Special & General Relativity Questions and Answers
Is it true that the speed of light is slowing down?
I don't think so. Between 1947 and 1967, the various independent measurements of the speed of light yielded 299,792.5 kilometers per second, with an uncertainty of about 0.1 kilometer per second if you took the entire sample of 32 studies together. Today, the adopted value was established to be 299,792.458 kilometers per second. There are no reported detections of any deviations from 299,792 kilometers per second at the 1 kilometer per second level over the last 45 years or so when the independent experimental uncertainties are factored in. If you group them into separate populations over this period, there is no evidence for a change by as much as 100 meters per second or so over this time span.
I know of no recent claims in the professional literature that identify any changes in the speed of light that are statistically significant relative to the unavoidable experimental measuring uncertainties.
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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.