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The pure number we call the fine-structure constant and denote by α is a combination of the electron charge, e, the speed of light, c, and the Planck constant, h. At first we might be tempted to think that a world in which the speed of light was slower would be a different world. But this would be a mistake.
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).
Unidirectional light path in the aberration of light. Although the average speed over a two-way path can be measured, the one-way speed in one direction or the other is undefined (and not simply unknown), unless one can define what is "the same time" in two different locations.
VSL should not be confused with faster than light theories, its dependence on a medium's refractive index or its measurement in a remote observer's frame of reference in a gravitational potential. In this context, the "speed of light" refers to the limiting speed c of the theory rather than to the velocity of propagation of photons.
In special relativity, an object that has nonzero rest mass cannot travel at the speed of light. As the object approaches the speed of light, the object's energy and momentum increase without bound. In the first years after 1905, following Lorentz and Einstein, the terms longitudinal and transverse mass were still in use.
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. [7]
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.