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286 CON'IRIBUTIONS TO SCIENCE
When by the special theory of relativity I had arrived at the
equivalence of all so-called inertial systems for the formulation
of natural laws (1905), the question whether there was not a
further equivalence of coordinate systems followed naturally,
to say the least oE it. To put it in another way, if only a relative
meaning can be attached to the concept of velocity, ought we
nevertheless to persevere in treating acceleration as an absolute
concept?
From the purely kinematic point of view there was no doubt
about the relativity of all motions whatever; but physically
speaking, the inertial system seemed to occupy a privileged
position, which made the use of coordinate systems moving in
other ways appear artificial.
I was of course acquainted with Mach's view, according to
which it appeared conceivable that what inertial resistance
counteracts is not acceleration as such but acceleration with
respect to the masses of the other bodies existing in the world.
There was something fascinating about this idea to me, but it
provided no workable basis for a new theory.
I first came a step nearer to the solution of the problem when
I attempted to deal with the law of gravity within the framework
of the special theory of relativity. Like most writers at the
time, I tried to frame a field-law for gravitation, since it was no
longer possible, at least in any natural way, to introduce direct
action at a distance owing to the abolition of the notion of
absolute simultaneity.
The simplest thing was, of course, to retain the Laplacian
scalar potential of gravity, and to complete the equation of
Poisson in an obvious way by a term differentiated with respect
to time in such a way that the special theory of relativity was
satisfied. The law of motion of the mass point in a gravitational
field had also to be adapted to the special theory of relativity.
The path was not so unmistakably marked out here,
since the inert mass of a body might depend on the gravitational
potential. In fact, this was to be expected on account of tbe
principle of the inertia of energy.
These investigations, however, led to a result which raised
my strong suspicions. According to classical mechanics, the