D O C . 1 3 D I A L O G U E A B O U T R E L A T I V I T Y T H E O R Y 7 3
did not collapse when its environment suffered the jolt—why such consequences
of the jolt show themselves so one-sided only in the train, while still no one-sided
unequivocal conclusion about the placement of the change in motion should be
possible—for this question the principle apparently has no answer that could sat-
isfy the simple mind.”
Rel.: For several reasons we willingly have to accept the complications to
which the theory leads us. For one, it is a great satisfaction for a consistently
thinking man to understand that the concept of absolute movement—which kine-
matically makes no sense anyway—need not be introduced into physics. It cannot
be denied that by the avoidance of this concept the foundation of physics benefits
in its logical structure. Furthermore, the fact of the equivalence of inertia and
gravity of bodies irresistibly demands clarification. Aside from this, physics needs
a method of getting to a local field theory for gravitation. Theoreticians could not
attack this problem without an effective limiting principle because one could pos-
tulate a large number of theories which all agree with the rather limited experi-
ence in this field. Embarras de richesse is one of the most malicious foes making
the life of a theoretician difficult. The postulate of relativity limited these possibil-
ities such that the road which the theory had to go was marked. Finally, the secular
movement of the perihelion of the planet Mercury had to be explained. Its exist-
ence had been established with certainty by the astronomers, but the Newtonian
theory could not find a satisfying explanation for it.—The postulate of the princi-
pal equivalence of coordinate systems does not claim that every coordinate system
is equally convenient for the investigation of a specific physical system; this is
already the case in classical mechanics. Strictly speaking, one should, for exam-
ple, not say the earth moves in an ellipse around the sun, because this statement
assumes a coordinate system in which the sun is at rest, whereas classical mechan-
ics also allows for systems relative to which the sun moves uniformly in a straight
line. But nobody would seriously want to use the latter coordinate system for an
investigation of the motion of the earth. Similarly, nobody would conclude from
this example that coordinate systems in which the center of gravity of the system
under consideration remains at rest at the origin are in principle distinguished over
other coordinate systems. The same applies to the example you mentioned.
Nobody would investigate our solar system in a coordinate system in which the
earth is at rest—because this would be impractical. But in principle, such a coor-
dinate system would still be equivalent to any other one in the general theory of
relativity. The phenomenon that in such coordinate systems fixed stars would race
around at tremendous velocities is no argument against its admissibility, but
merely against the usefulness of this choice of coordinates. The same is true of the
complicated structure of the gravitational field relative to such a coordinate
{2}
[p. 701]