![]() ![]() ![]() George throws a baseball at 90 mph (140 km/h) in the direction the train is moving. There is a train rushing by at 30 mph (48 km/h). George is standing on the ground next to some train tracks (railroad). This behavior is different from our common ideas about motion as shown by this example: This is sometimes expressed as "the speed of light is independent of the reference frame."Įxample explaining how speed does not depend on reference frame The special theory of relativity is based on the prediction, so far upheld by observations, that the measured speed of light in a vacuum is the same whether or not the source of the light and the person doing the measuring are moving relative to each other. In the theory of relativity, c interrelates space and time, and appears in the famous equation of mass–energy equivalence E = mc 2. Such particles and waves travel at c regardless of the motion of the source or the inertial frame of reference of the observer. It is predicted by the current theory to be the speed of gravity (that is, gravitational waves). ![]() It is the speed of all massless particles such as photons, and associated fields-including electromagnetic radiation such as light-in a vacuum. A photon (particle of light) travels at this speed in a vacuum.Īccording to special relativity, c is the maximum speed at which all energy, matter, and physical information in the universe can travel. It is exactly 299,792,458 metres per second (983,571,056 feet per second) by definition. The speed of light, in any medium,which is usually denoted by c, is a physical constant important in many areas of physics.It is denoted by 'c^0' especially in vacuum medium, although the symbol 'c' can be used to refer to that in any medium. ![]()
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