The speed of light is constant? Physicists plan to test a new theory that questions Einstein's assumptions

One of the oddest things about our universe is that it has some fundamental constants. These constants—such as the speed of light in a vacuum—have numerical values that don’t change no matter what conditions you test them under.

The speed of light in a vacuum had been measured by many physicists in the 19th century and found to always have the same value (299,792,458 meters per second, though it took until the 1970s to reach that level of precision). But physicists had to wait until Albert Einstein developed his theory of relativity to understand why this was the case....
http://qz.com/846498/the-speed-of-light-is-constant-physicists-plan-to-test-a-new-theory-that-questions-einsteins-assumptions/