Speed of light (c)

How to read it

The number is exact because the metre is defined by it. OpenStax’s University Physics explains that in 1983 the metre became the distance light travels in a vacuum in 1/299,792,458 of a second, after the speed of light had come to be known as exactly 299,792,458 m/s. The BIPM’s SI Brochure states the definition the same way, which makes c = 299 792 458 m s⁻¹ an exact relation, and NIST’s 2022 CODATA table gives c with its uncertainty as “exact”. Einstein’s second postulate of special relativity, in OpenStax’s words, is that light travels in a vacuum with the same speed c in any direction in all inertial frames, and the equation E = mc², which relates energy to mass, is an outcome of his theory.

Also searched as: c, c in E=mc2, c in physics, speed of light constant, speed of light symbol, what does c stand for in physics.

Other c’s

In print, c for the speed of light is italic, because it is a symbol for a quantity, while the upright c is the prefix centi, one hundredth, as in cm (BIPM). The Green Book also lists c among the letters for the speed of anything, beside v, u and w, so a c in a formula is the speed of light only when the text says so.