Eli5: how come photon doesn’t have mass?
In: Physics
If the photon had mass, it cannot travel at the speed of light as it would require infinite energy according to the equations of relativity.
Since photons travel at the speed of light, they have to have no mass and all their energy comes from their momentum.
This feels incorrect to anyone only familiar with classical physics because classically momentum is mass times velocity (speed with a direction). But with relativity, you have to consider momentum as a fundamental concept independent of mass.
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