[rel-uh-tiv-i-tee] 
| 1. | the state or fact of being relative. |
| 2. | Physics. a theory, formulated essentially by Albert Einstein, that all motion must be defined relative to a frame of reference and that space and time are relative, rather than absolute concepts: it consists of two principal parts. The theory dealing with uniform motion (special theory of relativity or special relativity) is based on the two postulates that physical laws have the same mathematical form when expressed in any inertial system, and the velocity of light is independent of the motion of its source and will have the same value when measured by observers moving with constant velocity with respect to each other. Derivable from these postulates are the conclusions that there can be no motion at a speed greater than that of light in a vacuum, mass increases as velocity increases, mass and energy are equivalent, and time is dependent on the relative motion of an observer measuring the time. The theory dealing with gravity (general theory of relativity or general relativity) is based on the postulate that the local effects of a gravitational field and of acceleration of an inertial system are identical. |
| 3. | dependence of a mental state or process upon the nature of the human mind: relativity of values; relativity of knowledge. |
| rel·a·tiv·i·ty (rěl'ə-tĭv'ĭ-tē) n.
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A theory concerning time, space, and the motion of objects, proposed first in 1905 by Albert Einstein in his special theory of relativity.
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.
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