One-way speed of light

When using the term "the speed of light" it is sometimes necessary to make the distinction between its one-way speed and its two-way speed. The "one-way" speed of light, from a source to a detector, cannot be measured independently of a convention as to how to synchronize the clocks at the source and the detector. What can however be experimentally measured is the round-trip speed (or "two-way" speed of light) from the source to a mirror (or other method of reflection) and back again to detector. Albert Einstein chose a synchronization convention (see Einstein synchronization) that made the one-way speed equal to the two-way speed. The constancy of the one-way speed in any given inertial frame is the basis of his special theory of relativity, although all experimentally verifiable predictions of this theory do not depend on that convention.^[1]^[2]

Experiments that attempt to directly probe the one-way speed of light independent of synchronization have been proposed, but none have succeeded in doing so.^[3] Those experiments directly establish that synchronization with slow clock-transport is equivalent to Einstein synchronization, which is an important feature of special relativity. However, those experiments cannot directly establish the isotropy of the one-way speed of light since it has been shown that slow clock-transport, the laws of motion, and the way inertial reference frames are defined already involve the assumption of isotropic one-way speeds and thus, are equally conventional.^[4] In general, it was shown that these experiments are consistent with anisotropic one-way light speed as long as the two-way light speed is isotropic.^[1]^[5]

The "speed of light" in this article refers to the speed of all electromagnetic radiation in vacuum.

^ ^a ^b Yuan-Zhong Zhang (1997). Special Relativity and Its Experimental Foundations. World Scientific. ISBN 978-981-02-2749-4.
^ Anderson, R.; Vetharaniam, I.; Stedman, G. E. (1998), "Conventionality of synchronisation, gauge dependence and test theories of relativity", Physics Reports, 295 (3–4): 93–180, Bibcode:1998PhR...295...93A, doi:10.1016/S0370-1573(97)00051-3
^ Michael Tooley (2000). Time, tense, and causation. Oxford University Press. p. 350. ISBN 978-0-19-825074-6.
^ Cite error: The named reference sep was invoked but never defined (see the help page).
^ Jong-Ping Hsu; Yuan-Zhong Zhang (2001). Lorentz and Poincaré Invariance: 100 Years of Relativity. World Scientific. ISBN 978-981-02-4721-8.

[zhang-1] Yuan-Zhong Zhang (1997). Special Relativity and Its Experimental Foundations. World Scientific. ISBN 978-981-02-2749-4.

[anderson-2] Anderson, R.; Vetharaniam, I.; Stedman, G. E. (1998), "Conventionality of synchronisation, gauge dependence and test theories of relativity", Physics Reports, 295 (3–4): 93–180, Bibcode:1998PhR...295...93A, doi:10.1016/S0370-1573(97)00051-3

[3] Michael Tooley (2000). Time, tense, and causation. Oxford University Press. p. 350. ISBN 978-0-19-825074-6.

[sep-4] Cite error: The named reference sep was invoked but never defined (see the help page).

[5] Jong-Ping Hsu; Yuan-Zhong Zhang (2001). Lorentz and Poincaré Invariance: 100 Years of Relativity. World Scientific. ISBN 978-981-02-4721-8.

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