22.1. Galilean Relativity http://www.ck12.org
FIGURE 22.2
How sound moves in different frames of
referenceThe Speed of Light Paradox
Towards the end of the 19th century, there was growing evidence of a paradox that threw this into doubt. The
essence of the paradox is simple:
No matter how fast you are moving, light always appears to move away at the same speed, equal toc=
3. 0 × 108 m/s in a vacuum.This appears simple, but the consequences of this discovery were enormous. The principles are illustrated in the
diagram below, which ignores the effect of air on the speed of light.
FIGURE 22.3
How light moves in different frames of
referenceThis appears on the surface to be logically inconsistent. In the ground frame of reference, the car is traveling in the
same direction as the light, and so their relative motion is less thanc. However, in the car frame of reference, the
light is moving away at exactlyc.
How the Paradox Was Proven: The Michaelson-Morley Experiment
Physicists carried out experiments in order to determine the Earth’s motion through the ether. The most famous
of these experiments, designed by Albert Michelson (1852-1931) and Edward W. Morley (1838-1923), called
the Michelson-Morley experiment, was performed in 1887 at what is now Case Western Reserve University in
Cleveland, Ohio. Using the interferometer, developed by Michelson, discussed earlier, they attempted to measure
the relative velocity of the earth with respect to the velocity of light through theether. It is important to remember
that, like the velocity of sound, the velocity of light is independent of the velocity of the source. Therefore, the
thinking was: if we move through the air while sound is traveling toward us in one instance and away from us in