Speed of light
The speed of light in a vacuum (postulated to be constant for all inertial observers by the Special Theory of Relativity) is 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.
In physics, it is often represented in equations by the letter c, as in
- λ = c / f
(wavelength of an electromagnetic wave in vacuum = the speed of light divided by the wave's frequency).
The speed of light is about one foot per nanosecond. The late computer pioneer Admiral Grace Hopper was fond of keeping foot-long lengths of wire in her purse; she used them as props for her talks, referring to them as "nanoseconds," and using them to explain how the speed of light set limitations on computing systems: no signal could possibly propagate in any wire faster than the speed of light.
The speed of light is slower in any medium which is not a vacuum, and varies from medium to medium. This variation gives rise to (as a result of quantum mechanics, particularly the concept of a path of least action) the phenomenon of refraction. When a charged particle exceeds the speed of light in the medium in which it is travelling, it emits Cherenkov Radiation.
Since the speed of light in a vacuum is observed to be constant, it can be used to define distances as well. The distance that light travels in one year is known as a light-year, which is about 6 million million (6x1012) miles.
The speed of light raises questions regarding the age of the universe, which are usually summed up under the term "starlight problem".