In some cases, objects or waves may appear to travel faster than light (e.g., phase velocities of waves, the appearance of certain high-speed astronomical objects, and particular quantum effects). The expansion of the universe is understood to exceed the speed of light beyond a certain boundary. a b c d e Gibbs, P (1997). "How is the speed of light measured?". Usenet Physics FAQ. University of California, Riverside. Archived from the original on 21 August 2015 . Retrieved 13 January 2010. Aoki, K; Mitsui, T (2008). "A small tabletop experiment for a direct measurement of the speed of light". American Journal of Physics. 76 (9): 812–815. arXiv: 0705.3996. Bibcode: 2008AmJPh..76..812A. doi: 10.1119/1.2919743. S2CID 117454437. Cajori, Florian (1922). A History of Physics in Its Elementary Branches: Including the Evolution of Physical Laboratories. Macmillan. p.76. Barger, R.; Hall, J. (1973). "Wavelength of the 3.39-μm laser-saturated absorption line of methane". Applied Physics Letters. 22 (4): 196. Bibcode: 1973ApPhL..22..196B. doi: 10.1063/1.1654608. S2CID 1841238.

Nevertheless, at this degree of precision, the effects of general relativity must be taken into consideration when interpreting the length. The metre is considered to be a unit of proper length, whereas the AU is usually used as a unit of observed length in a given frame of reference. The values cited here follow the latter convention, and are TDB-compatible. [102] Resolution 1 of the 15th CGPM". BIPM. 1967. Archived from the original on 11 April 2021 . Retrieved 14 March 2021. Lauginie, P (2004). Measuring Speed of Light: Why? Speed of what? (PDF). Fifth International Conference for History of Science in Science Education. Keszthely, Hungary. pp.75–84. Archived from the original (PDF) on 4 July 2015 . Retrieved 12 August 2017.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. Bibcode: 1999PhRvL..82.4964S. doi: 10.1103/PhysRevLett.82.4964. S2CID 119339066. Liu, Xiaoshu; He, Vincent F.; Mikulski, Timothy M.; Palenova, Daria; Williams, Claire E.; Creighton, Jolien; Tasson, Jay D. (7 July 2020). "Measuring the speed of gravitational waves from the first and second observing run of Advanced LIGO and Advanced Virgo". Physical Review D. 102 (2): 024028. arXiv: 2005.03121. Bibcode: 2020PhRvD.102b4028L. doi: 10.1103/PhysRevD.102.024028. S2CID 220514677. Empedocles (c. 490–430 BCE) was the first to propose a theory of light [124] and claimed that light has a finite speed. [125] He maintained that light was something in motion, and therefore must take some time to travel. Aristotle argued, to the contrary, that "light is due to the presence of something, but it is not a movement". [126] Euclid and Ptolemy advanced Empedocles' emission theory of vision, where light is emitted from the eye, thus enabling sight. Based on that theory, Heron of Alexandria argued that the speed of light must be infinite because distant objects such as stars appear immediately upon opening the eyes. [127]

Because neutrinos have a small but non-zero mass, they travel through empty space very slightly more slowly than light. However, because they pass through matter much more easily than light does, there are in theory occasions when the neutrino signal from an astronomical event might reach Earth before an optical signal can, like supernovae. [25] Milonni, PW (2004). Fast light, slow light and left-handed light. CRC Press. p.25. ISBN 978-0-7503-0926-4. 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.II. From 1673 to 1682. London: C. & R. Baldwin. pp.397–398.

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Stauffer, RH (April 1997). "Finding the Speed of Light with Marshmallows". The Physics Teacher. 35 (4): 231. Bibcode: 1997PhTea..35..231S. doi: 10.1119/1.2344657 . Retrieved 15 February 2010.

