|
Faster-than-light (also superluminal or FTL) communication and travel refer to the propagation of information or matter faster than the speed of light. Under the special theory of relativity, a particle (that has rest mass) with subluminal velocity needs infinite energy to accelerate to the speed of light, although special relativity does not forbid the existence of particles that travel faster than light at all times (tachyons). On the other hand, what some physicists refer to as "apparent" or "effective" FTL〔 〕〔 〕〔 〕〔 〕 depends on the hypothesis that unusually distorted regions of spacetime might permit matter to reach distant locations in less time than light could in normal or undistorted spacetime. Although according to current theories matter is still required to travel subluminally with respect to the locally distorted spacetime region, ''apparent'' FTL is not excluded by general relativity. Examples of FTL proposals are the Alcubierre drive and the traversable wormhole, although their physical plausibility is uncertain. ==FTL travel of non-information== In the context of this article, FTL is the transmission of information or matter faster than ''c'', a constant equal to the speed of light in a vacuum, which is 299,792,458 m/s (by definition) or about 186,282.4 miles per second. This is not quite the same as traveling faster than light, since: * Some processes propagate faster than ''c'', but cannot carry information (see examples in the sections immediately following). * Light travels at speed ''c/n'' when not in a vacuum but travelling through a medium with refractive index = ''n'' (causing refraction), and in some materials other particles can travel faster than ''c/n'' (but still slower than ''c''), leading to Cherenkov radiation (see phase velocity below). Neither of these phenomena violates special relativity or creates problems with causality, and thus neither qualifies as ''FTL'' as described here. In the following examples, certain influences may appear to travel faster than light, but they do not convey energy or information faster than light, so they do not violate special relativity. 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「Faster-than-light」の詳細全文を読む スポンサード リンク
|