Hi there! Question on momentum in SR. I'm trying to understand this holistically... From what I understand... p=mv is an approximation. when we look at the energy equation for a photon E = pc. Since a photo is massless, this momentum p is an intrinsic feature of a particle apart from its mass... So... 1. Why is it that we can use the mass of electrons/chairs/planets to calculate their momentum? Is there an a prior reason that the two are equivalent? 2. How does this momentum in SR relate to the operator momentum in QM? Thank you!