Einstein's theory of special relativity is based in part on which of the following postulates? mass and energy are equivalent. space and time are absolutes. energy is conserved only in elastic collisions. speed of light in vacuum is the same for all observers regardless of source velocity.

The answer to your question is speed of light in a vacuum is the same for all observers regardless of source velocity, it was determined by Albert Einstein 1905, he then spent 10 years on incorporating acceleration in the theory and in 1915 he published his theory of general relativity, he explained what gravity was. massive objects cause a distortion in space-time, which is known as gravity,