Brian Greene is now offering an online course on the theory of relativity. There are two versions of the class, one with math and one without. Additional information was provided during Greene's recent Ask Me Anything on reddit, during which he agrees with a redditor who recommends Leonard Susskind's "Theoretical Minimum"(previously) as a good preliminary.
Albert Einstein's Theory of Relativity in words of four letters or less
c not a constant! Over a period of billions of years of course. If this theory is true, then E=mc2 might not apply to a large period of time.