for this reason electrostatics is overthrown by the field-equations of
Maxwell in electrodynamics? Not in the least. Electrostatics is
contained in electrodynamics as a limiting case; the laws of the latter
lead directly to those of the former for the case in which the fields
are invariable with regard to time. No fairer destiny could be allotted
to any physical theory, than that it should of itself point out the way
to the introduction of a more comprehensive theory, in which it lives
on as a limiting case.
In the example of the transmission of light just dealt with, we have
seen that the general theory of relativity enables us to derive
theoretically the influence of a gravitational field on the course of
natural processes, the laws of which are already known when a
gravitational field is absent. But the most attractive problem, to the
solution of which the general theory of relativity supplies the key,
concerns the investigation of the laws satisfied by the gravitational
field itself. Let us consider this for a moment.
We are acquainted with space-time domains which behave (approximately)
in a Galileian fashion under suitable choice of reference-body,
_i.e._ domains in which gravitational fields are absent. If we now
refer such a domain to a reference-body _K _ possessing any kind of
motion, then relative to _K _ there exists a gravitational field which
is variable with respect to space and time.[17] The character of this
field will of course depend on the motion chosen for _K ._ According to
the general theory of relativity, the general law of the gravitational
field must be satisfied for all gravitational fields obtainable in this
way. Even though by no means all gravitationial fields can be produced
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