Is there a limit to how many photons can fit into a space.. let's say a cubic millimeter? Let's say you pointed 20 very powerful lasers so that they all crossed paths within the same cubic millimeter... would they all fit? Would it be crowded? Would they bounce of of each other, scattering at angles other than the original path of the lasers?

Photons are bosons, arbitrary many can be placed in the same state. So there is no limit on the number you can place in a given volume.

Until you switch on gravity or possibly even other interactions. Then it is not so clear that you can put any number of photons in any volume.

 

Regarding the question of interactions: if you shoot two photons onto another then in principle, the outgoing photons might have a different direction than the incoming (although I am not sure if this has been measured). So in this sense photons do interact with another.

Edited November 2, 201015 yr by timo

Until you switch on gravity or possibly even other interactions. Then it is not so clear that you can put any number of photons in any volume.

 

Sure, at some point you would have enough energy localised so that the curvature of space-time would play a role. You could imagine space-time engineering being accomplished using huge lasers.