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Q: How can photons have energy and momentum, but no mass?
Physicist: Classically (according to Newton) kinetic energy is given by and momentum is given by , where m is mass and v is velocity. is very accurate for slow moving (compared to light) objects with mass, but it’s not true in general. Anything with zero mass always travels at the speed of light. Notice that you can never have zero momentum, since something with zero mass and zero energy isn’t something, it’s nothing. Say you have an object with mass m, that isn’t moving (P=0).
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