Why does light interact with normal matter but not with other light?
Assumptions:
- Light does not interact with other light at all.
- Light does interact with other matter, i.e reflection/refraction.
Why does light interact with normal matter but not with other light?
Assumptions:
Actually it's not correct that light does not interact with light.
Photon-Photon scattering is a recently observed phenomenon that was predicted by QED under the name of Euler-Heisenberg Scattering.
It can be achieved by integrating out fermions from QED path intergral and gives you an effective action that depends only on the electromagnetic field.
More can be found here:Two-Photon Scattering
Bastam's answer is absolutely correct that in QED photons do interact with other photons.
But you can see why this effect is negligible in every-day scenarios for a couple of reasons:
One is that photons do not carry electric charge, and photons only directly interact with particles which carry electric charge. Contrast this with gluons which carry color charge and do interact with other gluons.
Alternatively, you could see this as a result of the fact the electromagnetic field equations are linear, which roughly means that different states of the EM field can be added together and evolve independently.
While it is true that in a QED, photons do interact with each other, if we consider light as a wave for our present purposes, light waves do interact with each other, such as in constructive or destructive interferences with other light waves. This results in a change in amplitude of the light wave.
An example of this is the two-slit experiment which proves the dual nature of light, but shows that light waves constructively interfere on the other side of the board.
If you were talking about photons not interacting with each other, read Bastam Tajik's answer.