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My understanding is that a photon is pure kinetic energy and has no inertial mass of its own (or probably too low to be significant). But for a box with two photons whose momentums cancel, the photons will affect the inertial mass, because now the energy is stationary when seen from outside.

Except for gluons and photons all elementary particles get their mass from Higgs potential energy.

Is this the only reason why photons don't have inertial mass? That they are purely kinetic and don't interact with any field to gain potential energy?

So the picture I have in my head is that "stationary energy is what has the property of inertial mass and energy that has no form of stationary energy involved has no inertial mass."

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I think your understanding is more or less correct, although I'm not entirely sure about some of the terminology you use.

Massive elementary particles get their masses from the Higgs mechanism. Only particles that interact with the Higgs (by gauge interactions in the case of gauge bosons or Yukawa in the case of fermions) obtain masses in this way.

As the Higgs is electrically neutral and colorless, it doesn't interact with the photon or the gluon, and thus those particles don't acquire masses from the Higgs.

I wouldn't, however, describe the gluon or photon as purely kinetic, as both interact with other particles, just not the Higgs.

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  • $\begingroup$ I'm actually trying to understand this without the mathematical stuff (but I'll get into that eventually) so that's why my terminology is quite off. Can you explain how being purely kinetic contradicts that photons interact with other particles? $\endgroup$ – Foon Dec 14 '15 at 6:22
  • $\begingroup$ Purely kinetic implies to me only a kinetic energy term in the mathematical description of a photon. There are other terms, though, describing interactions. $\endgroup$ – innisfree Dec 14 '15 at 6:29
  • $\begingroup$ Does a photon use it energy (hf) to propagate? Then isn't that kinetic energy when it's moving? $\endgroup$ – Foon Dec 14 '15 at 7:27
  • $\begingroup$ No it doesn't use energy to propagate, but yes it has amongst other things KE $\endgroup$ – innisfree Dec 14 '15 at 7:57

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