What is the theory of relativity

Albert Einstein, in his theory of special relativity, determined that the laws of physics are the same for all non-accelerating observers, and he showed that the speed of light within a vacuum is the same no matter the speed at which an observer travels.

Albert Einstein's general theory of relativity is one of the towering achievements of 20th-century physics. Published in 1916, it explains that what we perceive as the force of gravity in fact arises from the curvature of space and time. Einstein proposed that objects such as the sun and the Earth change this geometry.

Special relativity is a theory proposed by Albert Einstein that describes the propagation of matter and light at high speeds. ... Special relativity reduces to Newtonian mechanics in the limit of small speeds. According to special relativity, no wave or particle may travel at a speed greater than the speed of light c.