A pulse with different group and phase velocities (which occurs if the phase velocity is not the same for all the frequencies of the pulse) smears out over time, a process known as dispersion. Certain materials have an exceptionally low (or even zero) group velocity for light waves, a phenomenon called slow light. [73] Taylor, EF; Wheeler, JA (1992). Spacetime Physics: Introduction to Special Relativity (2nded.). Macmillan. p.59. ISBN 978-0-7167-2327-1. Whereas moving objects are measured to be shorter along the line of relative motion, they are also seen as being rotated. This effect, known as Terrell rotation, is due to the different times that light from different parts of the object takes to reach the observer. [27] [28]

Translated as "A Demonstration concerning the Motion of Light". Philosophical Transactions of the Royal Society. 12 (136): 893–894. 1677. Bibcode: 1677RSPT...12..893.. doi: 10.1098/rstl.1677.0024. Archived from the original on 29 July 2007. 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. O'Connor, JJ; Robertson, EF (November 1997). "James Clerk Maxwell". School of Mathematics and Statistics, University of St Andrews. Archived from the original on 28 January 2011 . Retrieved 13 October 2010. a b c "Demonstration tovchant le mouvement de la lumiere trouvé par M. Rŏmer de l'Académie Royale des Sciences"[Demonstration to the movement of light found by Mr. Römer of the Royal Academy of Sciences] (PDF). Journal des sçavans (in French): 233–236. 1676. a b Ellis, George F. R.; Williams, Ruth M. (2000). Flat and Curved Space-times (2nded.). Oxford: Oxford University Press. p.12. ISBN 0-19-850657-0. OCLC 44694623.Similarly, communications between the Earth and spacecraft are not instantaneous. There is a brief delay from the source to the receiver, which becomes more noticeable as distances increase. This delay was significant for communications between ground control and Apollo 8 when it became the first crewed spacecraft to orbit the Moon: for every question, the ground control station had to wait at least threeseconds for the answer to arrive. [82] 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. 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]

a b "Resolution B2 on the re-definition of the astronomical unit of length" (PDF). International Astronomical Union. 2012. 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). In exotic materials like Bose–Einstein condensates near absolute zero, the effective speed of light may be only a few metres per second. However, this represents absorption and re-radiation delay between atoms, as do all slower-than- c speeds in material substances. As an extreme example of light "slowing" in matter, two independent teams of physicists claimed to bring light to a "complete standstill" by passing it through a Bose–Einstein condensate of the element rubidium. The popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrarily later time, as stimulated by a second laser pulse. During the time it had "stopped", it had ceased to be light. This type of behaviour is generally microscopically true of all transparent media which "slow" the speed of light. [68] Toll, JS (1956). "Causality and the Dispersion Relation: Logical Foundations". Physical Review. 104 (6): 1760–1770. Bibcode: 1956PhRv..104.1760T. doi: 10.1103/PhysRev.104.1760. Although this distance is largely irrelevant for most applications, latency becomes important in fields such as high-frequency trading, where traders seek to gain minute advantages by delivering their trades to exchanges fractions of a second ahead of other traders. For example, traders have been switching to microwave communications between trading hubs, because of the advantage which radio waves travelling at near to the speed of light through air have over comparatively slower fibre optic signals. [80] [81] Spaceflight and astronomy A beam of light is depicted travelling between the Earth and the Moon in the time it takes a light pulse to move between them: 1.255 seconds at their mean orbital (surface-to-surface) distance. The relative sizes and separation of the Earth–Moon system are shown to scale.Between 1960 and 1983 the metre was defined as "the length equal to 1 650 763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p 10 and 5d 5 of the krypton 86 atom." [156] It was discovered in the 1970s that this spectral line was not symmetric, which put a limit on the precision with which the definition could be realized in interferometry experiments. [157]

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. Mendelson, KS (2006). "The story of c". American Journal of Physics. 74 (11): 995–997. Bibcode: 2006AmJPh..74..995M. doi: 10.1119/1.2238887. 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.International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8thed.), p.126, ISBN 92-822-2213-6, archived (PDF) from the original on 4 June 2021 , retrieved 16 December 2021 Time is money when it comes to microwaves". Financial Times. 10 May 2013. Archived from the original on 10 December 2022 . Retrieved 25 April 2014. Ben-Menahem, Shahar (November 1990). "Causality between conducting plates". Physics Letters B. 250 (1–2): 133–138. Bibcode: 1990PhLB..250..133B. doi: 10.1016/0370-2693(90)91167-A. OSTI 1449261. Theoretical vs real-world speed limit of Ping". Pingdom. June 2007. Archived from the original on 2 September 2010 . Retrieved 5 May 2010.

The Essen–Gordon-Smith result, 299 792 ±9km/s, was substantially more precise than those found by optical techniques. [108] By 1950, repeated measurements by Essen established a result of 299 792.5 ±3.0km/s. [111] Conover, Emily (2 November 2016). "Units of measure are getting a fundamental upgrade". Science News . Retrieved 6 February 2022. 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] Wolf, Emil (2001). "Analyticity, Causality and Dispersion Relations". Selected Works of Emil Wolf: with commentary. River Edge, N.J.: World Scientific. pp.577–584. ISBN 978-981-281-187-5. OCLC 261134839.Bajcsy, M; Zibrov, AS; Lukin, MD (2003). "Stationary pulses of light in an atomic medium". Nature. 426 (6967): 638–641. arXiv: quant-ph/0311092. Bibcode: 2003Natur.426..638B. doi: 10.1038/nature02176. PMID 14668857. S2CID 4320280. Michelson, A. A. (1927). "Measurement of the Velocity of Light Between Mount Wilson and Mount San Antonio". The Astrophysical Journal. 65: 1. Bibcode: 1927ApJ....65....1M. doi: 10.1086/143021. Ellis, GFR; Uzan, J-P (2005). " 'c' is the speed of light, isn't it?". American Journal of Physics. 73 (3): 240–227. arXiv: gr-qc/0305099. Bibcode: 2005AmJPh..73..240E. doi: 10.1119/1.1819929. S2CID 119530637. The possibility that the fundamental constants may vary during the evolution of the universe offers an exceptional window onto higher dimensional theories and is probably linked with the nature of the dark energy that makes the universe accelerate today.