# Why can't massless particle exceed speed of light?

Why massless particle can't exceed speed of light?

It's just what it means for a particle to be massless. For a general motion of a particle in the vacuum, the rest mass $m_0$ satisfies $$m_0^2 c^4 = E^2-p^2 c^2$$ If the left hand side is zero, it follows that $$E=|p|c$$ which means that the energy-momentum vector is null (light-like). In general, the speed may be calculated from the direction of the energy-momentum vector as $v=pc^2/E$ and it equals $v=c$ if the formula above is satisfied. Massless particles are not only unable to move faster than light; they're unable to move slower than light, too. The massless particles must move exactly by the speed of light.