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However, the frequency of light can depend on the motion of the source relative to the observer, due to the Doppler effect. 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.

Consoli, Maurizio; Pluchino, Alessandro (2018). Michelson-Morley Experiments: An Enigma for Physics & The History of Science. World Scientific. pp.118–119. ISBN 978-9-813-27818-9 . Retrieved 4 May 2020.

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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] 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. Liu, Xiaoshu; He, Vincent F.; Mikulski, Timothy M.; Palenova, Daria; Williams, Claire E.; Creighton, Jolien; Tasson, Jay D. (7 July 2020). "Measuring the speed of gravitational waves from the first and second observing run of Advanced LIGO and Advanced Virgo". Physical Review D. 102 (2): 024028. arXiv: 2005.03121. Bibcode: 2020PhRvD.102b4028L. doi: 10.1103/PhysRevD.102.024028. S2CID 220514677.

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. 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.Panofsky, WKH; Phillips, M (1962). Classical Electricity and Magnetism. Addison-Wesley. p. 182. ISBN 978-0-201-05702-7. In branches of physics in which c appears often, such as in relativity, it is common to use systems of natural units of measurement or the geometrized unit system where c = 1. [17] [18] Using these units, c does not appear explicitly because multiplication or division by 1 does not affect the result. Its unit of light-second per second is still relevant, even if omitted. Rees, M (1966). "The Appearance of Relativistically Expanding Radio Sources". Nature. 211 (5048): 468. Bibcode: 1966Natur.211..468R. doi: 10.1038/211468a0. S2CID 41065207. O'Connor, JJ; Robertson, EF. "Abu han Muhammad ibn Ahmad al-Biruni". MacTutor History of Mathematics archive. University of St Andrews . Retrieved 12 January 2010. Gibbs, P (2004) [1997]. "Why is c the symbol for the speed of light?". Usenet Physics FAQ. University of California, Riverside. Archived from the original on 25 March 2010 . Retrieved 16 November 2009.

Since 1983, the constant c has been defined in the International System of Units (SI) as exactly 299 792 458m/s; this relationship is used to define the metre as exactly the distance that light travels in vacuum in 1⁄ 299 792 458 of a second. By using the value of c, as well as an accurate measurement of the second, one can thus establish a standard for the metre. [14] As a dimensional physical constant, the numerical value of c is different for different unit systems. For example, in imperial units, the speed of light is approximately 186 282 miles per second, [Note 4] or roughly 1 foot per nanosecond. [Note 5] [15] [16]

The results of special relativity can be summarized by treating space and time as a unified structure known as spacetime (with c relating the units of space and time), and requiring that physical theories satisfy a special symmetry called Lorentz invariance, whose mathematical formulation contains the parameter c. [29] Lorentz invariance is an almost universal assumption for modern physical theories, such as quantum electrodynamics, quantum chromodynamics, the Standard Model of particle physics, and general relativity. As such, the parameter c is ubiquitous in modern physics, appearing in many contexts that are unrelated to light. For example, general relativity predicts that c is also the speed of gravity and of gravitational waves, [30] and observations of gravitational waves have been consistent with this prediction. [31] In non-inertial frames of reference (gravitationally curved spacetime or accelerated reference frames), the local speed of light is constant and equal to c, but the speed of light can differ from c when measured from a remote frame of reference, depending on how measurements are extrapolated to the region. [32] 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. a b Lester, PM (2005). Visual Communication: Images With Messages. Thomson Wadsworth. pp.10–11. ISBN 978-0-534-63720-0.

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