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{{dated prod|concern = No reason for this to be a separate article. This appears to be mainly about the speed of light rather than the Metre. The main article on ] already mentions this redefinition as is the appropriate place for this.|month = July|day = 30|year = 2010|time = 12:29|timestamp = 20100730122906}}
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In 1983 for both practical and philosophical reasons, the 17th meeting of the ] (CGPM) decided to change the definition of the meter to "the length of the path travelled by light in vacuum during a time interval of {{frac|1|{{val|299792458}}}} of a second". In 1983 for both practical and philosophical reasons, the 17th meeting of the ] (CGPM) decided to change the definition of the meter to "the length of the path travelled by light in vacuum during a time interval of {{frac|1|{{val|299792458}}}} of a second".


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In 1983 for both practical and philosophical reasons, the 17th meeting of the Conférence Générale des Poids et Mesures (CGPM) decided to change the definition of the meter to "the length of the path travelled by light in vacuum during a time interval of 1⁄299792458 of a second".

Increased accuracy and redefinition of the metre

See also: Metre
History of measurements of c
Year Author and method Value (km/s)
1675 Rømer and Huygens, moons of Jupiter 220000
1729 James Bradley, aberration of light 301000
1849 Hippolyte Fizeau, toothed wheel 315000
1862 Léon Foucault, rotating mirror 298000±500
1907 Rosa and Dorsey, EM constants 299710±30
1926 Albert Michelson, rotating mirror 299796±4
1950 Essen and Gordon-Smith, cavity resonator 299792.5±3.0
1958 K.D. Froome, radio interferometry 299792.50±0.10
1972 Evenson et al., laser interferometry 299792.4562±0.0011
1983 17th CGPM, definition of the metre 299792.458 (exact)

In the second half of the 20th century much progress was made in increasing the accuracy of measurements of the speed of light, first by cavity resonance techniques and later by laser interferometer techniques. In 1972, using the latter method, a team at the US National Institute of Standards and Technology (NIST) laboratories in Boulder, Colorado determined the speed of light to be c = 299792456.2±1.1 m/s. Almost all the uncertainty in this measurement of the speed of light was due to uncertainty in the length of the metre.

Since 1960, the metre had been defined as a given number of wavelengths of the light of one of the spectral lines of krypton, but it turned out that the profile of the chosen spectral line was not perfectly symmetrical. This made its wavelength, and hence the length of the metre, ambiguous, because the definition did not specify what point on the line profile it referred to.

In 1975, considering that similar measurements of c agreed with each other and their uncertainty was comparable to that in the realization of the metre, the 15th Conférence Générale des Poids et Mesures (CGPM) recommended using 299792458 m/s for "the speed of propagation of electromagnetic waves in vacuum". In 1983, because the definition of the metre did not allow a sufficiently precise realization of it for all requirements, the 17th CGPM decided on a new definition: "the length of the path travelled by light in vacuum during a time interval of 1⁄299792458 of a second". The effect of this definition gives the speed of light the exact value 299792458 m/s. As a result, in the SI system of units the speed of light is now a defined constant. Improved experimental techniques do not affect the value of the speed of light in SI units, but do result in a more precise realisation of the metre.

References

  1. "Touchant le mouvement de la lumiere trouvé par M. Rŏmer de l'Académie Royale des Sciences" (PDF). Journal des sçavans (in French): 233–36. 1676.
    Translated in "On the Motion of Light by M. Romer". Philosophical Transactions of the Royal Society. 12 (136): 893–95. 1677. (As reproduced in Hutton, C; Shaw, G; Pearson, R eds. (1809). "On the Motion of Light by M. Romer". The Philosophical Transactions of the Royal Society of London, from Their Commencement in 1665, in the Year 1800: Abridged. Vol. 2. London: C. & R. Baldwin. pp. 397–98. {{cite book}}: |first3= has generic name (help); External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help))
    The account published in Journal des sçavans was based on a report that Rømer read to the French Academy of Sciences in November 1676 (Cohen, 1940, p. 346).
  2. Huygens, C (1690). Traitée de la Lumière (in French). Pierre van der Aa. pp. 8–9.
  3. ^ Gibbs, P (1997). "How is the speed of light measured?". Usenet Physics FAQ. University of California, Riverside. Retrieved 2010-01-13.
  4. Essen, L; Gordon-Smith, AC (1948). "The Velocity of Propagation of Electromagnetic Waves Derived from the Resonant Frequencies of a Cylindrical Cavity Resonator". Proceedings of the Royal Society of London A. 194 (1038): 348–361. Bibcode:1948RSPSA.194..348E. doi:10.1098/rspa.1948.0085. JSTOR 98293.
  5. Rosa, EB; Dorsey, NE (1907). Bulletin of the Bureau of Standards. 3: 433. doi:10.1103/PhysRevSeriesI.22.367. {{cite journal}}: Missing or empty |title= (help)
  6. Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1086/143021, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1086/143021 instead.
  7. Essen, L (1950). "The Velocity of Propagation of Electromagnetic Waves Derived from the Resonant Frequencies of a Cylindrical Cavity Resonator". Proceedings of the Royal Society of London A. 204 (1077): 260–277. Bibcode:1950RSPSA.204..260E. doi:10.1098/rspa.1950.0172. JSTOR 98433.
  8. Froome, KD (1958). "A New Determination of the Free-Space Velocity of Electromagnetic Waves". Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences,. 247 (1248). The Royal Society: 109–-122. doi:10.1098/rspa.1958.0172.{{cite journal}}: CS1 maint: extra punctuation (link)
  9. ^ Evenson, KM (1972). "Speed of Light from Direct Frequency and Wavelength Measurements of the Methane-Stabilized Laser". Physical Review Letters. 29: 1346–49. doi:10.1103/PhysRevLett.29.1346. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  10. ^ "Resolution 1 of the 17th CGPM". Conférence Générale des Poids et Mesures. BIPM. 1983. Retrieved 2009-08-23.
  11. ^ Sullivan, DB. "Speed of Light from Direct Frequency and Wavelength Measurements" (PDF). NIST. pp. 191–93. Retrieved 2009-08-22.
  12. ^ Resolution 2 of the 15th CGPM. BIPM. 1975. Retrieved 2009-09-09. {{cite book}}: |work= ignored (help)
  13. Resolution 6 of the 11th CGPM. BIPM. 1960. Retrieved 2009-10-18. {{cite book}}: |work= ignored (help)
  14. Taylor, EF; Wheeler, JA (1992). Spacetime Physics: Introduction to Special Relativity (2nd ed.). Macmillan. ISBN 0716723271.
  15. Penzes, WB (2009). "Time Line for the Definition of the Meter". NIST. Retrieved 2010-01-11.
  16. Jespersen, J; Fitz-Randolph, J; Robb, J (1999). From Sundials to Atomic Clocks: Understanding Time and Frequency (Reprint of National Bureau of Standards 1977, 2nd ed.). Courier Dover. p. 280. ISBN 0486409139.
  17. Adams, S (1997). Relativity: An Introduction to Space-Time Physics. CRC Press. p. 140. ISBN 0748406212. One peculiar consequence of this system of definitions is that any future refinement in our ability to measure c will not change the speed of light (which is a defined number), but will change the length of the meter!
  18. Rindler, W (2006). Relativity: Special, General, and Cosmological (2nd ed.). Oxford University Press. p. 41. ISBN 0198567316. Note that improvements in experimental accuracy will modify the meter relative to atomic wavelengths, but not the value of the speed of light!


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