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What is the relativistic mass of one photon, i.e. what is $N$ here: $10^{-N}$ kg?

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  • $\begingroup$ Photons don't have mass. Can you elaborate on your question more? $\endgroup$ – Aaron Stevens Sep 23 '18 at 17:07
  • $\begingroup$ Photon in rest doesn't have rest mass, but still must have relativistic mass by $E=mc^2$.If $E$ is not zero as it is in fact not, $m$ cannot be zero as well. $\endgroup$ – user2925716 Sep 23 '18 at 17:10
  • $\begingroup$ For photons $E=pc$. They have momentum, but not mass. $\endgroup$ – Aaron Stevens Sep 23 '18 at 17:14
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First, photons do not have rest mass. Thus, they move in space with speed c, in vacuum, when measured locally.

Because they do not have rest mass, they do not move in the time dimension, their speed in the time dimension is 0. Thus, they are not experiencing time the way we do.

They do have energy, and it is related to their frequency. They do have stress-energy, so they do excert gravitational pressure.

They do not use the term relativistic mass any more, because in special relativity, mass is not a conserved quantity. It is not used in particle physics. It is not used anymore because it causes confusion. It connects Newtonian inertial mass with relativistic energies. It is only used for theoretical experiments with spaceships trying to reach relativistic speed and the fuel they would need.

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