Light Speed in Materials or Gravity

We know from relativity that the speed of light is constant but Einstein said when it comes near to a strong gravational field it slows down and in water or glass light also slows down so how could relativity be true while the speed of light is not constant.And thanks alot.
- Amr Mohsen (age 15)
Egypt, Live in Qatar

Nice questions.

The one about light speed in materials really isn't a problem. Light in materials gets mixed up with other things, mostly electron motions, and it's that whole collective phenomenon that travels slower than bare light would.  If you describe what's going on inside the material, the combination of electromagnetism and electron motions, etc., the electromagnetic part by itself still travels at the usual c. So the slowing of light by materials, and effect known long before Einstein, is no problem at all for relativity.

The one about the slowing of light (and other things) when they are downhill from us in a gravitational field (say near a big star or a black hole) has a completely different answer. To include gravity, Special Relativity had to be modified to General Relativity. Special Relativity only describes how things happen on small patches of spacetime near an observer. Including bigger distances with strong gravity requires the more general theory. So the surviving true statement about the speed is that all the light near you is traveling at c regardless of its direction or its source.

Mike W.

(published on 04/17/2015)