# Can we define the effective mass or the moving mass of a photon?

I know that the rest mass of a photon is zero. but the photon can be bent by gravity (which can also be explained by the curvature of space-time due to the effect of mass), this implies that it must have some effective mass, while in motion, therefore does it also bend space-time? can the mass of the photon be defined in common(SI) units, how?

• Use $E = m_{rel}c^2$. Or not, because the concept of relativistic mass is of dubious value. – ACuriousMind Oct 29 '14 at 11:31
• @ACuriousMind I did not quite understand your comment, can you please elaborate? – Quantum Sphinx Oct 29 '14 at 11:38

As an excitation of the electromagnetic field, a photon contributes to the stress-energy tensor $T_{\mu\nu}$, which, through the Einstein field equations, will distort the metric on spacetime, and thus exert gravity.