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The speed of light in vacuum, commonly denoted c, is a universal physical constant that is exactly equal to 299,792,458 metres per second (approximately 300,000 kilometres per second; 186,000 miles per second; 671 million miles per hour).
speed of light in vacuum 299 792 458 m ... While the values of the physical constants are independent of the system of units in use, each uncertainty as stated ...
This value is used to define the SI unit of mass, the kilogram: "the kilogram [...] is defined by taking the fixed numerical value of h to be 6.626 070 15 × 10 −34 when expressed in the unit J⋅s, which is equal to kg⋅m 2 ⋅s −1, where the metre and the second are defined in terms of speed of light c and duration of hyperfine ...
c, the speed of light in vacuum, G, the gravitational constant, ħ, the reduced Planck constant, and; k B, the Boltzmann constant. Variants of the basic idea of Planck units exist, such as alternate choices of normalization that give other numeric values to one or more of the four constants above.
c is the speed of light (299 792 458 m⋅s −1 [8]); ε 0 is the electric constant (8.854 187 8188 (14) × 10 −12 F⋅m −1 [9]). Since the 2019 revision of the SI, the only quantity in this list that does not have an exact value in SI units is the electric constant (vacuum permittivity).
with e being the elementary charge, h being the Planck constant, and c being the speed of light in vacuum, each with exactly defined values. The relative uncertainty in the value of ε 0 is therefore the same as that for the dimensionless fine-structure constant , namely 1.6 × 10 −10 .
For example, the speed of light is defined as having the numerical value of 299 792 458 when expressed in the SI unit metres per second, and as having the numerical value of 1 when expressed in the natural units Planck length per Planck time. While its numerical value can be defined at will by the choice of units, the speed of light itself is a ...
Depending on the value assumed for the astronomical unit, this yields the speed of light as just a little more than 300,000 kilometres per second. The first measurements of the speed of light using completely terrestrial apparatus were published in 1849 by Hippolyte Fizeau (1819–96). Compared to values accepted today, Fizeau's result (about ...