The Special Theory of Relativity 125
motion of astronomical bodies such as the Sun and the Moon, his
concept of time became more sophisticated. He began to describe the
intervals between events in terms of the number of times the Sun had
risen between the two events or in terms of the number of moons. Later,
he divided the interval between successive sunrises into smaller units
called hours. Each hour corresponded to a different position of the Sun in
the sky. He began describing the intervals between events in terms of the
change of the Sun’s position in the sky.
Finally, instead of always relating events to the motion of the Sun or
Moon, man created an instrument called the clock, which, like the Sun in
the sky, executed a regular type of motion. A correspondence was then
made between events in the real life and events on the clock. So, for
example, when we say that we arose at 8 o’clock in the morning, what
we are in essence saying is that the event of our arising and the event of
the hands of the clock indicating 8 o’clock coincided. If we think of a
clock as an instrument, which performs a regular and repetitive motion to
which we may refer other events, we will be more in the spirit of the
Einstein view of time. If we consider the clock as an instrument, which
measures the passage of time, we shall retain the Newtonian concept of
time.
The Newtonian concept of absolute motion is destroyed in the Theory
of Relativity because the velocity of light is a constant in all frames of
reference. As a consequence, all motion is relative and space is no longer
absolute as it was in Newtonian physics. It is also the constancy of the
speed of light, which destroys another concept central to Newtonian
physics, namely, the absoluteness of time. According to Newtonian
thinking, there is only one absolute time, which is valid for all frames of
reference. In the Theory of Relativity, on the other hand, time is also
relative to the particular frame of reference under consideration. In other
words, the time in two different frames of reference is not the same.
In order to illustrate how this follows from the constancy of light, we
will consider a hypothetical situation originally suggested by Einstein to
explain his ideas. In essence, we will conduct a thought experiment, that
is, create an experimental situation in thought. Let us consider two
observers, one sitting on a train moving at a constant velocity with
respect to our second observer sitting at rest along the track on which the
train is traveling as shown in Fig. 14.1. Two flashes of lightning strike
the front and the rear of the train when the train is in the position
represented by the dashed lines. The train has a length L and velocity v.