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, to nine significant figures, exactly 299,792,458 meters per second (approximately 186,282.3 miles per second). As the speed of light is used to define the SI meter, this is the value by definition.

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 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. For instance, the nearest large galaxy to our own Milky Way Galaxy is the Andromeda Galaxy, which astronomers have measured to be about 2.5 million light years away, meaning visible light from the galaxy has taken 2.5 million years to reach Earth.

The speed of light raises several questions for Young Earth Creationists including that of the starlight problem.