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
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.
Latest Answers