How to Measure the Speed of Light in Space?

relativity is the theory that tells us how to think about the universe as it curves and bends in response to mass. But there is no well defined definition of velocity for something that's far away. For example, a distant observer will see light near a black hole travel at all sorts of crazy speeds - from less than the speed of light down to zero. If space is curved, then the reference frame of some one else near by those photons would be very different,. and your measurement of velocity, ind theirs won't agree.