D O C . 4 9 R E P L Y T O R E I C H E N B Ä C H E R 2 0 5
Herr Reichenbächer misunderstood my consideration of the two celestial bodies
that rotate relative to each other. One of these bodies is to be imagined as rotating
in the sense of Newtonian mechanics and, consequently, oblate due to centrifugal
action, but not the other one. Inhabitants with rigid measuring rods would find this
out, and would communicate it to each other, whereupon they would ask for the real
cause of this behavior of the two celestial bodies. (This consideration has nothing
to do with the Lorentz contraction). Newton answers this question by declaring the
reality of absolute space relative to which one body rotates while the other one does
not. I myself am of Mach’s opinion, which can be formulated in the language of the
theory of relativity thus: all the masses in the universe determine the -field, and
this field is seen differently from the first celestial body than from the second one,
because the motion of the masses that generate the -field is quite different when
described from each one of the two masses. In my opinion, inertia is in the same
sense a (communicated) mutual action between the masses of the universe, just like
the actions that the Newtonian theory considers as gravitational actions. From this
point of view what Herr Reichenbächer says about the two-body problem is quite
incorrect. The fact that the action of all bodies in the universe, save the two under
consideration, can be approximated by a quasi-constant -field must not be con-
fused with the statement that these celestial bodies have no influence on the two
bodies considered.
It is completely incomprehensible to me how Herr Reichenbächer, toward the
end of his analysis—in the paragraph beginning with “If we correctly consider the
whole situation”—after all that has been said, arrives at the conclusion: all laws of
nature must be phrased in a generally covariant form. Because, if acceleration has
absolute meaning, then the nonaccelerated coordinate systems are preferred by na-
ture, i.e., the laws then must—when referred to them—be different (and simpler)
than the ones referred to accelerated coordinate systems. Then it makes no sense to
complicate the formulation of the laws by pressing them into a generally covariant
Vice versa, if the laws of nature are such that they do not attain a preferred form
through the choice of coordinate systems of a special state of motion, then one can-
not relinquish the condition of general covariance as a means of research. If one as-
sumes in addition that for an infinitesimal system of measurement (in the -small)
the theory of special relativity is valid and that the gravitational field is described
by the that follow from this assumption, then one stands on the ground of the
theory of general relativity. From the statements of Herr Reichenbächer, I cannot
see whether this is the case with him.
Berlin, November 20, 1920 A. Einstein

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