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User:Physchim62/Speed of light constancy

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Constancy

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The idea that the speed of light is a physical constant comes from Maxwell's equations, which describe light as electromagnetic radiation. Maxwell's 1865 theory predicted the existence of radio waves, which were discovered by Heinrich Hertz in 1887, a strong confirmation of the theory. By the end of the nineteenth century, the idea of a single and constant speed of light was well established, particularly by the work of Albert Michelson.

Michelson pioneered the technique of interferometry for measuring lengths, a technique that relies on the idea that the speed of light is constant. He calibrated his technique against the then U.S. metal-bar standards of both the yard (made out of bronze) and the metre (made out of a platinum–iridium alloy), and also against the International Prototype Metre in France. His measurements indicated that the length of the bronze standard yards was steadily getting shorter, a result that was later confirmed by comparison with other standard yards in the Anglosphere. His results persuaded the United States to officially adopt the metric system in 1893 with the Mendenhall Order.[1]

Some scientists have questioned why the fundamental constants of nature, including c, have the values they do, and whether they are changing as the universe evolves.[2][3][4][5] These questions remain an interest of on-going research.[6][7][8]

Constancy over time

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Main article: Variable speed of light

There have been several theoretical speculations as to what might occur if the speed of light were slowly changing over time, although none has received general support within the physics community. Nevertheless, there are experimental tests which place an upper limit on any variation.

Several tests of general relativity consider a possible variation in the Newtonian gravitational constant G, following the Dirac large numbers hypothesis (1937).[4] From the Lunar laser ranging experiment, the maximum possible change in G is 1.1 parts in 1012 per year,[9] while analysis of more than 250,000 ranging observations of U.S. and Russian spacecraft has lowered the possible variation to 7 parts in 1014 per year.[10] Any change in the speed of light over the timescale of these observations would appear as an apparent change in G,[11] that is the data would not distinguish between a change in G and a change in c. In fact the maximum possible change in c that is derived from the data is one third of the maximum possible variation in G, or about 2 parts in 1014 per year.

Other methods for detecting a possible change in the speed of light look for changes in the fine structure constant α.[12][13] This constant is a pure number, and so its value is independent of the system of units used to measure it. It can also be expressed as a ratio of other physical constants, one of which is the speed of light:

 

Single-ion optical atomic clocks to place a very stringent constraint on the present time variation of α, with a maximum possible variation (2008) of 4 parts in 1017 per year.[14] This is consistent with data from the Oklo natural nuclear fission reactor, which suggest a possible change in α of 4.5 parts in 108 over the past 2 billion years, or 2 parts in 1017 per year.[15][16][17]

Independence of frequency

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Others have suggested that the speed of light may exhibit a small dispersion—that is, different frequencies may travel at slightly different velocities. (The invariant speed c of special relativity would the be the upper limit of the speed of light in vacuum.[18])

However, to date, such variations, if any, are smaller than experimental errors of observation—the measured speed of electromagnetic radiation is the same for different frequencies to within very stringent experimental limits.[19][20][21]

A reason for light to travel at a frequency-dependent speed would be a non-zero rest mass of the photon, the carrier of the electromagnetic force or field.[22] At present, the rest mass of the photon is taken to be zero, implying that it always travels at speed c. If the hypothetically massive photon is described by Proca theory,[23] the experimental upper bound for its mass is about 10−57 grams.[24] If photon mass is generated by a Higgs mechanism, the experimental upper limit is less sharp, m ≤ 10−14 eV/c2 (equivalent to of the order of 10−47 g).[23]

Equivalence of constants

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In modern physics, the "speed of light" is far more than simply the speed of propagation of electromagnetic radiation.[6] For example

  • special relativity implies that there is a maximum speed which all observers will measure to be the same, and this is generally assumed to be the same as the speed of light;
  • Hermann Minkowski realized in 1907 that special relativity implies that space and time are not separate concepts, but a combined in what has come to be known as Minkowski spacetime, where the speed of light plays the role of the equivalence factor between our measurements of time and our measurements of space;
  • developments in quantum theory, especially quantum electrodynamics (QED), link the speed of light to phenomena at the atomic and sub-atomic scale, particularly through the dimensionless fine structure constant.[25]

In each case, the constancy of the speed of light is used as an assumption in constructing the theory: if the assumption is incorrect, the theory will be incorrect, yet the relativity and quantum theories have survived many, many experimental tests.

