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
The traditional way of explaining this is that while photons don’t have mass they do have *momentum*.
And it is momentum that matters for bouncing off things and making them move.
So photons can push against things. It just takes a lot of them to generate even a small amount of push.
The more complex answer is that mass isn’t as big of a deal as we sometimes thing – it is more of an expression of energy. Energy is generally what matters, and photons have energy.
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