The German physicist Albert Einstein revolutionized our concepts of space and time with his theories of relativity.
In 1905 Einstein published hisspecial theory of relativity.The foundation of the theory is the observation that light moves at a constant speed as measured by all observers, whatever their state of motion. One consequence is that measurements of distance and time are not absolute quantities but vary relative to the motion of an observer. Another is that energy (E) can be converted into mass (m), and vice versa, according to the formula
E = mc2, where c is the speed of light.
Ten years later, Einstein’sgeneral theory of relativityextended this work to include acceleration and gravity—stating that the two are equivalent. Einstein showed that gravity is actually the warping of space by matter. One consequence is that light will follow a curved path when it passes a massive body. Observations of a small apparent shift in the positions of stars near the sun during a solar eclipse in 1919 showed that the theory was correct. General relativity was later used to interpret Hubble’s discovery of the expanding universe.