122 The Poetry of Physics and The Physics of Poetry
are identical in a frame that is stationary with respect to the Earth and in
the frame that is in the interior of a train moving at a constant velocity
with respect to the Earth. (We are ignoring, for the purposes of this
discussion, the motion of the Earth in its orbit about the Sun or its
rotation). If the train was accelerating or decelerating, the laws of physics
in the frame of the train would not be the same as those in the frame at
rest with respect to the Earth. The reason for this is a mass with no force
acting upon it would experience a fictitious force in the accelerating train
but not in the frame at rest with respect to the Earth. The fictitious force
arises because the mass will continue to move at a constant velocity due
to its inertia independent of the motion of the train. If the train
accelerates then the constant motion of the mass will no longer be
constant with respect to the accelerating train. With respect to the train,
the mass will appear to be accelerated by a fictitious force. This is a
familiar experience to all who have been thrown forward in a moving
train or bus, which suddenly decelerates. When describing the laws of
physics it is, therefore, wisest to remain in a non-accelerating frame of
reference so as to avoid the presence of friction forces. In our discussion
of special relativity, therefore, we will automatically limit ourselves to
frames of references for which there are no fictitious forces. Therefore,
unless otherwise mentioned, the reader may assume that the frame of
reference under discussion is one undergoing uniform motion. Einstein’s
principle of relativity states that all such frames are equivalent and the
laws of physics described in each of these frames are identical.
The idea that all motion is relative was as devastating to Einstein’s
contemporaries as Copernicus’ notion that the Earth was no longer
the center of the universe. It took people over one hundred years to
accept the idea that the Earth actually moved. Once this idea was
accepted, thinkers became used to the idea that the Earth was moving
with respect to some frame of reference absolutely at rest. So, although
the Earth was not at rest, at least there was some place in the universe at
rest, a place where one could anchor one’s thoughts. The concept of
absolute motion and absolute space, which evolved essentially from the
physics of Copernicus, Galileo, and Newton was elevated to the heights
of an a priori truth by the philosopher, Immanuel Kant. This is an
indicator of how engrained the concept of absolute motion became in the
minds of Western thinkers. Kant could not conceive of the possibility
that space could be structured in any other way. Einstein’s principle of
relativity, based on experimental fact, completely destroyed the validity
of this so-called a priori or absolute truth. Instead of there being one