D O C . 7 5 N O B E L L E C T U R E 7 7
with regard to it are presumed. Time is measured, for each inertial frame, by iden-
tically constructed clocks at rest relative to it.
The transformation laws for space coordinates and time for the transition from
one inertial frame to another, the so-called Lorentz transformations, are unequivo-
cally determined by these definitions and the hypotheses inherent in the presump-
tion that they are free of contradiction. The immediate physical content of these
transformations lies in the effect of the motion relative to the inertial frame used on
the shape of rigid bodies (Lorentz contraction) and on the rate of the clocks. Ac-
cording to the special principle of relativity, the laws of nature must be covariant
with respect to Lorentz transformations; the theory thus provides a criterion for
general laws of nature. In particular, it leads to a modification of the Newtonian law
of point motion, in which the vacuum velocity of light figures as a limiting velocity;
and it leads to the insight of an essential equality of energy and inertial mass.
The special theory of relativity brought substantial advances. It reconciled me-
chanics with electrodynamics. It reduced the number of logically mutually inde-
pendent hypotheses of the latter. It enforced an epistemological clarification of the
fundamental concepts. It unified the laws of momentum and energy conservation;
it demonstrated the essential unity of mass and energy. Yet it was not entirely sat-
isfactory—quite apart from the quantum problems, which all theory so far has been
incapable of solving properly. The special theory of relativity, just like classical
mechanics, gives preference to certain states of motion—namely, to those of the in-
ertial frames—over all other states of motion. This was actually harder to tolerate
than preferring a single state of motion, as did the theory of a stationary light ether;
for it conceived a real reason for this preference, namely, the light ether. A theory
which from the outset prefers no state of motion must appear more satisfactory.
Moreover, the aforementioned ambiguity in the definition of an inertial frame, re-
spectively in the formulation of the law of inertia, raises doubts. These obtain their
decisive importance due to the empirical finding of the equality of inertial and grav-
itational mass, on the grounds of the following consideration.
Let K be an inertial frame without a gravitational field, K′ a coordinate system
uniformly accelerated relative to K. Then the behavior of material points with re-
spect to K′ is the same as if K′ were an inertial frame, with respect to which a ho-
mogeneous gravitational field exists. Because of the empirically known properties
of the gravitational field, the definition of an inertial frame thus proves to be obso-
lete. It seems likely that any given arbitrarily moving reference system is equivalent
to any other for the formulation of the laws of nature, that there are thus no physi-
cally preferred states of motion at all with respect to regions of finite extension
(general relativity principle).
[p. 5]