Special & General Relativity Questions and Answers
Has the constancy of the speed of light been measured?
Yes, in fact it was measured in a decay process that involved particles accelerated to some very high fraction of the speed of light like 99.999 percent. The photon produced in the decay was generated in the 'proper' reference frame of the decaying particle where the particle was at rest, but the speed of the photon measured in OUR reference frame where the particle was traveling at 99.999 percent the speed of light showed the photon had the standard speed of 300,000 kilometers/sec. The test of this key principle of special relativity established that the speed of light was indeed a constant independent of reference frame motion to better than 1 part in 10,000. I will have to search a bit for the exact reference, but I recall reading about this test about 1 year ago.
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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.