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he speed of light in vacuum, denoted c, is a physical constant that is exactly equal to 299,792,458 metres per second (approximately 186,282 miles per second, or one foot per nanosecond). According to standard modern physics, visible light and all other electromagnetic radiation moves at this constant speed in vacuum, and c is the fastest speed at which matter, energy or any signal carrying information can travel through space. Ole Rømer first demonstrated in 1676 that light does not travel instantaneously by studying the apparent motion of Jupiter's moon Io (diagram shown). In an 1865 paper, James Clerk Maxwell proposed that light was an electromagnetic wave, and therefore travelled at the speed c. According to the theory of special relativity, which interrelates space and time, all observers will measure the speed of light as being the same, regardless of the inertial reference frame of the observer or the velocity of the object emitting the light

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