How would we know if the speed of light was varying with time? (FAQ - Quantum)
The speed of light is assumed to be a fundamental physical constant and has now been assigned a defined value. How would we know if it was varying with time?
The very term fundamental physical constants invites two questions: are they fundamental and are they constant. The speed of light c is undoubtedly regarded as fundamental since it features prominently in our best physical theories concerning the nature of the physical universe. The non-trivial problem which arises when we try to answer the second question involves finding something of assured constancy against which we can measure any possible change in any fundamental `constant'. Because of this the most meaningful experiments must involve dimensionless constants such as the fine structure constant (α=μ0ce2/h∼1/137) or the ratio of rest masses of fundamental particles (e.g. mp/me). There is a growing body of laboratory based experiments which have set tighter and tighter limits on the rate of change of α with time.
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