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In modern quantum physics, the electromagnetic field is described by the theory of quantum electrodynamics (QED). In this theory, light is described by the fundamental excitations (or quanta) of the electromagnetic field, called photons. In QED, photons are massless particles and thus, according to special relativity, they travel at the speed of light in vacuum. [24] 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. Bibcode: 2007PhRvL..98a0402A. doi: 10.1103/PhysRevLett.98.010402. PMID 17358459. S2CID 31249827.

Penrose, R (2004). The Road to Reality: A Complete Guide to the Laws of the Universe. Vintage Books. pp. 410–411. ISBN 978-0-679-77631-4. ... the most accurate standard for the metre is conveniently defined so that there are exactly 299 792 458 of them to the distance travelled by light in a standard second, giving a value for the metre that very accurately matches the now inadequately precise standard metre rule in Paris. 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] Hartle, JB (2003). Gravity: An Introduction to Einstein's General Relativity. Addison-Wesley. p.332. ISBN 978-981-02-2749-4. Mendelson, KS (2006). "The story of c". American Journal of Physics. 74 (11): 995–997. Bibcode: 2006AmJPh..74..995M. doi: 10.1119/1.2238887. 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.

Another method is to use the aberration of light, discovered and explained by James Bradley in the 18th century. [95] This effect results from the vector addition of the velocity of light arriving from a distant source (such as a star) and the velocity of its observer (see diagram on the right). A moving observer thus sees the light coming from a slightly different direction and consequently sees the source at a position shifted from its original position. Since the direction of the Earth's velocity changes continuously as the Earth orbits the Sun, this effect causes the apparent position of stars to move around. From the angular difference in the position of stars (maximally 20.5 arcseconds) [96] it is possible to express the speed of light in terms of the Earth's velocity around the Sun, which with the known length of a year can be converted to the time needed to travel from the Sun to the Earth. In 1729, Bradley used this method to derive that light travelled 10 210 times faster than the Earth in its orbit (the modern figure is 10 066 times faster) or, equivalently, that it would take light 8minutes 12seconds to travel from the Sun to the Earth. [95] Astronomical unit A detailed discussion of the interferometer and its use for determining the speed of light can be found in Vaughan (1989). [114] The speed of light in vacuum is usually denoted by a lowercase c, for "constant" or the Latin celeritas (meaning 'swiftness, celerity'). In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch had used c for a different constant that was later shown to equal √ 2 times the speed of light in vacuum. Historically, the symbol V was used as an alternative symbol for the speed of light, introduced by James Clerk Maxwell in 1865. In 1894, Paul Drude redefined c with its modern meaning. Einstein used V in his original German-language papers on special relativity in 1905, but in 1907 he switched to c, which by then had become the standard symbol for the speed of light. [10] [11] 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.

Imbs, D; Raynal, Michel (2009). Malyshkin, V (ed.). Software Transactional Memories: An Approach for Multicore Programming. 10th International Conference, PaCT 2009, Novosibirsk, Russia, 31 August – 4 September 2009. Springer. p.26. ISBN 978-3-642-03274-5. Ellis, J; Mavromatos, NE; Nanopoulos, DV; Sakharov, AS (2003). "Quantum-Gravity Analysis of Gamma-Ray Bursts using Wavelets". Astronomy & Astrophysics. 402 (2): 409–424. arXiv: astro-ph/0210124. Bibcode: 2003A&A...402..409E. doi: 10.1051/0004-6361:20030263. S2CID 15388873. HKUST Professors Prove Single Photons Do Not Exceed the Speed of Light". The Hong Kong University of Science and Technology. 19 July 2011. Lindberg, DC (1996). Roger Bacon and the origins of Perspectiva in the Middle Ages: a critical edition and English translation of Bacon's Perspectiva, with introduction and notes. Oxford University Press. p.143. ISBN 978-0-19-823992-5. a b c 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. S2CID 121261770.

Knotts, Sandra; Mohr, Peter J.; Phillips, William D. (January 2017). "An Introduction to the New SI". The Physics Teacher. 55 (1): 16–21. Bibcode: 2017PhTea..55...16K. doi: 10.1119/1.4972491. ISSN 0031-921X. S2CID 117581000.

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] Rees, M (1966). "The Appearance of Relativistically Expanding Radio Sources". Nature. 211 (5048): 468. Bibcode: 1966Natur.211..468R. doi: 10.1038/211468a0. S2CID 41065207. Tolman, RC (2009) [1917]. "Velocities greater than that of light". The Theory of the Relativity of Motion (Reprinted.). BiblioLife. p.54. ISBN 978-1-103-17233-7.

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All forms of electromagnetic radiation, including visible light, travel at the speed of light. For many practical purposes, light and other electromagnetic waves will appear to propagate instantaneously, but for long distances and very sensitive measurements, their finite speed has noticeable effects. Any starlight viewed on Earth is from the distant past, allowing humans to study the history of the universe by viewing distant objects. When communicating with distant space probes, it can take minutes to hours for signals to travel. In computing, the speed of light fixes the ultimate minimum communication delay. The speed of light can be used in time of flight measurements to measure large distances to extremely high precision. Lindberg, DC (1974). "Late Thirteenth-Century Synthesis in Optics". In Edward Grant (ed.). A source book in medieval science. Harvard University Press. p.396. ISBN 978-0-674-82360-0.