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Special relativity. 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).
With a rotational speed of 600-800 revolutions per second, the displacement was 0.2 to 0.3 mm. [5]: 128–129 Guided by similar motivations as his former partner, Foucault in 1850 was more interested in settling the particle-versus-wave debate than in determining an accurate absolute value for the speed of light. [2]
Since 1983 the metre has been defined as the distance traveled by light in vacuum in 1 ⁄ 299,792,458 second. [7] This means that the speed of light can no longer be experimentally measured in SI units, but the length of a meter can be compared experimentally against some other standard of length.
The speed of light in vacuum is defined to be exactly 299 792 458 m/s (approx. 186,282 miles per second). The fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at exactly this same speed in vacuum. Different physicists ...
In the context of this article, "faster-than-light" means the transmission of information or matter faster than c, a constant equal to the speed of light in vacuum, which is 299,792,458 m/s (by definition of the metre) [ 3] or about 186,282.397 miles per second. This is not quite the same as traveling faster than light, since:
Speed of gravity; Exact values; metres per second: 299 792 458: Approximate values (to three significant digits) kilometres per hour: 1 080 000 000: miles per second: 186 000: miles per hour [1] 671 000 000: astronomical units per day: 173 [Note 1] parsecs per year: 0.307 [Note 2] Approximate light signal travel times; Distance: Time: one foot ...
Slow light. Slow light is the propagation of an optical pulse or other modulation of an optical carrier at a very low group velocity. Slow light occurs when a propagating pulse is substantially slowed by the interaction with the medium in which the propagation takes place. Group velocities below the speed of light in vacuum c were known to be ...
In this table, c = 29 979 245 800 is the numeric value of the speed of light in vacuum when expressed in units of centimetres per second. The symbol "≘" is used instead of "=" as a reminder that the units are corresponding but not equal.