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
A simple way to think about this is that air doesn’t have much weight. You walk through it all the time (how rude!) and for most day-to-day things we treat air pretty much like it has no real mass. It does, but it’s really small.
But we know that air when it is moving fast can move ships. This isn’t up for debate, anyone who has been down to the docks can see moving air pushing along boats that are really quite big and heavy. And we know that in the past they moved ships that were crazy big. The Golden Horizon sailing ship was almost 9,000 tons and was moved by sails.
Solar sails work the same way. Individual photons (light) are so small and light (pun intended) that for most practical purposes they’re treated as having no mass. However they’re also moving incredibly fast, literally at the speed of light, and speed matters. Why?
A useful analogy for your friend at the pub would be to bring a BB round. It’s tiny and light. Ask him if he’s okay with with you throwing it at him. He’ll probably laugh and say he’s not afraid. Okay, now ask if he’s happy for you to put it in a BB gun and fire it at him. He’ll probably say no thank you, because it would HURT. Now ask how he’d feel if you put it in handgun cartridge and fired it at him, and he’d admit that could kill him.
The amount of force something exerts, even something really, really small and light, is influenced by how fast it is going. And while photons have a mass that is **close to zero** they’re also moving at the speed of light, which is really, really fast.
So like air moving 9,000 ton sailing ships so it is also possible for light to move a huge space ship. Now don’t get me wrong, you’re not going to get a Ferrari. However in space there’s very little to slow you down, so you’ll start slow, but move faster and faster and you won’t need engines or fuel. Just huge solar sails. And in theory you could reach almost the speed of light this way.
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