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In physics, length contraction is the phenomenon of a decrease in length of an object as measured by an observer which is traveling at any non-zero velocity relative to the object. This contraction (more formally called Lorentz contraction or Lorentz–FitzGerald contraction after Hendrik Lorentz and George FitzGerald) is usually only noticeable at a substantial fraction of the speed of light. Length contraction is only in the direction parallel to the direction in which the observed body is travelling. This effect is negligible at everyday speeds, and can be ignored for all regular purposes. Only at greater speeds does it become relevant. At a speed of 13,400,000 m/s (30 million mph, 0.0447), the contracted length is 99.9% of the length at rest; at a speed of 42,300,000 m/s (95 million mph, 0.141), the length is still 99%. As the magnitude of the velocity approaches the speed of light, the effect becomes dominant, as can be seen from the formula: : where : is the proper length (the length of the object in its rest frame), : is the length observed by an observer in relative motion with respect to the object, : is the relative velocity between the observer and the moving object, : is the speed of light, and the ''Lorentz factor'', , is defined as :. In this equation it is assumed that the object is parallel with its line of movement. For the observer in relative movement, the length of the object is measured by subtracting the simultaneously measured distances of both ends of the object. For more general conversions, see the Lorentz transformations. An observer at rest viewing an object travelling very close to the speed of light would observe the length of the object in the direction of motion as very near zero. ==History== (詳細はGeorge FitzGerald (1889) and Lorentz–FitzGerald contraction hypothesis). Eventually, Albert Einstein (1905) was the first〔 to completely remove the ad hoc character from the contraction hypothesis, by demonstrating that this contraction did not require motion through a supposed aether, but could be explained using special relativity, which changed our notions of space, time, and simultaneity.〔. See also: (English translation ).〕 Einstein's view was further elaborated by Hermann Minkowski, who demonstrated the geometrical interpretation of all relativistic effects by introducing his concept of four-dimensional spacetime.〔 : *Various English translations on Wikisource: Space and Time〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Length contraction」の詳細全文を読む スポンサード リンク
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