Limit to EM energy density?

[easyrider]

Hi yall, I was just wondering, seeing as how photons are massless, would there be a limit to the amount of electromagnetic energy you can fit in a given (finite) area? Of course excluding the practical reasons that would stop you. And if there is a limit, how come? Because blackholes are technically infinite density and yet they are massive so to speak.

[zhermes]

The answer isn't entirely known as it requires a combination of general relativity with field theory which is difficult, tenuous, and uncertain.
Gravity responds to energy in general, not just rest mass, so (presumably) a black-hole would still be formed above some critical energy (density).

[easyrider]

It seems that, at a point it would become too much and start forming particle-antiparticle pairs. Although I would think during the very early parts of the big bang, if it wasnt already an infinite energy density then an unthinkably high density, that it would atleast be possible to attain ever-increasing energy densities since there was no matter quite yet?