1834, "fact or condition of being relative" (apparently coined by Coleridge, of God, in "Notes on Waterland's Vindication of Christ's Divinity"), from relative (adj.) + -ity. In scientific use, connected to the theory of Albert Einstein (1879-1955), published 1905 (special theory of relativity) and 1915 (general theory of relativity), but the word was used in roughly this sense by J.C. Maxwell in 1876.
The theory of space and time developed by Albert Einstein, based on the postulates that all the laws of physics are equally valid in all reference frames moving at a constant speed relative to each other (that is, in all inertial frames), and that the speed of light is observed to be the same in all reference frames. To compare measurements of length or time made in different inertial frames, their values must be regarded as components of vectors in a 4-dimensional space called Minkowski space-time. Different frames are related to each other mathematically by Lorentz transformations. Special Relativity predicts that all massless particles, such as photons, are always necessarily moving at the speed of light, while all particles with mass are always necessarily moving slower than the speed of light, and only particles with mass can be at rest. The theory also states that velocities are not simply additive. For example, if an object (such as a spacecraft) moving with velocity v1 observes another faster object (such as a rocket fired from the spacecraft) moving ahead of it in the same direction with velocity v2, an observer at rest observes the faster object moving at velocity that is not just the sum of v1 and v2, but at the slower speed v1 + v2/1 + v1v2/c2 . See also General Relativity, time dilation.
Either of two theories in physics developed by Albert Einstein, General Relativity or Special Relativity. See Notes at Einstein, gravity, space-time.
Our Living Language : 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=mc2, 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.
The “special theory of relativity” is based on the principle of special relativity, which states that all observers moving at constant velocities with respect to each other should find the same laws of nature operating in their frames of reference. It follows from this principle that the speed of light would have to appear to be the same to every observer. The theory predicts that moving clocks will appear to run slower than stationary ones (see time dilation), that moving objects will appear shorter and heavier than stationary ones, and that energy and mass are equivalent (see E = mc2). There is abundant experimental confirmation of these predictions.
The general theory of relativity is the modern theory of gravitation, proposed in 1915, also by Albert Einstein. The central point of the theory is the principle of general relativity, which states that all observers, regardless of their state of motion, will see the same laws of physics operating in the universe. The most famous prediction of the theory is that light rays passing near the sun will be bent — a prediction that has been well verified.
Note: The special and general theories of relativity have had important implications for thought in general. They show that no frame of reference for observation of nature is more correct than any other.
Note: It is important to distinguish between the theory of relativity, in which the laws of nature are the same for all observers anywhere in the universe, and the philosophical doctrine of relativism, which holds that there are no absolute truths. The similarity in their names has been a source of confusion.