Note A.
For example consider a massive particle resting on a circular disc. If
we set the disc rotating, a centrifugal force appears in the field. On
the other hand, if we transform to a set of rotating axes, we must
introduce a centrifugal force in order to correct for the change of
axes. This newly introduced centrifugal force is usually looked upon as
a mathematical fiction—as “geometrical” rather than physical. The
presence of such a geometrical force is usually interpreted as being due
to the adoption of a fictitious framework. On the other hand a
gravitational force is considered quite real. Thus a fundamental
distinction is made between geometrical and gravitational forces.
In the General Theory of Relativity, this fundamental distinction is
done away with. The very possibility of distinguishing between
geometrical and gravitational forces is denied. All axes of reference
may now be regarded as equally valid.
In the Restricted Theory, all “unaccelerated” axes of reference were
recognised as equally valid, so that physical laws were made independent
of uniform absolute velocity. In the General Theory, physical laws are
made independent of “absolute” motion of any kind.
Footnote 1:
See Note 1.
Footnote 2:
See Note 2.
Footnote 3:
See Note 4.
Footnote 4:
See Notes 9 and 12.
Footnote 5:
Note A.
On The Electrodynamics of Moving Bodies
By
A. Einstein.