Consider a collimated light beam in a vacuum (I am being unspecific about the frequency, anything from radio to gamma). If the beam power/cross section was increased indefinitely would new phenomena occur at certain energy densities?
I gather that pair-production couldn't occur due to momentum considerations but would there be some interaction with the quantum vacuum?
At sufficiently high power is there a self-gravitation effect of the beam, a point touched on by a previous question? What if the beam was reflected back on itself by a mirror?
Note: the motivation for this question relates to theoretical limits for beam weapons.