John, Norfolk asked:
How can massless photons impart momentum to objects like asteroids?
Dominic - This is actually a very common misconception because at school, of course, youíre taught a lot of Newtonian physics where something without mass can't exert a force. It doesnít have momentum. But, in fact, a photon is a relativistic particle; and in relativity, energy and mass are equivalent to another.
So, although the photon doesnít have any mass, it has momentum still, which is associated with its energy and thatís significant because the photon is traveling at this relative speed of the speed of light.
Einsteinís theory of relativity has completely taken over from Newtonian physics. And that explains how the photon can have this momentum which it can impart to an asteroid...