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By around 1900, the increased precision of new measuring instruments had shown that the laws of motion and gravity established by Galileo and Newton were unable to explain certain phenomena. The observed orbit of Mercury, for example, differed slightly from that predicted by Newton, and laws describing the motion of electromagnetic waves left many electrical effects unexplained. In 1905, an unknown 26-year-old patent office clerk named Albert Einstein published four papers that not only solved these problems, but revolutionized physics. The first two presented his Special Theory of Relativity, which departed from the classical Newtonian concepts of space and time in its assertion that all reference frames (all coordinate systems) do not measure space and time equivalently. That is, space and time are not the same throughout the universe, but depend on the motion of the observer. But for Einstein, not everything was relative. Following the electromagnetic theory of Maxwell, Einstein argued that the speed of light is the same for all observers, and introduced a new concept of space-time to reconcile this with concepts of relative motion. He also introduced the famous equation expressing a direct relation between mass and energy, E = mc ², known as mass-energy equivalence. A third paper analyzed electromagnetic radiation such as light in terms of particles called photons, and explained how some substances, when exposed to such radiation, eject electrons in a quantum process called the photoelectric effect. A fourth paper explained the random movement of particles suspended in a fluid, now known as Brownian motion. In 1916, in his General Theory of Relativity, Einstein described gravity as a warping of space-time (as opposed to Newton's force) caused by the mere presence of objects possessing mass. Einstein's new conception of gravity correctly predicted Mercury's observed orbit, and his work on photons led to a more accurate description of electromagnetic radiation. In his later years, Einstein devoted himself to a search for a theory that would unify gravity with the other three fundamental forces in nature: the strong force, the electromagnetic force, and the weak force. This search is still ongoing.