The Fisher Files

The phenomenon of stellar aberration was early evidence agains the ether.  What was observed was that, relative to fixed coordinates of the Earth, distant stars seems to move in a small ellipse, completing on revolution over the course of a year.  The first observations were carried out by Bradley in 1729 (*).

Stellar aberration results from the motion of the Earth perpendicular to the direction of light coming from a distant star.  To see how, it works, put yourself in the rest from of the star, so the telescope appears to be moving with a velocity v perpendicular the the vector from the telescope to the star.  A photon that makes it down the long tube of a telescope must have a component of velocity v parallel to the direction in which the telescope moves and a total velocity of c.  So, the photon makes an angle tan alpha~alpha=v/c with respect to the vector connecting the telescope and the star.  The telescope must be inclined at this angle for the photon to make it through.

Another way to think about stellar aberration is think about running through the rain.  If the rain falls with a velocity w and you run with a velocity v, the rain appears to you (the runner) as falling with an angle beta, tan beta=v/w.

Numerically, the velocity of the Earth around the sun is v=150×10^9×2xPi m/(365×24x60×60 s)=30000 m/s, so alpha=v/c=30000 m/s/3×10^8 m/s=10^-4 rad. 1rad=57.3 degrees=206,000 arc-src, so alpha=20 arc-sec, so over six months, the star shifts by an angle of 40 arc-sec, which is just what Bradley observed.

That’s an analysis from the light moves like little particles.  What about the ether?  Suppose there is an ether and the Earth is at rest in it (i.e. the Earth is “dragging” it along with it).  Light hitting the Earth’s ether form outside would be dragged along with the ether and stellar aberration would not be observed.  To be honest, I’m not sure how to analyze this if the stat is at rest in the ether frame.  To be really honest, I’m really not interested in figuring it out; the ether theory is just wrong!

This is another interesting aspect of the ether theory: I think it is the only thing we spend  lot of time teaching you that is just plain wrong.  You learn classical mechanics that later gets modified by quantum mechanics and relativity, but classical mechanics is still very useful and an active area of research in its own right.  We have to teach way the ether is wrong so you understand why relativity is right.

Stellar aberration is very nicely explained in French, p. 41-46.

*Astronomers frequently express distances as angles, since that is what they measure with a telescope.  They use degrees, minutes and seconds, with one minute being 1/60th of a degree and one second being 1/60th of a minute.  Thus, an arc-second is 1/3600th of a degree.  The best telescopes, like the Magellan Telescope in Chile (where several MIT astronomers work) have angular resolutions of 0.15 arc-sec.