photons have mass or not??
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No photons do not have mass, they have energy and momentum but not mass.
Albert01:
y photons do not have mass
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Photons do have kinetic mass, given by E=mc2E=mc2. The use of the term "kinetic mass" is out of favor these days, but it was the way many physicists thought about photons for many decades.
but
A photon does indeed have no rest energy as it is massless. However, that does not mean that it can't have energy by moving around (at the speed of light). The energy of a photon is given by E=pcE=pc, where pp is the momentum of the photon.
A photon obeys E=pcE=pc. Einstein (in 1905) showed in his seminal paper on Special Relativity that
E2=(pc)2+(m0c2)2E2=(pc)2+(m0c2)2
where m0m0 is the rest mass (or invariant mass) of the particle. Invariant means the mass in the rest frame of the particle. A photon has no rest mass. In other words, m0,γ=0m0,γ=0 (we usually denote photons by γγ in physics). Notice that by setting m0=0m0=0 above, we obtain E=pc
but
A photon does indeed have no rest energy as it is massless. However, that does not mean that it can't have energy by moving around (at the speed of light). The energy of a photon is given by E=pcE=pc, where pp is the momentum of the photon.
A photon obeys E=pcE=pc. Einstein (in 1905) showed in his seminal paper on Special Relativity that
E2=(pc)2+(m0c2)2E2=(pc)2+(m0c2)2
where m0m0 is the rest mass (or invariant mass) of the particle. Invariant means the mass in the rest frame of the particle. A photon has no rest mass. In other words, m0,γ=0m0,γ=0 (we usually denote photons by γγ in physics). Notice that by setting m0=0m0=0 above, we obtain E=pc
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