3 4 D O C . 4 F O U N D A T I O N S O F G E N E R A L R E L A T I V I T Y
energy tensor , it follows that the G-field is caused and determined by the
energy tensor of matter.
As to (a), Herr Kretschmann notes that the principle of relativity, phrased in this
manner, is not a statement about physical reality, i.e., not about the content of na-
ture’s laws, but it is rather a demand with respect to their mathematical formulation.
Because physical experience pertains only to coincidences, it must always be pos-
sible to represent the causal connections between these coincidences by generally
covariant equations. He thinks it necessary to associate a different meaning to the
demand of relativity. I consider the first argument of Herr Kretschmann as correct,
but I do not believe his suggested innovation is recommendable. While it is correct
that every empirical law can be brought into a generally covariant form, principle
(a) also carries considerable heuristic weight, which has already proven itself
splendidly in the problem of gravitation. The point is the following. Among two
theoretical systems, both compatible with experience, one will have to prefer the
one that is simpler and more transparent from the point of view of absolute differ-
ential calculus. One just should bring the mechanics of Newtonian gravitation in
the form of absolute-covariant equations (four-dimensional) and one will certainly
become convinced that principle (a) excludes this theory, not on theoretical
grounds, but on practical ones!
Principle (b) was the starting point of the entire theory, and has brought with it
the formulation of principle (a); it certainly cannot be abandoned as long as one
clings to the basic ideas of the theoretical system.
Mach’s principle (c) is a different story. The necessity to uphold it is by no means
shared by all colleagues; but I myself feel it is absolutely necessary to satisfy it.
With (c), according to the field equations of gravitation, there can be no G-field
without matter. Obviously, postulate (c) is closely connected to the space-time
structure of the world as a whole, because all masses in the universe will partake in
the generation of the G-field.
Originally, I suggested for the field equations of gravitation
, (1)
with the abbreviation
Tμν) (
[p. 243]
Gμν Tμν
-- -

–κ⎝ =
Gμν τν) ,
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