The fact that the speed of light is constant in a vacuum is one of the cornerstones of Einsteins theory of special relativity. The speed of light in vacuum commonly denoted c is a universal physical constant important in many areas of physics. C ϵ 0 m 0 1 2.
299792458 meters per second 186282 miles per seconda speed.
Amazingly special relativity looks right and so does this postulate of constant light speed until now. If however the other ship is traveling at c maybe its just a blip of light it just comes out to be c again. Time dilation the twin paradox moving clocks running slowly space-time and even E mc 2 all stem from just these two postulates. It is a fact that scientists have constantly found the speed of light in a perfect vacuum to be 299792458 ms using mathematics and tests one cant create a perfect vacuum to run tests instead tests with atomic clocks and lasers are used.