# The Speed of Light

**Episode #6 of the c****ourse “Most important numbers in the world”**

**299,792,458 meters per second**

For many thousands of years it was assumed that light traveled instantaneously, because its movement is so incredibly fast and difficult to measure. It wasn’t until the 17th century that the fact that light moves at a finite speed was demonstrated for the first time by Ole Romer. Then, in the 19th century, James Clerk Maxwell defined “c” as the symbol for the speed at which light traveled in an electromagnetic wave.

Finally, it was Albert Einstein in the 20th century who enhanced the understanding of “c” as a measurement of speed. Today, the constant value of “c” that is used to represent the speed of light is calculated at exactly 299,792,458 meters per second. The International Standard Unit of measurement, the SI “meter,” was redefined in the 1980s to precisely correlate with the most modern understandings of the speed of light.

The speed of light is so important because it is understood as the upper limit of speed. As far as physics understands, all matter and information break down when traveling at the speed of light, becoming only transfers of energy in a vacuum. The transfer of energy occurs regardless of the direction and wavelength of the initial source, as well as the reference point of the observer. The speed at which the breakdown of information into energy occurs—the speed at which light travels—is the connection between the concepts of “time” and “space” as well as being essential to understanding the limits of potential energy. Einstein’s famous energy-mass equivalence, expressed as *E* = *mc*^{2}*, *states how the constant speed of light is essential to understanding how mass and energy are equivalent to one another but various existences of the same physical potential.

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