according to the principle of relativity, be the same for the railway
carriage as reference-body as when the rails are the body of reference.
But, from our above consideration, this would appear to be impossible.
If every ray of light is propagated relative to the embankment with the
velocity _c_, then for this reason it would appear that another law of
propagation of light must necessarily hold with respect to the
carriage a result contradictory to the principle of relativity.
In view of this dilemma there appears to be nothing else for it than to
abandon either the principle of relativity or the simple law of the
propagation of light _in vacuo_. Those of you who have carefully
followed the preceding discussion are almost sure to expect that we
should retain the principle of relativity, which appeals so
convincingly to the intellect because it is so natural and simple. The
law of the propagation of light _in vacuo_ would then have to be
replaced by a more complicated law conformable to the principle of
relativity. The development of theoretical physics shows, however, that
we cannot pursue this course. The epoch-making theoretical
investigations of H. A. Lorentz on the electrodynamical and optical
phenomena connected with moving bodies show that experience in this
domain leads conclusively to a theory of electromagnetic phenomena, of
which the law of the constancy of the velocity of light in vacuo is a
necessary consequence. Prominent theoretical physicists were therefore
more inclined to reject the principle of relativity, in spite of the
fact that no empirical data had been found which were contradictory to
this principle.
At this juncture the theory of relativity entered the arena. As a
result of an analysis of the physical conceptions of time and space, it
became evident that _in reality there is not the least incompatibilitiy
between the principle of relativity and the law of propagation of
light_, and that by systematically holding fast to both these laws a
logically rigid theory could be arrived at. This theory has been called
the _special theory of relativity_ to distinguish it from the extended
theory, with which we shall deal later. In the following pages we shall
present the fundamental ideas of the special theory of relativity.
VIII.
16