eli5 – if photons have no mass, how can they be used to push hypothetical “solar sails”?
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Solar sails are not hypothetical. IKAROS launched in 2010 used solar sales and practically demonstrated they work.
The answer is that photons have momentum.
The momentum is what is preserved in collisions. That is what Newton’s laws of motion are about.
In our everyday life momentum is mass * velocity. Mass in this context is the rest mass or invariant mass and it is the only thing we need to care about at a speed low compared to the speed of light.
At high-speed what is called relativistic mass is also relevant. A photon does have a relativistic mass.
The idea of mass–energy equivalence from relativity is to some degree known to most people that is what the E =mc^2 equation stands for.
That equation is for an object at rest for a moving object the formula i
E ^2 = ( m * c^2 )^2 + (p * c)^2
p is the momentum. So even if the rest mass m is zero the energy of the photons results in momentum this is called the Energy–momentum relation.
So rest mass is not required for momentum, mass is just what results in the momentum at the speed we are used to on earth.
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