300,000,000

Product Information

Miller, AI (1981). Albert Einstein's special theory of relativity. Emergence (1905) and early interpretation (1905–1911). Addison–Wesley. ISBN 978-0-201-04679-3.

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. However, the frequency of light can depend on the motion of the source relative to the observer, due to the Doppler effect. Conover, Emily (2 November 2016). "Units of measure are getting a fundamental upgrade". Science News . Retrieved 6 February 2022.Supplement 2014: Updates to the 8th edition (2006) of the SI Brochure" (PDF). The International System of Units. International Bureau of Weights and Measures: 14. 2014. 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] The speed of light is of relevance to communications: the one-way and round-trip delay time are greater than zero. This applies from small to astronomical scales. On the other hand, some techniques depend on the finite speed of light, for example in distance measurements.

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] 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. a b "Resolution B2 on the re-definition of the astronomical unit of length" (PDF). International Astronomical Union. 2012.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. Fearn, H. (10 November 2006). "Dispersion relations and causality: does relativistic causality require that n (ω) → 1 as ω → ∞?". Journal of Modern Optics. 53 (16–17): 2569–2581. Bibcode: 2006JMOp...53.2569F. doi: 10.1080/09500340600952085. ISSN 0950-0340. S2CID 119892992. 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] a b Stachel, JJ (2002). Einstein from "B" to "Z" – Volume 9 of Einstein studies. Springer. p.226. ISBN 978-0-8176-4143-6.

Vaughan, JM (1989). The Fabry-Perot interferometer. CRC Press. pp.47, 384–391. ISBN 978-0-85274-138-2.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. Resolution 1 of the 15th CGPM". BIPM. 1967. Archived from the original on 11 April 2021 . Retrieved 14 March 2021. 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. 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]