Aperir le menu principal

Le velocitate de lumine, normalmente denotate per , es un constante physic que es importante in multe areas del physica. Lumine e omne altere radiation electromagnetic semper propaga a iste velocitate in spatio vacue (un vacuo), indifferente del motion del origine o le referentia inertial del observator. Le valor de equala exactemente 299.792.458 metros per secunda[1] (circa 186.282 millias per secunda o 1 pede per nanosecunda). In le theoria de relativitate, connecte spatio e tempore, e illo appare in le equation famose del equivalentia de massa e energia .[2] Le velocitate de lumine es le velocitate de omne particulas sin massa e campos associate in vacuo, e isto es predicite per le theoria currente de esser le velocitate de gravitate e de undas gravitational e un limite superior pro le velocitate del motion de energia, materia, e information.

Le velocitate de lumine in un material transparente, tal como aqua o vitro, es minus de . Le ratio inter e le velocitate a qual lumine move in un material es appellate le indice de refraction del material (). Pro exemplo, pro lumine visibile, le indice de refraction de vitro es circa 1,5; dunque, lumine in vitro propaga a ≈ 200.000 km/s. Le indice de refraction del aer pro lumine visibile es circa 1,0003, ergo le velocitate de lumine in aer es quasi .

In multe casos practic, lumine pote ser considerate como mover instantaneemente, sed pro distantias longe e mesurationes multe sensibile, le finite velocitate de lumine pote haber effectos observabile. In communicationes con distante sondas spatial, il pote requirer minutas a horas pro un message de ir ab le Terra al satellite e retornar.

Le lumine del stellas que nos vide ha partite los stellas multe annos passato; lumine ab le plus proxime stella, Alpha Centauri, require 4,4 annos de arrivar al Terra. Le velocitate finite de lumine limita le velocitate maximal theoric de computatores, depost information propaga in le computator in circuitos de dimension finite. Finalmente, le velocitate de lumine pote esser usate con mesurationes de tempore de volo de mesurar distantias grande a precision alte.

Un diagramma de 1676 per Ole Rømer monstra como calcular per le motion apparente de Io

Ole Rømer era le primo qui ha demonstrate (in 1676) que lumine move a un velocitate finite (in loco de instantaneemente) per le studio del motion apparente de Io, un luna de Jupiter. In 1905, Albert Einstein ha postulate que le velocitate de lumine in vacuo era independente del origine o referentia inertial, ha explorate le consequentias de lo postulato in le theoria de relativitate special, e ha monstrate que le parametro habeva pertinentia extra del contexto de lumine e electromagnetismo. Post seculos de accrescentemente precise mesurationes, in 1975 le velocitate de lumine era sapite de ser 299.792.458 m/s con un incertitude relative de mesuration de 4 partes per billion. In 1983, le metro era redefinite in le Systema International de Unitates (SI) como le distantia movite per lumine in un vacuo in 1/299792458 de un secunda. Como resultato, le valor numeric de in metros per secunda es nunc fixate exactemente per le definition del metro.[3]

Valor numeric, notation, e unitatesModificar

Le velocitate de lumine in vacuo es un dimensional constante physic, si su valor numeric depende sur le systema de unitates usate. In Systema International de Unitates (SI), le metro es definite per le distantia lumine move in vacuo in 1/299792458 de un secunda. Le effecto de iste definition es de fixar le velocitate de lumine in vacuo a exactemente 299.792.458 m/s. Le velocitate de lumine pote anque esser exprimite in unitates anglese, basate sur un uncia de exactemente 2,54 cm, como 186.282 millias, 698 yardes, 2 pedes, e 5 21/127 uncias per secunda. [4][5][6]

Le velocitate de lumine in un vacuo es normalmente denotate per  , pro "constante" o le latin celeritas ("celeritate"). Originalmente, le symbolo   era usate, introducite per James Clerk Maxwell in 1865. In 1856, Wilhelm Eduard Weber e Rudolf Kohlrausch habeva usate   pro un constante different, depost monstrate a equalar   vices le velocitate de lumine in vacuo. In 1894, Paul Drude redefinite   con su signification moderne. Einstein ha usate   in su original discursos (in germano) super relativitate special in 1905, sed in 1907, ille ha cambiate a  , que de alora habeva devenite le symbolo standard. [7][8] Aliquandos   es usate pro le velocitate de undas in alicun medio material, e   pro le velocitate in vacuo. [9] Iste subindicate notation, que es appoiate per official litteratura del SI, ha le mesme forma como altere connexe constantes: a saper,   pro le permeabilitate del vacuo o constante magnetic,   pro le permittivitate del vacuo o constante electric, e   pro le impedantia de spatio libere. Iste articulo use   exclusivemente pro le velocitate de lumine in vacuo. In areas de physica in que le velocitate de lumine es importante, pro exemplo in relativitate, il es commun de usar systemas de unitates natural de mesuration in que   = 1.0. Quando tal systema de mesuration es usate, le velocitate de lumine evanesce del equationes de physica, perque multiplication o division per 1 non affice le resulto.

