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Albert Einstein's two theories of relativity were the first successful revisions of Newtonian mechanics—a mechanics so simple and intuitive that it was held to be a permanent fixture of physics. Uniting the theories is the idea that two observers traveling relative to each other may have different perceptions of time and space, yet the laws of nature are still uniform, and certain properties always remain invariant. Einstein developed the first theory, the theory of Special Relativity (1905), to explain and extend certain consequences of Maxwell's equations describing electromagnetism, in particular, addressing a puzzle surrounding the speed of light in a vacuum, which was predicted always to be the same, whether the light source is stationary or moving. Special Relativity considers the laws of nature from the point of view of frames of reference upon which no forces are acting, and describes the way time, distance, mass, and energy must be perceived by observers who are in uniform motion relative to each other if the speed of light must always turn out the same for all observers. Two implications of Special Relativity are space and time dilation. As speed increases, space is compressed in the direction of the motion, and time slows down. A famous example is the space traveler who returns to Earth younger than his Earth-dwelling twin, his biological processes proceeding more slowly due to his relative speed. These effects are very small at the speeds we normally experience but become significant at speeds approaching the speed of light (known as relativistic speeds). Perhaps the best-known implication of Special Relativity is the equation E=mc ², which expresses a close relation between energy and mass. The speed of light is a large number (about 300,000 km per second, or 186,000 mi per second), so the equation suggests that even small amounts of mass can be converted into enormous amounts of energy, a fact exploited by atomic power and weaponry. Einstein's General Theory of relativity extended his Special Theory to include non-inertial reference frames, frames acted on by forces and undergoing acceleration, as in cases involving gravity. The General Theory revolutionized the way gravity, too, was understood. Since Einstein, gravity is seen as a curvature in space-time itself.