Photons do not have what is termed "rest mass". The rest mass of a particle is the mass that it would have if you could ever make one stop moving (Heisenberg prevents this, or at least makes them very hard to find if you do). Because photons have zero rest mass, they are in a class of particles that moves at light speed.
Alternative theories of the photon include a term that behaves like a mass, and this gives rise to the very advanced idea of a "massive photon". If the rest mass of the photon were non-zero, the theory of quantum electrodynamics would be "in trouble" primarily through loss of gauge invariance, which would make it non-renormalisable; also,
Does a photon have "resting" mass? The photon mass is zero and it is not very correct to talk about a photon rest mass as they cannot be at rest. For massive particles, rest mass is what physicists normally refer to when they say "mass". The term "relativistic mass" is not used much and you will have a hard time finding a physicist who refers to it as just "mass".
Why is the rest mass of a photon zero? The concept of “rest mass” hides the true nature of the photon’s mass. The photon does have mass, but photon mass is reciprocal to electron mass. Just as time is reciprocal to frequency and length is reciprocal to wave number, physical mass also has a reciprocal manifestation.
The photon – the quantum of light or other electromagnetic radiation – is normally considered to have zero mass. But some theories allow photons to have a small rest mass and one consequence of that would be that photons could then decay into lighter elementary particles.
What is the mass of a photon? If the rest mass of the photon were non-zero, the theory of quantum electrodynamics would be "in trouble" primarily through loss of gauge invariance, which would make it non-renormalisable; also, charge conservation would no longer be absolutely guaranteed, as it is if photons have zero rest mass.
How much does a photon weigh? In a sense, this problem is resolved by our earlier statement: since a photon is always moving at the speed of light, it has no “rest energy” and has no rest mass. A better way to put it was pointed out by Dr. Matthew Francis in his excellent 2013 post on The Meaning of Mass .
No. a photon has zero mass. m = 0. Its total energy is E = sqrt ((mc²)² + (pc)²) = pc = h/lambda hc/lambda = hc/f hf since m = 0. Energy and mass are not equivalent. Instead mass …
Does a Photon Have a Rest Mass? adshelp[at]cfa.harvard.edu The ADS is operated by the Smithsonian Astrophysical Observatory under NASA Cooperative Agreement NNX16AC86A
What if photons actually have mass? Large photon mass means the force acts only over short distances, while a small mass would lead to influences over larger distance. This is what we see with the particles carrying the weak force (memorably known as the W and Z gauge bosons): they have very large masses, and the weak force consequently extends over very short distances.
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