Eli5: If light has no mass, how does it gain momentum / have energy / reflect off a mirror?



Is it 100% confirmed that light has no mass? Because if it has mass the titled questions should be easier to answer the observed behaviour of light. Right? Or I’m way off… pls.

In: Physics

Light indeed has no mass; because of this, it always has to travel at, well, the speed of light, and can’t slow down. (Or speed up.) It does, however, have energy and momentum. But the forrmulas to caluculate those are edge cases of those for massive particles.

You’re probably thinking of ‘ e=mc^2 ‘ and “p=mv”, which are the low-energy approximations, for Einstein, of his real formulas. The actual formula (eek!) is E^2 = p^2 c^2 + m^2 c^4 , with v = p/E . Those aren’t too bad, right?

Notice what happens to the first one when m=0: it reduces to E=pc, which is the actual relation for massless things. Like I said, an edge case. And, indeed, if you multiply mv by c, and then set v=c, you get mc^2 – though for massless stuff that’s 0=0, not as useful.

Short version: photons can have energy and momentum, and exchange them with other stuff, without actually having to have a “rest mass” that’s nonzero.

–Dave, for them, energy is also directly related to frequency, E = hnu; high-energy photons are high-frequency radiation

Momentum is not exclusive to moving mass. The usual equation L = m*v is a special case for massive objects.

Waves carry momentum without actual net-movement of mass. This applies both to mechanical waves (where there is moving mass, but its average velocity is zero) and electromagnetic waves (which have no mass). The wave-part is simply negligibly small in most applications.

Cant explain the details of how exactly light obtains momentum, thats beyond me.