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CODATA value: Speed of Light in Vacuum". The NIST reference on Constants, Units, and Uncertainty. NIST . Retrieved 21 August 2009.

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. Penrose, R (1959). "The Apparent Shape of a Relativistically Moving Sphere". Proceedings of the Cambridge Philosophical Society. 55 (1): 137–139. Bibcode: 1959PCPS...55..137P. doi: 10.1017/S0305004100033776. S2CID 123023118. Cornish, Neil; Blas, Diego; Nardini, Germano (18 October 2017). "Bounding the Speed of Gravity with Gravitational Wave Observations". Physical Review Letters. 119 (16): 161102. arXiv: 1707.06101. Bibcode: 2017PhRvL.119p1102C. doi: 10.1103/PhysRevLett.119.161102. PMID 29099221. S2CID 206300556.

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Wynne, K (2002). "Causality and the nature of information". Optics Communications. 209 (1–3): 84–100. Bibcode: 2002OptCo.209...85W. doi: 10.1016/S0030-4018(02)01638-3. archive In 1983 the metre was defined as "the length of the path travelled by light in vacuum during a time interval of 1⁄ 299 792 458 of a second", [92] fixing the value of the speed of light at 299 792 458m/s by definition, as described below. Consequently, accurate measurements of the speed of light yield an accurate realization of the metre rather than an accurate value of c. Feigenbaum, Mitchell J.; Mermin, N. David (January 1988). "E = mc 2". American Journal of Physics. 56 (1): 18–21. Bibcode: 1988AmJPh..56...18F. doi: 10.1119/1.15422. ISSN 0002-9505. 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.

Wang, L. J.; Kuzmich, A.; Dogariu, A. (2000). "Gain-assisted superluminal light propagation". Nature. 406 (6793): 277–279. doi: 10.1038/35018520. PMID 10917523. S2CID 4358601. 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. a b c Evenson, KM; etal. (1972). "Speed of Light from Direct Frequency and Wavelength Measurements of the Methane-Stabilized Laser". Physical Review Letters. 29 (19): 1346–1349. Bibcode: 1972PhRvL..29.1346E. doi: 10.1103/PhysRevLett.29.1346. S2CID 120300510. An astronomical unit (AU) is approximately the average distance between the Earth and Sun. It was redefined in 2012 as exactly 149 597 870 700m. [97] [98] Previously the AU was not based on the International System of Units but in terms of the gravitational force exerted by the Sun in the framework of classical mechanics. [Note 12] The current definition uses the recommended value in metres for the previous definition of the astronomical unit, which was determined by measurement. [97] This redefinition is analogous to that of the metre and likewise has the effect of fixing the speed of light to an exact value in astronomical units per second (via the exact speed of light in metres per second). [100] Cromie, William J. (24 January 2001). "Researchers now able to stop, restart light". Harvard University Gazette. Archived from the original on 28 October 2011 . Retrieved 8 November 2011.Standish, EM (February 1982). "The JPL planetary ephemerides". Celestial Mechanics. 26 (2): 181–186. Bibcode: 1982CeMec..26..181S. doi: 10.1007/BF01230883. S2CID 121966516. 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. Mermin, N. David (2005). It's About Time: Understanding Einstein's Relativity. Princeton: Princeton University Press. p.22. ISBN 0-691-12201-6. OCLC 57283944. 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.