Light does gravitate --- there is plenty of sound evidence for this, such as gravitational lensing in astronomy. Gravity makes space refract, it seems. What theoretical explanation can be offered for this observation? The equivalence principle is often used, but invoking a principle smacks of avoiding something you don't really understand. Does special relativity require that light gravitates (because photons have measurable energy -- hence inertial mass -- and experiment shows that inertial masses gravitate) ? And if not, why not?