Taylor, EF; Wheeler, JA (1992). Spacetime Physics: Introduction to Special Relativity (2nded.). Macmillan. p.59. ISBN 978-0-7167-2327-1. 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. Radar systems measure the distance to a target by the time it takes a radio-wave pulse to return to the radar antenna after being reflected by the target: the distance to the target is half the round-trip transit time multiplied by the speed of light. A Global Positioning System (GPS) receiver measures its distance to GPS satellites based on how long it takes for a radio signal to arrive from each satellite, and from these distances calculates the receiver's position. Because light travels about 300 000kilometres ( 186 000mi) in one second, these measurements of small fractions of a second must be very precise. The Lunar Laser Ranging experiment, radar astronomy and the Deep Space Network determine distances to the Moon, [89] planets [90] and spacecraft, [91] respectively, by measuring round-trip transit times. 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] 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.

Massless particles and field perturbations, such as gravitational waves, also travel at speed c in vacuum. Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. Particles with nonzero rest mass can be accelerated to approach c but can never reach it, regardless of the frame of reference in which their speed is measured. In the special and general theories of relativity, c interrelates space and time and also appears in the famous equation of mass–energy equivalence, E = mc 2. [9]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] CODATA value: Speed of Light in Vacuum". The NIST reference on Constants, Units, and Uncertainty. NIST . Retrieved 21 August 2009. a b Gibbs, P (1997) [1996]. Carlip, S (ed.). "Is The Speed of Light Constant?". Usenet Physics FAQ. University of California, Riverside. Archived from the original on 2 April 2010 . Retrieved 26 November 2009. OPERA Collaboration (12 July 2012). "Measurement of the neutrino velocity with the OPERA detector in the CNGS beam". Journal of High Energy Physics. 2012 (10): 93. arXiv: 1109.4897. Bibcode: 2012JHEP...10..093A. doi: 10.1007/JHEP10(2012)093. S2CID 17652398.

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. 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. a b Galilei, G (1954) [1638]. Dialogues Concerning Two New Sciences. Crew, H; de Salvio A (trans.). Dover Publications. p.43. ISBN 978-0-486-60099-4. Archived from the original on 30 January 2019 . Retrieved 29 January 2019.

Translated in "A demonstration concerning the motion of light, communicated from Paris, in the Journal des Sçavans, and here made English". Philosophical Transactions of the Royal Society. 12 (136): 893–895. 1677. Bibcode: 1677RSPT...12..893.. doi: 10.1098/rstl.1677.0024. Resolution 1 of the 15th CGPM". BIPM. 1967. Archived from the original on 11 April 2021 . Retrieved 14 March 2021. 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. 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. Rees, M (1966). "The Appearance of Relativistically Expanding Radio Sources". Nature. 211 (5048): 468. Bibcode: 1966Natur.211..468R. doi: 10.1038/211468a0. S2CID 41065207.