• Question: How do you know that photons have no rest mass?

    Asked by stormwhite to James_M on 13 Jun 2011.
    • Photo: James M Monk

      James M Monk answered on 13 Jun 2011:

      You are right to ask this question because we can only ever know photons are massless to within (very good) experimental limits. If the photon had a mass then the range of the electromagnetic force would be limited (rather than being infinite as we currently understand it). So the 1/r^2 law of attraction/repulsion for charged objects would not be 1/r^2 anymore. Furthermore, I think that things like the light from distant galaxies would be affected because the photon, having a mass, could decay on its way here.

      If the photon has mass then it would be possible to trap a stationary photon – electromagnetism would be completely different in that situation.

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