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To do this, they redefined the metre as "the length of the path traveled by light in vacuum during a time interval of 1/ 299 792 458 of a second". [93] As a result of this definition, the value of the speed of light in vacuum is exactly 299 792 458 m/s [163] [164] and has become a defined constant in the SI system of units. [13]
The light-second is a unit of length useful in astronomy, telecommunications and relativistic physics.It is defined as the distance that light travels in free space in one second, and is equal to exactly 299 792 458 m (approximately 983 571 055 ft or 186 282 miles).
A value of 9.460 536 207 × 10 15 m found in some modern sources [15] [16] is the product of a mean Gregorian year (365.2425 days or 31 556 952 s) and the defined speed of light (299 792 458 m/s). Another value, 9.460 528 405 × 10 15 m , [ 17 ] is the product of the J1900.0 mean tropical year and the defined speed of light.
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, as this would require an infinite amount of energy.
The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of 1 / 299 792 458 of a second, where the second is defined by a hyperfine transition frequency of caesium.
The speed of light in IAU is the defined value c 0 = 299 792 458 m/s of the SI units. In terms of this speed, the old definition of the astronomical unit of length had the accepted value: [3] 1 au = c 0 τ A = (149 597 870 700 ± 3) m, where τ A is the transit time of light across the astronomical unit.
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 ...
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 the caesium frequency Δν Cs. The metre may be expressed directly in terms of the defining constants: 1 m = 9 192 631 770 / 299 792 458 c / Δν Cs .