Lawrie, ID (2002). "Appendix C: Natural units". A Unified Grand Tour of Theoretical Physics (2nded.). CRC Press. p.540. ISBN 978-0-7503-0604-1. According to Galileo, the lanterns he used were "at a short distance, less than a mile." Assuming the distance was not too much shorter than a mile, and that "about a thirtieth of a second is the minimum time interval distinguishable by the unaided eye", Boyer notes that Galileo's experiment could at best be said to have established a lower limit of about 60 miles per second for the velocity of light. [119]a b c Gibbs, P (1997). "Is Faster-Than-Light Travel or Communication Possible?". Usenet Physics FAQ. University of California, Riverside. Archived from the original on 10 March 2010 . Retrieved 20 August 2008. As a result of this definition, the value of the speed of light in vacuum is exactly 299 792 458m/s [159] [160] and has become a defined constant in the SI system of units. [14] Improved experimental techniques that, prior to 1983, would have measured the speed of light no longer affect the known value of the speed of light in SI units, but instead allow a more precise realization of the metre by more accurately measuring the wavelength of krypton-86 and other light sources. [161] [162] Herrmann, S; etal. (2009). "Rotating optical cavity experiment testing Lorentz invariance at the 10 −17 level". Physical Review D. 80 (100): 105011. arXiv: 1002.1284. Bibcode: 2009PhRvD..80j5011H. doi: 10.1103/PhysRevD.80.105011. S2CID 118346408. Füllekrug, M (2004). "Probing the Speed of Light with Radio Waves at Extremely Low Frequencies". Physical Review Letters. 93 (4): 043901. Bibcode: 2004PhRvL..93d3901F. doi: 10.1103/PhysRevLett.93.043901. PMID 15323762.

Halley, E (1694). "Monsieur Cassini, his New and Exact Tables for the Eclipses of the First Satellite of Jupiter, reduced to the Julian Stile and Meridian of London". Philosophical Transactions of the Royal Society. 18 (214): 237–256. Bibcode: 1694RSPT...18..237C. doi: 10.1098/rstl.1694.0048. Bartlett, D. J.; Desmond, H.; Ferreira, P. G.; Jasche, J. (17 November 2021). "Constraints on quantum gravity and the photon mass from gamma ray bursts". Physical Review D. 104 (10): 103516. arXiv: 2109.07850. Bibcode: 2021PhRvD.104j3516B. doi: 10.1103/PhysRevD.104.103516. ISSN 2470-0010. S2CID 237532210. Gibbs, P (2004) [1997]. "Why is c the symbol for the speed of light?". Usenet Physics FAQ. University of California, Riverside. Archived from the original on 25 March 2010 . Retrieved 16 November 2009. Ole Rømer first demonstrated in 1676 that light does not travel instantaneously by studying the apparent motion of Jupiter's moon Io. Progressively more accurate measurements of its speed came over the following centuries. In a paper published in 1865, James Clerk Maxwell proposed that light was an electromagnetic wave and, therefore, travelled at speed c. [7] In 1905, Albert Einstein postulated that the speed of light c with respect to any inertial frame of reference is a constant and is independent of the motion of the light source. [8] He explored the consequences of that postulate by deriving the theory of relativity and, in doing so, showed that the parameter c had relevance outside of the context of light and electromagnetism.Sriranjan, B (2004). "Postulates of the special theory of relativity and their consequences". The Special Theory to Relativity. PHI Learning Pvt. Ltd. pp.20ff. ISBN 978-81-203-1963-9. Michelson, AA; Morley, EW (1887). "On the Relative Motion of the Earth and the Luminiferous Ether". American Journal of Science. 34 (203): 333–345. doi: 10.1366/0003702874447824. S2CID 98374065. Outer space is a convenient setting for measuring the speed of light because of its large scale and nearly perfect vacuum. Typically, one measures the time needed for light to traverse some reference distance in the Solar System, such as the radius of the Earth's orbit. Historically, such measurements could be made fairly accurately, compared to how accurately the length of the reference distance is known in Earth-based units. Day 4: Lunar Orbits 7, 8 and 9". The Apollo 8 Flight Journal. NASA. Archived from the original on 4 January 2011 . Retrieved 16 December 2010.