Notes on Relativity and Cosmology - Physics Department, UCSB

50 CHAPTER 2. MAXWELL, E&M, AND THE ETHER
If we want light from the straight star in front of us to make it all the way
down, we have to tilt the telescope. In other words, what we do see though the
telescope is not the region of space straight in front of the telescope opening,
but a bit of space slightly to the right.
Light Ray hits side instead of reaching bottom
Telescope moves through ether
Must tilt telescope to see star
This phenomenon had been measured, using the fact that the earth first moves in
one direction around the sun and then, six months later, it moves in the opposite
direction. In fact, someone else (Fizeau) had measured the effect again using
telescopes filled with water. The light moves more slowly through water than
through the air, so this should change the angle of aberration in a predictable
way. While the details of the results were actually quite confusing, the fact that
the effect occurred at all seemed to verify that the earth did move through the
ether and, moreover, that the earth did not drag very much of the ether along
with it.
You might wonder how Fizeau could reach such a conclusion. After all, as
you can see from the diagram below, there is also and effect if the ether is
dragged along by the earth. In the region far from the earth where the ether is
not being dragged, it still provides a ‘current’ that affects the path of the light.
The point, however, is that the telescope on the Earth must now point at the
place where the light ray enters the region of ether being dragged by the earth.
Note that this point does not depend on whether the telescope is filled with air
or with water! So, Fizeau’s observation that filling the telescope with water
increases the stellar aberration tells us that the ether is not strongly dragged
along by the earth.
Ether here
at rest wrt Earth
Earth
Out here, an ether ‘‘current’’ flows
(as measured by Earth)