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/Light waves interact with things and you don’t to have mass to have momentum to transfer – its an energy transfer which all momentum transfer is.
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.
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.
Ok, I’m probably going to butcher this, but it was answered a while ago on this sub and I’ll give it a shot from what I recall of that. Photons do not have mass, but as your drunkard friend explained, they *do* have energy in the form of momentum. That energy/momentum is imparted onto the solar sail when they bounce off of it to give it momentum. Also, there’s some math involving E=MC^(2) That someone will fill in below.
Solar sails are designed to use radiation pressure to pull a space craft through space by reflecting radiation from the Sun or another source back to where it came from the solar sail can power a craft either round our own solar system or even to another star. https://youtu.be/0sHNaXE-aco
They do not have mass, and therefore cannot have weight.
Solar sails are possible, because light does have momentum. We account for solar pressure all the time on normal spacecraft, but it’s pretty weak.
The energy of a photon is E=hf, that’s Planck’s constant and frequency
E^2 = (mc^(2))^2 + (pc)^2 (thanks Einstien)
We already established that m=0
E^2 = (pc)^2
E = pc
hf = pc
p=hf/c = h/λ (lambda is wavelength)
The light from a star comes in and hits the sail, so the photon has to reverse its direction of travel, so it must reverse its momentum. The law of conservation of momentum means the spacecraft must gain momentum.
Other posters have explained the science of photons, but I just wanted to mention this “physical proof” that photons can move things: look up videos of “radiometers”. They’re mostly in the context of a desk toy (I had one we a kid) and you can shine a flashlight on one and it’ll start to spin purely from the light hitting it.
Photons are massless, they have no weight. They do however have momentum, so solar sails are not sci-fi, they are very much real. An actual solar sail satellite that has flown [https://en.wikipedia.org/wiki/IKAROS](https://en.wikipedia.org/wiki/IKAROS)
Solar sails are certainly not science fiction. Japan launched IKAROS in 2010 with a 14m square sail that carried it to Venus in just 7 months. NASA’s ACS3 with an 80m square sail was completely unfurled just a few days ago and is currently in orbit. It may even be visible from your backyard (go to [NASA.gov](http://NASA.gov) for details).
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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