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The obvious example is that, to bring something that has mass to the speed $c$ requires infinite energy due to special relativity. But what if a kid asked me "What if light/photos had a super small mass, and actually traveled slightly less than the speed $c$?" This seems to explain a lot of simple facts to me, and doesn't break any obvious simple facts as far as I can tell.

Some things simple things it would explain:

  1. light has a momentum, because obviously it has a mass and a velocity.
  2. light is affected by gravity, because obviously it has a mass

Is there some simple example I can give to the kid for why light shouldn't have mass and should be massless?

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  • $\begingroup$ There is no such thing because we are still doing experiments to set an upper limit on the rest mass of the photon. It is like with the situation with neutrinos; before the phenomenon of neutrino oscillations was discovered and realised to be insisting that neutrinos have rest mass, we would have no good reason to assign a rest mass to the neutrinos and photons, but we have to keep doing experiments to check that the masslessness is correct. $\endgroup$
    – naturallyInconsistent
    Commented Jul 13 at 8:35
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    $\begingroup$ Photons could have a tiny mass, but there's no evidence for this yet. The "simple things" you think a photon mass would explain actually are not issues. The modern theory of gravity (general relativity) says everything is affected by gravity the same, regardless of mass, because "gravity" is spacetime curvature. And Newtonian momentum is just part of a 4 dimensional momentum that is well defined for massless particles. $\endgroup$
    – Eric Smith
    Commented Jul 13 at 10:05
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    $\begingroup$ You are asking if light can be stationary (because "speed less than $c$" in one frame is "speed zero" in another). $\endgroup$
    – WillO
    Commented Jul 13 at 11:22
  • $\begingroup$ Light has infinite range because the photon is massless. If you give the photon a little mass, then the photon will have a finite lifetime and there will be regions of the universe we cannot look into with our telescopes. $\endgroup$
    – niels nielsen
    Commented Jul 13 at 20:09

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