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In particle data group's (PDG) The Review of Particle Physics (2016) page number 29, the mass of the photon is given to be $<1\times10^{-18}eV$ and that of graviton as $<6\times10^{-32}eV$.

  • In case those limits are experimental upper bounds, what makes them give theoretical value for gluons and experimental limits for photon and graviton?
  • Why aren't we sure that photon's mass is zero, even though we are sure that gluons mass is zero (as given in it)?
  • In case they have mass, why do we think graviton may have lesser mass that of the photon even though they both are carriers of forces of similar range?
Vidya Sagar V
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    I VTC because of duplication (and echos of old texts telling me the neutrino has no mass) –  Dec 09 '16 at 19:08
  • @AccidentalFourierTransform In case you mean that those limits are experimental upper bounds, what makes them give theoretical value for gluons and experimental limits for photon and graviton? – Vidya Sagar V Dec 09 '16 at 19:10

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