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A collective term for two theories, special relativity and general relativity, developed by A. Einstein. The special theory of relativity, published in 1905, is concerned with the laws of physics as viewed by observers moving relative to each other at constant speed (i.e. not subject to acceleration). It describes how the motion of one observer relative to another affects measurements made by these observers. At low speeds special relativity reduces to situations describable by classical physics, embodied in Newton's laws of motion. Differences between Newton's and Einstein's physics become apparent only at velocities towards that of light. The general theory of relativity (1915) describes how the relationship between space and time is affected by the gravitational effects of matter (see gravitation) and how space and time change as seen by an observer on an accelerating object. The theory concludes that gravitational fields created by the presence of matter cause spacetime to become curved. This curvature controls the motion of bodies in space.

Subjects: physics — astronomy and cosmology.

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