| fundamental physical constant determining the general properties of space-time and affecting all particles and fields, whose value is set to 299 792 458 m/s
Note 1 to entry: Any electromagnetic wave or gravitational wave propagates in vacuum with the speed of light in vacuum. The speed of light in vacuum is also the upper limit of speed for any transfer of matter, energy, or information.
Note 2 to entry: The speed of light in vacuum is used in the SI to define the metre.
Note 3 to entry: The term “luminal speed" is used in relativity theory. Related concepts are "subluminal speed" and "superluminal speed".
The term "speed of light" shall be used only when the context indicates without ambiguity that the full term “speed of light in vacuum” is meant, because "speed of light" is also observed in matter where its value is always less than that in vacuum.
The term “light speed” is deprecated because it is not a short form of the preferred term.
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