The speed of light in free space (that is, in a vacuum) is a constant that has been measured to considerable accuracy. To nine significant figures, it is 299,792,458 meters per second (2.99792458 x 10 8 m/s). This is the speed with which all electromagnetic field s, including radio waves, infrared (IR), ultraviolet (UV), X rays, and gamma rays, propagate in a vacuum. The constant is symbolized c .
The speed of light seems, for most everyday purposes, infinite. But the true finiteness of its speed becomes apparent in satellite communications, particularly when geostationary satellites are used. These satellites orbit approximately 36,000 kilometers (km) above the Earth. The propagation delay can be noticed as increased latency when satellite Internet services are used. Electromagnetic waves, including visible light, take approximately 1.3 s to traverse the distance between the Earth and the moon, and about eight minutes (8 min) to get from the sun to the Earth. The EM waves from stars in our galaxy can take up to thousands of years to reach us; the EM waves from the most distant known objects in the Cosmos have spent billions of years on their way to us (where one billion defined as 1,000,000,000 or 10 9 ).
The speed of light in free space is independent of the relative velocity between the source and the observer. Some people find this counterintuitive, but it has been demonstrated by experiments. The most famous such experiment was conducted by two physicists, Albert Michelson and Edward Morley, in the late 1800s. They found that the speed of light is the same in all directions, despite the fact that the Earth moves through space.
Also see our Table of Physical Units and Constants.