Rolo fundamental in physicaModificar

Le velocitate de lumine in vacuo es independente ambe del motion del origine del lumine e del referentia inertial del observator. Iste invariantia del velocitate de lumine era postulate per Einstein in 1905, motivate per le theoria de electromagnetismo de Maxwell e le manco de evidentia pro le luminifere ether. [10] Le invariantia ha depost essite consistentemente confirmate per multe experimentos. [11][12] Le theoria de relativitate special explora le consequentias del realitate del invariantia de   e le hypothese que le leges de physica son le mesme in omne referentias inertial. [13][14] Un consequentia es que   es le velocitate al qual omne particulas e undas sin massa, includente lumine, debe mover.

Le factor de Lorentz   es un function de velocitate  . Illo comencia a   e approcha infinitate como   approcha  . Relativitate special ha multe contra-intuitive implicationes, que ha essite verificate in multe experimentos. [15] Istos include le equivalentia de massa e energia ( ), contraction de longitude (objectos movente abbrevia), e dilation de tempore (horologios movente decelera). Le factor   per le qual longitudes contrahe e tempores dilata, sapite como le Lorentz factor, es date per  , ubi   es le velocitate del objecto. A quotidian velocitates ( ),   e relativitate special es approximate per relativitate de Galileo. Comocunque, como   accresce,   e effectos relativistic appare, e   como  .

Le resultatos de relativitate special pote ser summarisate si on considera spatio e tempore como un structura unificate sapite como spatio-tempore (con   refere le unitates de spatio e tempore), e require que theorias physic satisface un symmetria special appellate invariantia de Lorentz, cuje formulation mathematic contine le parametro  . [16] Lorentz invariantia ha devenite un quasi universal supposition de moderne theorias physic, tal como electrodynamica quantic, chromodynamica quantic, le Modello Standard del physica de particulas, e relativitate general. Como tal, le parametro   es omnipresente in physica moderne, que appare in multe contextos que sembla esser non connexe a lumine. Pro exemplo, relativitate general predice que   es anque le velocitate de gravitate e de undas gravitational. [17][18] In referentias non-intertial, (spatio curvate per gravitate o accelerate referentias), le velocitate local de lumine es constante e equal de  , sed le velocitate de lumine preter un trajectoria de longitude finite pote differer ab  , dependente sur como le distantias e durations es definite. [19]

Il es supponite generalmente in physica que constantes fundamental tal como   ha le mesme valor in omne partes de spatio-tempore, que significa illos es independente de location e invariante in tempore. Comocunque, alicun physicos propone le possibilitate que le velocitate de lumine ha alterate durante tempore. [20] Necun evidentia conclusive pro tal alterationes ha essite fundate, sed illes remane le subjecto de recerca. [21][22] Il es anque supponite generalmente in physica que   ha le mesme valor indifferente del direction de propagation, significante que illo es isotropic. Le experimento de Michelson-Morely prescribe un limite super le anisotropia de circa 10-4. Comocunque, plus recente observationes del emissiones ab nivellos energetic nucleari per un function del orientation del nucleos emittente in un campo magnetic ha reducite iste limite a 10-21. [23] Provas de isotropia continua a ser de experimental interesse. Pro exemplo, un accostamento nove al provas de Lorentz invariantia pro undas electromagnetic compara le resonante frequentias de resonatores optical como un function de orientation in spatio. Tal provas ha constatate un limite sur anisotropia de circa 10-10. [24]

