22.2. Time Dilation and Length Contraction http://www.ck12.org
According to the special theory of relativity, as the beam of light passes the space ship, an observer in the space ship
will measure the velocity of the light beam as
a. 0. 6 c
b.c
c. 1. 4 c
Answer: The correct answer is B. The second postulate of special relativity asserts that the velocity of light in a
vacuum will always be measured ascregardless of the motion of the source or observer.
Time dilation
The first result of relativity is that time is not the same for all frames of reference. The simplest case for relativity is
light traveling independently of the direction of motion.
To answer the question, we will describe a rather simple clock. The clock functions when a beam of light (shown in
orange) is emitted from a source/clock and travels to a mirror. Upon striking the mirror, the light is reflected back,
directly opposite the mirror, and both observersAandBrecord the event, as inFigure22.5.
FIGURE 22.5
Light clock setupFrom the point of view of observerA, the light goes straight up and down, while the ground and observerBare
moving off to the left at velocityv.
If the distance between the mirror and the source isd, then the round-trip distance the light travels is 2d. Since the
light travels with speedc, the one-way travel timetAas measured by observerAis