# Question #7af2a

$m = \frac{\sqrt{{E}^{2} - {\left(p c\right)}^{2}}}{c} ^ 2$
Where $m$ is the relativistic mass, $E$ is the total energy, $p$ is the total momentum, and $c$ is the speed of light. A massive object traveling at the speed of light would need to increase to infinite mass. Since light does travel at light speed, it can't have mass. We therefore infer that the rest mass of a photon must therefore be zero.