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Einstein

Relativity: The Special and General Theory
Albert Einstein: Relativity
Part II: The General Theory of Relativity
Part II
The General Theory of Relativity
Special and General Principle of Relativity
The basal principle, which was the pivot of all our previous considerations, was the special principle
of relativity, i.e. the principle of the physical relativity of all uniform motion. Let as once more
analyse its meaning carefully.
It was at all times clear that, from the point of view of the idea it conveys to us, every motion must
be considered only as a relative motion. Returning to the illustration we have frequently used of the
embankment and the railway carriage, we can express the fact of the motion here taking place in
the following two forms, both of which are equally justifiable :
(a) The carriage is in motion relative to the embankment,
(b) The embankment is in motion relative to the carriage.
In (a) the embankment, in (b) the carriage, serves as the body of reference in our statement of the
motion taking place. If it is simply a question of detecting or of describing the motion involved, it is
in principle immaterial to what reference−body we refer the motion. As already mentioned, this is
self−evident, but it must not be confused with the much more comprehensive statement called "the
principle of relativity," which we have taken as the basis of our investigations.
The principle we have made use of not only maintains that we may equally well choose the
carriage or the embankment as our reference−body for the description of any event (for this, too, is
self−evident). Our principle rather asserts what follows : If we formulate the general laws of nature
as they are obtained from experience, by making use of
(a) the embankment as reference−body,
(b) the railway carriage as reference−body,
then these general laws of nature (e.g. the laws of mechanics or the law of the propagation of light
in vacuo) have exactly the same form in both cases. This can also be expressed as follows : For
the physical description of natural processes, neither of the reference bodies K, K1 is unique (lit. "
specially marked out ") as compared with the other. Unlike the first, this latter statement need not of
necessity hold a priori; it is not contained in the conceptions of " motion" and " reference−body "
and derivable from them; only experience can decide as to its correctness or incorrectness.
Up to the present, however, we have by no means maintained the equivalence of all bodies of
reference K in connection with the formulation of natural laws. Our course was more on the
following Iines. In the first place, we started out from the assumption that there exists a
reference−body K, whose condition of motion is such that the Galileian law holds with respect to it :
A particle left to itself and sufficiently far removed from all other particles moves uniformly in a
straight line. With reference to K (Galileian reference−body) the laws of nature were to be as simple
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