The mass of a photon

  • Thread starter swampwiz
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  • #1
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I read an internet meme:

Photons have mass? I didn't even know they were Catholic.

I was going to respond that photons are "massless" particles, but then I realized that photons have energy, and thus must have mass.

What's the proper explanation here?
 

Answers and Replies

  • #3
Mister T
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then I realized that photons have energy, and thus must have mass.

What's the proper explanation here?

The mass ##m## is not equivalent to the energy ##E##. In other words, ##m \neq E/c^2##. Instead, ##m^2=(E/c^2)^2-(p/c)^2##

A system consisting of a single photon has zero mass because its energy ##E## is equal to its momentum ##pc##.
 

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