A New Figure for the Cosmic Speed Limit – Science News, December 2, 1972
A group at the National Bureau of Standards at Boulder, Colo., now reports an extremely accurate [speed of light] measurement using the wavelength and frequency of a helium-neon laser.… The result gives the speed of light as 299,792.4562 kilometers per second.
Update
That 1972 experiment measured the two-way speed of light, or the average speed of photons that traveled from their source to a reflective surface and back. The result, which still holds up, helped scientists redefine the standard length of the meter (SN: 10/22/83, p. 263). But they weren’t done putting light
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