Re: What experimental proof is there that gravity bends light?

Greetings, Daniel --

Thanks for the excellent question!  I'll answer the part about experimental 
evidence first.  You already know about Sir Arthur Eddington's solar 
eclipse observations of 1919, in which the apparent positions of stars 
behind the Sun were seen to be shifted by just the amount Einstein had 
predicted.  This measurement of the deflection of light by the Sun 
constituted the first successful test of general relativity; it made the 
front pages of all the major newspapers, and turned "Einstein" into a 
household name pretty much overnight.  This experiment was repeated a 
number of times over the following decades, always producing results 
consistent with Einstein's prediction, but with a fair bit of experimental 
uncertainty.

It turns out that you can greatly reduce the experimental uncertainty by 
looking for this deflection in radio waves instead of in optical light.  
(In addition, you don't need to wait for an eclipse to do the observation 
in radio waves!)  However, in this case the solar corona is potentially 
problematic: when radio waves pass through the corona, they are refracted 
(bent) due to electromagnetic interactions between the radio waves and the 
material in the corona.  So how can we tell if the bending that we see is 
due to gravity or refraction by the corona?  It turns out that the amount 
of bending by the corona depends on the frequency of the radio waves you're 
considering, whereas the amount of bending by the Sun's gravity does *not*.  
So if you observe at two different radio frequencies, then you can tell how 
much effect the corona is having by looking at the *difference* in the 
degree of bending between the two frequencies.  What's left after this 
difference is accounted for is the bending due to the Sun's gravity.  A 
messy problem, but I believe it works!

Now, over the past couple of decades, we've seen many other instances of 
the bending of light in the presence of gravity, in the guise of 
"gravitational lensing".  This occurs when a relatively nearby galaxy or 
cluster of galaxies (the "lens") passes in front of a more distant galaxy 
or quasar (the "source"); the light from the distant source is bent around 
the intervening lens due to the lens's gravitational influence.  The visual 
results can be spectacular - distorted images of the source, or even 
multiple images.  See http://bustard.phys.nd.edu/PH308/lensing/taxonomy.html for a few examples 
of what this looks like.

Hope this helps!

Cheers,
Eric