Goolag's "light-in-the-box" question provoked this question, but I thought it might be worthy of its own thread.

How much electromagnetic radiation (light, radio, etc.) can pass through a volume of space? Is there some theoretical limit at which spacetime "saturates" (for want of a better word), or is there no known limit?

There are a couple of interesting things that should happen. At some point, you begin to deviate from Maxwell's equations since light can interact with itself through weird quantum processes. From what I understand, this happens at very high energy densities. If you Google for phrases like "nonlinear light interaction in vacuum" you'll find a bunch of papers. Just one example is here:

http://prola.aps.org/abstract/PR/v129/i5/p2354_1As for a more fundamental limit, there is certainly a limit at which our current theories break down. I don't know exactly where that is, but it probably relates to the Planck density. At that point, we don't know how to accurately predict the effects, which would involve quantum effects due to gravity interacting with the light.

What I don't know is if you could somehow concentrate the light enough so that you form a black hole. It's easy to think about stuffing enough mass into an area to form a black hole, since mass will play nicely and sit still until you get enough of it together. Light has the unhappy tendency to go flying off at light speed as soon as you turn your back, so getting all your photons in the same place at the same time is going to be difficult.