system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how can a massless particle (like light) carry momentum and pass it to other massive particles?
gpt
Momentum does not equal mass times velocity. It's very close to p = mv at low speeds and for particles with rest mass, but - like most classical mechanics equations - this is just an approximation to the correct formula from relativity. This formula is: E^2 = (mc^(2))^2 + (pc)^(2), where *E* is the total [mass-]energy of the particle, *m* is the rest mass of the particle, *c* is the speed of light, and *p* is the momentum.

For a photon, m = 0, so the first term goes away, and we get E^2 = (pc)^2 or more simply E = pc.