Prelude to Relativity 115
More accurate measurements of c were performed without recourse to
astronomical observations using terrestrial-based apparatus, which utilize
either a rotating mirror or else a rotating cogwheel. A very accurate
measurement was made by Michelson using a rotating mirror involved
an optical path of 22 miles between Mt. Wilson and Mt. San Antonio in
California in 1880.
One of the triumphs of Maxwell’s equations was that the velocity of
the electromagnetic waves that he predicted was in agreement with the
measured value of the velocity of light. Maxwell’s identification of light
as electromagnetic radiation explained a great deal about the nature of
light. One mystery still remained, however, namely, how is it that the
light wave can propagate through empty space? It was impossible for
physicists to conceive of a wave without some medium in which it could
travel. One need only consider the wave motion of water or sound waves
to see that these waves could not exist without their medium. Using these
analogies, Maxwell postulated the existence of a transmitting medium for
light, which he called the luminiferous aether. The properties of the
aether had to be somewhat unusual in order for it to serve the role for
which it was invented. It had to fill all space uniformly since the velocity
of light is the same throughout space. It also had to penetrate substances
such as air, water and glass since light propagates through these
materials. Furthermore, this transparent substance could not, in any way,
interfere with the motion of corporeal bodies such as the planets since no
evidence for the resistance to the motion of corporeal bodies in empty
space could be found. In fact, the law of inertia clearly states that the
velocity of a body will remain constant as long as no force is acting upon
it. The aether can therefore only act on matter through their electric
charge. The aether was purported to be the medium through which light
propagates and through which electric and magnetic forces between
charged particles are transmitted.
The experimental detection of the aether became a challenge to the
imagination of the nineteenth century scientists, in particular, Albert
Michelson, the man who had so accurately measured the speed of light.
Michelson argued that since the Earth is in motion about the sun it must
be moving through the aether. Therefore, a beam of light propagating in
the direction of the Earth’s motion through the aether would have the
velocity c – v relative to the Earth. The Earth moves through the aether
with the velocity v and the light moves through the aether in the same
direction with the velocity c and therefore, their relative velocity is c – v.