# Fundamental question

go quantum!
Why must the 4-momentum for photons $$p^\mu =(\frac{h\nu}{c},\frac{h\nu}{c} \textbf{e})$$ transform as a 4-vector in Special Relativity?

You can even derive it (Let $p^\mu = (p^0, \vec{p})$, apply a Lorentz transformation and you will see what p0 should be).