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 don’t have rest mass but they do have momentum, and that creates a radiation pressure which pushes the sail. It’s a rather weak effect but in space where you’re floating around it’s noticeable.’’
Comets are a great example of evidence of this radiation pressure: their tails always point away from the sun. Even though the photons are weightless and there’s no wind, something “blows” comet tails away from the sun. If there wasn’t radiation pressure acting on it the comet should just sorta steam out in a blob instead of forming a tail.
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