If I shine my new yottawatt laser onto a black hole (yotta = 10^24), I expect that the mass of the black hole will increase, along with it's gravity. I know that 'massless' photons can also be converted into electron/positron pairs, (and vice versa). But can the light itself generate it's own gravitational field? If so, I assume that it's own gravity can't affect itself, since gravity can't propagate faster than light, and I expect the beam won't 'gravity focus' itself (not sure though), but how about an observer who is close to the beam? I had planned to try these experiments myself, but found the electricity costs to run the laser are a bit on the high side.