121Chapter 14The Special Theory of Relativity
The Special Theory of Relativity is based upon the assumption that the
velocity of light is independent of the motion of both the observer and
the source of light. With this assumption, Einstein destroyed the notion
of absolute motion. Before he formulated his Theory of Relativity,
physicists believed that there existed a frame of reference absolutely at
rest and that the motion of all objects in the universe was to be taken
with respect to this absolutely stationary frame of reference. It was in this
frame of reference that the aether sat motionless and, as a consequence, it
was in this frame and only in this frame, that the velocity of light was
exactly equal to c. If an observer was in a frame of reference and she
wanted to determine the absolute motion of her frame of reference,
all she had to do was to measure the velocity of light in her frame
of reference. If the velocity of the light in her frame was exactly equal
to c, then her frame was absolutely at rest. From the deviations of this
velocity from c, she could determine the absolute motion of her frame of
reference with respect to the stationary aether. This was the aim of the
Michelson–Morley experiment, which as we know failed to detect any
motion of the Earth with respect to the aether.
The negative result of the Michelson–Morley experiment led to
Einstein’s formulation of the Theory of Relativity in which the velocity
of light is the same in all frames of reference independent of their
motion. It is, therefore, impossible to determine the absolute motion of a
frame of reference since the velocity of light will always be the same,
namely c. All motion is relative. No frame of reference is preferred over
any other. Einstein formulated this concept in terms of his principle of
relativity, i.e. the laws of nature are identical in all uniformly moving
frames of reference.
By a uniformly moving frame, we simply mean a frame of reference,
which is not undergoing acceleration. In other words, the laws of nature