If light has energy but no mass, how can it have momentum?

Question

If light has energy but no mass, how can it have momentum?

In: 55

Answer 1

“P=mv” is from Newtonian mechanics, which is known to be an incomplete model of reality, but which sticks around because it’s an excellent approximation for everyday purposes. The momentum of light is one of the cases where Newtonian mechanics fails and we need to upgrade to relativistic physics, in which the momentum of a massless particle is Planck’s constant ÷ the particle’s wavelength, or *p = h / λ*.