Vide etiamModificar

Wikimedia Commons ha files multimedia de: Velocitate de lumine

ReferentiasModificar

  1. Penrose, R (2004). The Road to Reality: A Complete Guide to the Laws of the Universe. Vintage Books. pp. 410–1. ISBN 978-0-679-77631-4.
  2. Uzan, J-P; Leclercq, B (2008). The Natural Laws of the Universe: Understanding Fundamental Constants (http:/ / books. google. com/ ?id=dSAWX8TNpScC& pg=PA43). Springer. pp. 43–4. ISBN 0-387-73454-6.
  3. International Bureau of Weights and Measures (2006), The International System of Units (SI) (http:/ / www. bipm. org/ utils/ common/ pdf/ si_brochure_8_en. pdf) (8th ed.), p. 112, ISBN 92-822-2213-6
  4. Sydenham, PH (2003). "Measurement of length" (http://books.google.com/books?id=sarHIbCVOUAC& pg=PA56).In Boyes, W. Instrumentation Reference Book (3rd ed.). Butterworth–Heinemann. p. 56. ISBN 0-7506-7123-8
  5. "CODATA value: Speed of Light in Vacuum" (http://physics.nist.gov/cgi-bin/cuu/Value?c).The NIST reference on Constants, Units, and Uncertainty. NIST. . Retrieved 2009-08-21.
  6. Jespersen, J; Fitz-Randolph, J; Robb, J (1999). From Sundials to Atomic Clocks: Understanding Time and Frequency (http://books.google.com/?id=Z7chuo4ebUAC& pg=PA280) (Reprint of National Bureau of Standards 1977, 2nd ed.). Courier Dover. p. 280. ISBN 0-486-40913-9
  7. Gibbs, P (2004). "Why is c the symbol for the speed of light?" (http://www.webcitation.org/5lLMPPN4L). Usenet Physics FAQ. University of California, Riverside. Archived from the original (http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/c.html) on 2009-11-17. Retrieved 2009-11-16.
  8. Mendelson, KS (2006). "The story of c" (subscription required). American Journal of Physics 74 (11): 995–997. doi:10.1119/1.2238887.
  9. Lawrie, ID (2002). "Appendix C: Natural units" (http://books.google.com/books?id=9HZStxmfi3UC& pg=PA540). A Unified Grand Tour of Theoretical Physics (2nd ed.). CRC Press. p. 540. ISBN 0-7503-0604-1.
  10. Einstein, A (1905). "Zur Elektrodynamik bewegter Körper" (http://www.pro-physik.de/Phy/pdfs/ger_890_921.pdf) (in german). Annalen der Physik 17: 890–921.
  11. Hsu, L (2006). "Appendix A: Systems of units and the development of relativity theories" (http://books.google.com/books?id=amLqckyrvUwC& pg=PA428). A Broader View of Relativity: General Implications of Lorentz and Poincaré Invariance (2nd ed.). World Scientific. pp. 427–8. ISBN 981-256-651-1.
  12. Zhang, YZ (1997). Special Relativity and Its Experimental Foundations (http://www.worldscibooks.com/physics/3180.html). Advanced Series on Theoretical Physical Science. 4. World Scientific. pp. 172–3. ISBN 981-02-2749-3.
  13. d'Inverno, R (1992). Introducing Einstein's Relativity. Oxford University Press. pp. 19–20. ISBN 0-19-859686-3.
  14. Sriranjan, B (2004). "Postulates of the special theory of relativity and their consequences" (http://books.google.com/books?id=FsRfMvyudlAC& pg=PA20#v=onepage& q=& f=false). The Special Theory to Relativity. PHI Learning. pp. 20 ff. ISBN 81-203-1963-X.
  15. Roberts, T; Schleif, S; Dlugosz, JM (ed.) (2007). "What is the experimental basis of Special Relativity?" (http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html). Usenet Physics FAQ. University of California, Riverside.
  16. Hartle, JB (2003). Gravity: An Introduction to Einstein's General Relativity. Addison-Wesley. pp. 52–9. ISBN 981-02-2749-3.
  17. Hartle, JB (2003). Gravity: An Introduction to Einstein's General Relativity. Addison-Wesley. p. 332. ISBN 981-02-2749-3.
  18. Schäfer, G; Brügmann, MH (2008). "Propagation of light in the gravitational filed of binary systems to quadratic order in Newton's gravitational constant: Part 3: ‘On the speed-of-gravity controversy’" (http://books.google.com/?id=QYnfdXOI8-QC& pg=PA111). In Dittus, H; Lämmerzahl, C; Turyshev, SG. Lasers, clocks and drag-free control: Exploration of relativistic gravity in space. Springer. ISBN 3-540-34376-8.
  19. Gibbs, P (1997). "Is The Speed of Light Constant?" (http://www.webcitation.org/5lLQD61qh). In Carlip, S. Usenet Physics FAQ. University of California, Riverside.
  20. "‘c’ is the speed of light, isn’t it?". American Journal of Physics 73: 240–7.
  21. Uzan, J-P (2003). "The fundamental constants and their variation: observational status and theoretical motivations". Reviews of Modern Physics 74: 403.
  22. Farrell, DJ; Dunning-Davies, J (2007). "The constancy, or otherwise, of the speed of light" (http:/ / books. google. com/ books?id=CZzOKIcQqxMC& pg=PA71). In Ross, LV. New Research on Astrophysics, Neutron Stars and Galaxy Clusters. Nova Publishers. pp. 71ff. ISBN 1-60021-110-0.
  23. Lang, KR (1999). Astrophysical formulae (http://books.google.com/?id=OvTjLcQ4MCQC& pg=PA152) (3rd ed.). Birkhäuser. p. 152. ISBN 3-540-29692-1.
  24. Herrmann, S; Senger, A; Kovalchuk, E; Müller, H; Peters, A (2005). "Test of the isotropy of the speed of light using a continuously rotating optical resonator". Phys Rev Lett 95: 150401