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In 1983 the metre was defined as "the length of the path travelled by light in vacuum during a time interval of 1 ⁄ 299 792 458 of a second", [94] fixing the value of the speed of light at 299 792 458 m/s by definition, as described below.
It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s −1, where the second is defined in terms of ∆ν Cs." [1] 1 / 10 000 000 of the distance from the Earth's equator to the North Pole measured on the meridian arc through Paris. L kilogram: kg mass
The two-way speed of light is the average speed of light from one point, such as a source, to a mirror and back again. Because the light starts and finishes in the same place, only one clock is needed to measure the total time; thus, this speed can be experimentally determined independently of any clock synchronization scheme.
The speed of light in vacuum is defined to be exactly 299 792 458 m/s (approximately 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.
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 ...
(The reason for the change was an improved method of measuring the speed of light.) The speed of light could then be expressed exactly as c 0 = 299 792 458 m/s, a standard also adopted by the IERS numerical standards. [19] From this definition and the 2009 IAU standard, the time for light to traverse an astronomical unit is found to be τ A ...
The four universal constants that, by definition, have a numeric value 1 when expressed in these units are: c, the speed of light in vacuum, G, the gravitational constant, ħ, the reduced Planck constant, and; k B, the Boltzmann constant.
The fastest possible speed at which energy or information can travel, according to special relativity, is the speed of light in vacuum c = 299 792 458 metres per second (approximately 1 079 000 000 km/h or 671 000 000 mph). Matter cannot quite reach the speed of light