References

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  1. ^ Barbrow, Louis E.; Judson, Lewis V. (1976), "The Mendenhall Order", Weights and Measures Standards of the United States: A brief history (PDF), Washington, D.C.: National Bureau of Standards, Special Publication 447.
  2. ^ Weyl, Hermann (1917), "Zur Gravitationstheorie", Ann. Phys., 359 (18): 117–45, doi:10.1002/andp.19173591804. Weyl, H. (1919), "Eine neue Erweiterung der Relativitätstheorie", Ann. Phys., 364 (10): 101–33, doi:10.1002/andp.19193641002.
  3. ^ Eddington, A. S. (1931), "On the instability of Einstein's spherical world", Month. Not. R. Astron. Soc., 90 (7): 668–78, Bibcode:1930MNRAS..90..668E, doi:10.1093/mnras/90.7.668{{citation}}: CS1 maint: unflagged free DOI (link)
  4. ^ a b Dirac, P. A. M. (1937), "The Cosmological Constants", Nature, 139 (3512): 323, doi:10.1038/139323a0, S2CID 4106534.
  5. ^ Farrell, Daniel J.; Dunning-Davies, Jeremy (2007), "The constancy, or otherwise, of the speed of light", in Ross, L. V. (ed.), New Research on Astrophysics, Neutron Stars and Galaxy Clusters, Nova Publishers, pp. 67–85, ISBN 978-1600211102.
  6. ^ a b Ellis, George F. R.; Uzan, Jean-Philippe (2005), "'c' is the speed of light, isn't it?", Am. J. Phys., 73 (3): 240–47, arXiv:gr-qc/0305099, doi:10.1119/1.1819929, S2CID 119530637.
  7. ^ Uzan, Jean-Philippe (2003), "The fundamental constants and their variation: observational status and theoretical motivations", Rev. Mod. Phys., 74: 403–55, arXiv:hep-ph/0205340, doi:10.1103/RevModPhys.75.403, S2CID 118684485.
  8. ^ Kafatos, Menas; Roy, Sisir; Roy, Malabika (2005), "Variation of Physical Constants, Redshift and the Arrow of Time", Acta Phys. Pol. B, 36: 3139–62, arXiv:astro-ph/0305117.
  9. ^ Williams, J. G.; Boggs, D. H.; Dickey, J. O.; Folkner, W. M. (2002), "Lunar Laser Tests of Gravitational Physics", in Gurzadyan, V. G.; Jantzen, R. T.; Ruffini, R. (eds.), Proceedings of the Ninth Marcel Grossman Meeting on recent developments in theoretical and experimental general relativity, gravitation and relativistic field theories, Rome, July 2–8, 2000, Singapore: World Scientific, p. 1797, ISBN 9812380108.
  10. ^ Pitjeva, E. V. (2005), "Relativistic effects and solar oblateness from radar observations of planets and spacecraft" (PDF), Astron. Lett., 31 (5): 340–49, doi:10.1134/1.1922533, S2CID 54645688.
  11. ^ Lammerzahl, C.; Preuss, O.; Dittus, H. (2006), "Is the physics within the Solar system really understood?", Proceedings of the 359th WE-Heraeus Seminar on Lasers, Clocks, and Drag-Free Technologies for Future Exploration in Space and Tests of Gravity, arXiv:gr-qc/0604052.
  12. ^ How would we know if the speed of light was varying with time?, UK National Physical Laboratory, retrieved 2009-10-18.
  13. ^ Mota, D. F. (2006), Variations of the fine structure constant in space and time, arXiv:astro-ph/0401631, Ph.D. Thesis, University of Cambridge..
  14. ^ Rosenband, T.; Hume, D.; Schmidt, P. O.; Chou, C. W.; Brusch, A.; Lorini, L.; Oskay, W. H.; Drullinger, R. E.; Fortier, T. M.; Stalnaker, J. E.; Diddams, S. A.; Swann, W. C.; Newbury, N. R.; Itano, W. M.; Wineland, D. J.; Berquist, J. C. (2008), "Frequency Ratio of Al+ and Hg+ Single-Ion Optical Clocks; Metrology at the 17th Decimal Place", Science, 319 (5871): 1808–, doi:10.1126/science.1154622, PMID 18323415, S2CID 206511320.
  15. ^ Lamoreaux, S. K.; Torgerson, J. R. (2004), "Neutron Moderation in the Oklo Natural Reactor and the Time Variation of α" (PDF), Phys. Rev. D, 69 (12): 121701, arXiv:nucl-th/0309048v3, doi:10.1103/PhysRevD.69.121701, S2CID 119337838.
  16. ^ Reich, E. S. (30 June 2004), "Speed of light may have changed recently", New Scientist, retrieved 2009-01-30.
  17. ^ Scientists Discover One Of The Constants Of The Universe Might Not Be Constant, ScienceDaily, 12 May 2005, retrieved 2009-01-30.
  18. ^ "Is The Speed of Light Constant?". Usenet Physics FAQ. Retrieved 2009-09-12.
  19. ^ Schaefer, BE (1999). "Severe limits on variations of the speed of light with frequency". Physical Review Letters. 82 (25): 4964–4966. arXiv:astro-ph/9810479. doi:10.1103/PhysRevLett.82.4964. S2CID 119339066.
  20. ^ Ellis, J; Mavromatos, DV; Sakharov, AS (2003). "Quantum-Gravity Analysis of Gamma-Ray Bursts using Wavelets". Astronomy & Astrophysics. 403 (2): 409–424. arXiv:astro-ph/0210124. doi:10.1051/0004-6361:20030263. S2CID 15388873.
  21. ^ Füllekrug, M (2004). "Probing the Speed of Light with Radio Waves at Extremely Low Frequencies". Physical Review Letters. 93 (4): 043901. doi:10.1103/PhysRevLett.93.043901. PMID 15323762.
  22. ^ Pradhan, T (2001). The Photon. Nova Press. p. 44. ISBN 1560729287.
  23. ^ a b Adelberger, E; Dvali, G; Gruzinov, A (2007). "Photon Mass Bound Destroyed by Vortices". Physical Review Letters. 98 (1): 010402. arXiv:hep-ph/0306245. doi:10.1103/PhysRevLett.98.010402. PMID 17358459. S2CID 31249827.
  24. ^ Sidharth, BG (2008). The Thermodynamic Universe. World Scientific. p. 134. ISBN 978-9812812346.
  25. ^ Itzykson, C.; Zuber, J.-B. (1980), Quantum Field Theory, McGraw-Hill, p. 705.

Further reading

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