The Fizeau Experiment
I was reading about the Fizeau experiment in which the Fresnel drag coefficient was measured. The idea was that if light propagated in water (index of refraction n=1.33), the light would propagte with a velocity of c/n+w(1-1/n^2), where w=water velocity. This was in fact observed as predicted by Fresnel.
The index of refraction comes from the polarization of atoms and molecules in the medium. An applied electric field polarizes the dipoles, lining them up in the same direction. This alignment cancels some of the field, slowing it down. It is a very simple picture, but a very powerful way of looking at the effect of an electric field on matter.
The Fizeau experiment made me think of how this might be applied to looking for dark matter. Dark matter, which comprises 23% of the matter of the universe, could have a very small dipole moment and this would lead to an index of refraction type effect in vacuum. It owuld be very tiny, but maybe measurable over galactic distances.
