7 6 D O C . 7 5 N O B E L L E C T U R E
of the laws and to apply the “reality postulate” to this essence first, that is to say, to
place the unambiguous relation to the world of experience at the end, instead of al-
ready effectuating it in incomplete form for an artificially isolated part, namely, for
the space-time metric. Yet we are not far enough along in our knowledge of the fun-
damental laws of nature to go down this more perfect path without losing a firm
foothold. We shall see at the end of our considerations that there has already been
an attempt to realize this logically purer method in the latest research based on
ideas by Levi-Civita, Weyl, and
Eddington.[4]
Based on what has been said above, it becomes clearer now how “preferred
states of motion” should be understood. They are preferred with respect to the laws
of nature. States of motion are preferred if coordinate systems in said states of mo-
tion are distinguished with respect to the formulation of the laws of nature when in
said coordinate systems these laws assume a form preferred by its simplicity. Ac-
cording to classical mechanics, the states of motion of inertial frames are physically
preferred in this sense. According to classical mechanics, one can distinguish be-
tween (absolutely) unaccelerated and accelerated motions; furthermore, according
to it, velocities have only relative existence (dependent on the choice of inertial
frame), whereas accelerations and rotations have absolute existence (independent
of the choice of inertial frame). Let us express it this way: according to classical
mechanics, “relativity of velocity” holds but “relativity of acceleration” does not.
Following these preparations, we move on to the actual topic to be considered, to
relativity theory; we shall characterize its development up to now from the point of
view of its principles.
The special theory of relativity is an adaptation of the foundations of physics to
Maxwell-Lorentz electrodynamics. It takes from earlier physics the assumption of
the validity of Euclidean geometry for the possible positions of rigid bodies, the in-
ertial frame, and the law of inertia. It assumes as valid for the whole of physics the
principle of the equivalency of inertial frames in the formulation of the laws of na-
ture (special relativity principle). From Maxwell- Lorentz electrodynamics, it takes
the postulate of the constancy of the velocity of light in a vacuum (light principle ).
In order for the special relativity principle and the light principle to harmonize,
the assumption that an absolute (agreeing for all inertial frames) time exists must
be abandoned. Thus the hypothesis that clocks built in the same way, moving arbi-
trarily, and suitably set, function in such a way that when they meet their readings
agree, is abandoned. A specific time is assigned to each inertial frame; the state of
motion of the inertial frame and its time are defined in compliance with the reality
postulate, such that the light principle applies with regard to that frame. The exis-
tence of the inertial frame thus defined as well as the validity of the law of inertia
[p. 4]