The '''speed of light''' in a vacuum (postulated to be constant for all inertial observers by the [[Theory of Relativity|special theory of relativity]]) is exactly 299,792,458 meters per second (approximately 186,282.3 miles per second). As the speed of light is now used to define the [[SI]] meter, this is now the value ''by definition''. | The '''speed of light''' in a vacuum (postulated to be constant for all inertial observers by the [[Theory of Relativity|special theory of relativity]]) is exactly 299,792,458 meters per second (approximately 186,282.3 miles per second). As the speed of light is now used to define the [[SI]] meter, this is now the value ''by definition''. |