Speed of light

In physics, the speed of light in vacuum, commonly denoted by c, is seen as one of the constants of nature. The main postulate of special relativity states that the velocity of light is independent of the motion of the light source; its speed is the same in all inertial frames (coordinate systems moving with constant velocity with respect to each other).

Already Galileo Galilei suspected that light has a finite velocity and tried in vain to measure it. About forty years later, in 1675, the Danish atronomer Olaf Roemer estimated that it takes about 11 minutes (660 seconds) for light to travel from the Sun to the Earth. He could make this estimate by observing eclipses of the first satellite of the planet Jupiter. Later this value was refined, by similar astronomical observations, to 499 seconds. In 1849 Hippolyte Louis Fizeau determined by an experiment on Earth that c is 3.15⋅10¹⁰ cm/s. Later work brought this down to just under 3⋅10¹⁰ cm/s.

In 1975 the 15th CGPS (Conférence Générale des Poids et Mesures), noticing that the uncertainty in the determination of the meter was about as large as the uncertainty in the measurements of the speed of light, recommended the definition

c ≡ 299 792 458 m/s.

At the same time the metre was redefined as the length of the path travelled by light in vacuum during a time interval of 1/c of a second. This definition of the meter was possible because earlier, in 1968, the second was defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.