Wikipedia:Today's featured article/October 29, 2004

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Diagram of a light cone

The speed of light in a vacuum is exactly equal to 299,792,458 metres per second (approximately 186,282 miles per second). This exact speed is a definition, not a measurement, as the metre is defined in terms of the speed of light and not vice versa. According to standard modern physical theory, all electromagnetic radiation, including visible light, propagates (or moves) at a constant speed in vacuo, known as the speed of light, which is a physical constant denoted as c. According to the theory of special relativity, all observers will measure the speed of light as being the same, regardless of the reference frame of the observer or the velocity of the object emitting the light. (more...)

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