New numbers for cosmic speed limit – science newsDecember 2, 1972
A team at the National Bureau of Standards in Boulder, Colorado has now reported a remarkably accurate [speed of light] Measured using the wavelength and frequency of a HeNe laser… which gives a speed of light of 299,792.4562 kilometers per second.
That 1972 experiment measured the two-way speed of light, or the average speed at which photons travel from a light source to a reflective surface and back again.The results are still valid, helping scientists redefine standard length in meters (SN: 10/22/83, p. 263). But they haven’t finished exploring it yet.In the late 1990s and early 2000s, photonics achieved slowest speed of light measurement at a speed of 17 meters per second and frozen in their tracks thousandths of a second (SN: 1/27/01, p. 52). Despite all these successes, one major hurdle remained: direct testing of the one-way speed of light. The measurement, considered impossible by many scientists, could resolve the long-standing question of whether the speed of light is uniform in all directions.