Notes on Relativity and Cosmology - Physics Department, UCSB

2.2. THE ELUSIVE ETHER 49
particular reference frame with respect to which this speed should be measured:
the reference frame of the water itself. Moreover, experiments with moving
water did in fact show that 1/ √ ǫµ gave the speed of light through water only
when the water was at rest 6 . The same thing, by the way, happens with regular
surface waves on water (e.g., ocean waves, ripples on a pond, etc.). There is
a wave equation not unlike (2.5) which controls the speed of the waves with
respect to the water.
So, clearly, c should be just the speed of light ‘as measured in the reference
frame of the vacuum.’ Note that there is some tension here with the idea we
discussed before that all inertial frames are fundamentally equivalent. If this
is so, one would not expect empty space itself to pick out one as special. To
reconcile this in their minds, physicists decided that ‘empty space’ should not
really be completely empty. After all, if it were completely empty, how could it
support electromagnetic waves? So, they imagined that all of space was filled
with a fluid-like substance called the “Luminiferous Ether.” Furthermore, they
supposed that electromagnetic waves were nothing other than wiggles of this
fluid itself.
So, the thing to do was to next was to go out and look for the ether. In
particular, they wanted to determine what was the ether’s frame of reference.
Was the earth moving through the ether? Was there an ‘ether wind’ blowing
by the earth or by the sun? Did the earth or sun drag some of the ether with
it as it moved through space?
The experiment that really got people’s attention was done by Albert Michelson
and Edward Morley in 1887. They were motivated by issues about the nature of
light and the velocity of light, but especially by a particular phenomenon called
the “aberration” of light. This was an important discovery in itself, so let us
take a moment to understand it.
2.2.1 The Aberration of Light
Here is the idea: Consider a star very far from the earth. Suppose we look at
this star through a telescope. Suppose that the star is “straight ahead” but
the earth is moving sideways. Then, we will not in fact see the star as straight
ahead. Note that, because of the finite speed of light, if we point a long thin
telescope straight at the star, the light will not make it all the way down the
telescope but will instead hit the side because of the motion of the earth. A bit
of light entering the telescope and moving straight down, will be smacked into
by the rapidly approaching right wall of the telescope, even if it entered on the
far left side of the opening (see diagram below). The effect is the same as if
the telescope was at rest and the light had been coming in at a slight angle so
that the light moved a bit to the right. The only light that actually makes it
to the bottom is light that is moving at an angle so that it runs away from the
oncoming right wall as it moves down the telescope tube.
6 However, physicists like Fizeau did find some odd things when they performed these
experiments. We will talk about them shortly.