# Definition:Speed of Light

## Physical Law

The speed of light (in a vacuum) is a physical constant.

Information cannot travel faster.

### Symbol

$c$

The symbol for the speed of light is $c$.

Its $\LaTeX$ code is c .

### Dimension

The speed of light has the dimension $\mathsf {L T^{-1} }$.

### Units

The SI unit for the speed of light is given as metres per second: $\mathrm {m \, s^{-1} }$

The CGS unit for the speed of light is given as centimetres per second: $\mathrm {cm \, s^{-1} }$

### Value

The value of the speed of light is:

 $\ds c$ $=$ $\ds 299 \, 792 \, 458$ $\mathrm {m \, s^{-1} }$ $\quad$ (exact) $\ds$ $=$ $\ds 2 \cdotp 99792 \, 458 \times 10^{10}$ $\mathrm {cm \, s^{-1} }$ $\quad$ (exact)

## Also known as

The speed of light is also seen referred to as velocity of light.

However, it can be argued that speed of light is better, as in this context we are concerned only with the magnitude of that velocity.

Sometimes the words in vacuo are added, which is Latin for in a vacuum.

## Historical Note

Until the $1983$ definition of the metre as exactly $\dfrac 1 {299 \, 792 \, 458}$ of the distance travelled by light in $1$ second, the speed of light has been the subject of measurement.

This measurement has been accomplished to greater and greater accuracy over the course of history.

Historical Measurements of Speed of Light
Date Technique Value $(\mathrm {km} \, \mathrm s^{-1})$ Tolerance Error
$1675$ Ole Christensen Rømer and Christiaan Huygens observing the moons of Jupiter $220 \, 000$ $-27 \%$
$1729$ James Bradley aberration of light $301 \, 000$ $+0 \cdotp 40 \%$
$1849$ Armand Hippolyte Louis Fizeau toothed wheel $315 \, 000$ $+5 \cdotp 1 \%$
$1862$ Jean Bernard Léon Foucault rotating mirror $298 \, 000$ $\pm 500$ $−0 \cdotp 60 \%$
$1907$ Rosa and Dorsey EM constants $299 \, 710$ $\pm 30$ $−280$ ppm
$1926$ Albert Abraham Michelson rotating mirror $299 \, 796$ $\pm 4$ $+12$ ppm
$1950$ Essen and Gordon-Smith cavity resonator $299 \, 792 \cdotp 5$ $\pm 3 \cdotp 0$ $+0 \cdotp 14$ ppm
$1958$ K.D. Froome radio interferometry $299 \, 792 \cdotp 50$ $\pm 0 \cdotp 10$ $+0 \cdotp 14$ ppm
$1972$ Evenson et al. laser interferometry $299 \, 792 \cdotp 4562$ $\pm 0 \cdotp 0011$ $−0 \cdotp 006$ ppm