Thx! So here are my next questions :) (I know nothing about physics etc... - I'm just a simple IT-employee) > "Yes, the target object gains velocity due to co

Thx! So here are my next questions :) (I know nothing about physics etc... - I'm just a simple IT-employee) > "Yes, the target object gains velocity due to conservation of momentum. The photon goes from momentum p to -p, so the target object needs to gain 2p of momentum to balance it out." 1) If the target object gains velocity, then the photon slows down? Meaning: a) are there then slow and fast photons and b) is the speed-of-light constant of ~300KM/s in vacuum valid only for not-sped-down-photons? 2) Then if I would put in orbit a bunch of mirrors and make them all reflect the sun's light to point to a fixed object, that fixed object would start moving because of the photons hitting it? (forgetting any other factor that does not have something to do with photons) > For perfect reflection, no. What you're referring to is light absorption, which is a different effect that raises the temperature of the target object and red-shifts the incoming photon. So, ignoring "perfect reflection", some photons will be reflected, some will be absorbed, which will raise the temp of the target object? > Pretty much. Photons are usually described by their momentum and energy. So, a remix of the original question, haha: if we say that a photon has no mass, but at the same time we say that momentum needs mass, then if we say that a photon has momentum we're screwed, no?