Just as the second forms the basis for other units of time, the light-second can form the basis for other units of length, ranging from the light-nanosecond' (just under one U.S. or imperial foot) to the light-minute, light-hour and light day' that are sometimes used in popular science publications.
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Definition of the metre
| “ | The metre is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second. | ” |
This definition fixes the speed of light at exactly 299 792 458 m/s, and hence the light-second at exactly 299 792 458 m.[1]
Use in telecommunications
Communications signals on Earth rarely travel at precisely the speed of light in free space, but distances in fractions of a light-second are still useful for planning telecommunications networks as they indicate the minimum possible delay between sender and receiver.
- One light-nanosecond is 29.98 centimetres, or almost exactly one foot, which limits the speed of data transfer between different parts of a large computer.
- One light-microsecond is about 300 metres.
- The mean distance, over land, between opposite sides of the Earth is about 66.8 light-milliseconds.
- Communications satellites are typically 1.334 light-milliseconds (low earth orbit) to 0.1194 light-seconds (geostationary orbit) from the surface of the Earth. Hence there will always be a delay of at least 0.24 s in a communication via geostationary satellite: this delay is just perceptible in a transoceanic telephone conversation routed by satellite.
Use in astronomy
The light-second is a convenient unit for measuring distances in the inner Solar System, because it corresponds very closely to the radiometric data used to determine them (the match is not exact for an Earth-based observer because of a very small correction for the effects of relativity). The value of the astronomical unit in light seconds is a fundamental measurement for the calculation of modern ephemerides (tables of plantary positions): it is usually quoted as "light-time for unit distance" in tables of astronomical constants, and its currently accepted value is 499.004 786 385(20) s.[2][3]
- The mean diameter of the Earth is about 0.0425 light-seconds.
- The average distance from the Earth to the Moon is about 1.282 light-seconds.
- The diameter of the Sun is about 4.643 light-seconds.
- The average distance from the Earth to the Sun is 499.0 light-seconds.
Multiples of the light-second can be defined, although they are more used in popular science publications than in research works. For example, a light-minute is 60 light-seconds and the average distance from the Earth to the Sun is 8.317 light-minutes.
| light-minute | 60 light-seconds | 1.799 × 1010 m | average distance from the Earth to the Sun is 8.317 light-minutes |
| light-hour | 60 light-minutes = 3600 light-seconds |
1.079 × 1012 m | semi-major axis of Pluto's orbit is about 5.473 light-hours |
| light-day | 24 light-hours = 86400 light-seconds |
2.590 × 1013 m | Sedna is currently 0.52 light-days from the Sun |
| light-week | 7 light-days = 604800 light-seconds |
1.813 × 1014 m | The Oort cloud is thought to extend between 41 and 82 light-weeks out from the Sun |
Relativity and geometrized units
The light-second is a natural unit of both length and time for calculations in special and general relativity. The standard definitions of coordinates in a four-dimensional spacetime involve either multiplying time intervals by the speed of light or dividing length intervals by the speed of light:
References
- ^ International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), p. 112, ISBN 92-822-2213-6, http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf
- ^ Standish, E. M. (1998), JPL Planetary and Lunar Ephemerides, DE405/LE405, JPL IOM 312.F-98-048, http://iau-comm4.jpl.nasa.gov/de405iom/de405iom.pdf.
- ^ McCarthy, Dennis D.; Petit, Gérard, ed. (2004), "IERS Conventions (2003)", IERS Technical Note No. 32, Frankfurt: Bundesamts für Kartographie und Geodäsie, ISBN 3-89888-884-3, http://www.iers.org/MainDisp.csl?pid=46-25776.
See also
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