National Physical Laboratory

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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  • Prestage J D, Tjoelker R L & Maleki L, `Atomic clocks and variations of the fine structure constant', Phys. Rev. Lett. Vol. 74 pp. 3511-4, 1995
  • Bradley E. Schaefer, `Severe Limits on Variations of the Speed of Light with Frequency' Phys. Rev. Lett. Vol. 82 pp. 4964-6 1999
  • Antoinette Songaila & Lennox L. Cowie, `Astronomy: Fine-structure variable?' Nature Vol. 398 pp. 667-8, 1999
  • J K. Webb, V V. Flambaum, C W. Churchill, M J. Drinkwater & J D. Barrow, `Search for Time Variation of the Fine Structure Constant' Phys. Rev. Lett. Vol. 82 pp. 884-7 1999
Last Updated: 25 Mar 2010
Created: 9 Aug 2007


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