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- Measuring the Speed of LightFrom
*The Hutchinson Unabridged Encyclopedia with Atlas and Weather guide*

The speed of light in a vacuum, c, is a fundamental constant of nature. It occurs in many scientific equations, such as Einstein's famous E = mc2. Numerous attempts have been made to determine c as accurately as possible. - Michelson-Morley experimentFrom
*Astronomy Encyclopedia*

The experiment attempted to detect the motion of the Earth through the luminiferous ether. Their apparatus split a beam from a common source into two parts, one travelling at right angles to the other. Both beams were reflected and recombined to produce an interference pattern. - SpacetimeFrom
*Collins Dictionary of Astronomy*

The single physical entity into which the concepts of space and time can be unified such that an event may be specified mathematically by four coordinates, three giving the position in space and one the time. The path of a particle in spacetime is called its world line. - Special relativity: Topic PageA system of mechanics applicable at high velocities (approaching the velocity of light) in the absence of gravitation; a generalization of Newtonian mechanics, due almost entirely to Albert Einstein (1905). Its fundamental postulates are that the velocity of light c is the same for all observers, no matter how they are moving; that the laws of physics are the same in all inertial frames; and that all such frames are equivalent.

- Relativity: Topic PagePhysical theory, introduced by Albert Einstein, that discards the concept of absolute motion and instead treats only relative motion between two systems or frames of reference. One consequence of the theory is that space and time are no longer viewed as separate, independent entities but rather are seen to form a four-dimensional continuum called space-time.
- Lorentz contraction: Topic PageIn physics, contraction or foreshortening of a moving body in the direction of its motion, proposed by H. A. Lorentz on theoretical grounds and based on an earlier suggestion by G. F. Fitzgerald; it is sometimes called the Fitzgerald, or Lorentz-Fitzgerald, contraction.
- TimeFrom
*Astronomy Encyclopedia*

Property of the Universe determined by the observed order of events and in which effects follow causes. Newtonian or classical physics allowed the existence of an absolute time framework, whereby different observers could agree on the time of occurrence of an event by making an allowance for the travel times of light over different distances. - Geometry, Riemannian : Topic PageA type of non-Euclidean geometry developed by Riemann in 1854. In Euclidean geometry, the distance between two neighbouring points on a plane is given by a relationship of the form

ds2 = dx2 + dy2