  Light speed is assumed as a constant in Einstein's general relativity theory.
In this paper, the polarized light from black hole Cygnus X-1 is found fit well
with Cherenkov radiation generated by Faster Than Light movement in vacuum
under gravity. Thus, proved the gravitational lens is a real existence, the
refractive index of space did increase with the gravity field, and light speed
is changeable with gravitational potential energy. Possible experiments to
double verify the theory on the earth is discussed. A new way to find more
advanced alien civilizations in lower gravitational potential energy area is
proposed.
