I understand that it’s impossible to bring a photon to a state of rest, therefore impossible to collect them into a cup to weigh them and calculate their mass. A guy in a pub explained to me, quite smugly, that photons are just expressions of energy and that’s that. From my understanding solar sails would be just large surface areas being hit by photons, pushing the spacecraft in a desired direction, just like normal ship sails are being pushed by wind. But air particles do have mass. How could photons push the spacecraft if they don’t weigh anything?
In: Physics
Photons do not have a rest mass, otherwise they could not travel at the speed of light. They do have a momentum, defined by the most famous equation ever(‘s extended form) E^2 =c^2 p^2 +(m^2 c^4 ). This means that the energy of a photon equals the momentum of the photon times the speed of light. As a result, they can impart that momentum on objects when they reflect or are absorbed by them. So in reality solar sails are something that can exist. And not in theory, but in practice. As in we’ve made them. [and they work](https://en.m.wikipedia.org/wiki/IKAROS).
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