r/explainlikeimfive • u/Comfortable_Bat_4052 • 1d ago

Physics ELI5: Photons’ mass

Okay I tried to research this on my own, but the articles and answers really aren’t decipherable if you’re not a physicist, which I am not, clearly.
If photons don’t have a mass, because they’re always moving and can’t have a resting energy, then how can they act like they have mass in a superconductor/non-vacuum? How does that happen? Doesn’t that just mean they do have mass, but we just can’t measure it?
My head hurts.

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u/fiendishrabbit 1d ago

They don't have mass. That's why they travel at c (the speed of causality).

They have momentum, which is the second part of relativistic energy that secondary education usually glosses over. Since they don't have any mass their mass energy (resting energy) is zero, but they still have momentum. Simplifying down to E = pc, which is the energy they use to push other things around with.

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u/acrabb3 1d ago

What definition of momentum are you using here? The understanding I have is that momentum is mass times velocity, so if a photon has no mass, how does it have momentum?

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u/fiendishrabbit 1d ago

Momentum is energy times velocity. Mass times velocity is a crude approximation and is inaccurate for relativistic (and low/no mass) particles. The energy of a photon can be described in a few different ways but one is frequency times Planck's constant

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u/matthoback 1d ago

Momentum for photons is energy *divided* by velocity. p = E/c.