Why does light interact with normal matter but not with other light?

Why does light interact with normal matter but not with other light?

Assumptions:

1. Light does not interact with other light at all.
2. Light does interact with other matter, i.e reflection/refraction.
• What do you mean by light does not interact with other light? Aug 12, 2023 at 2:13
• i think using photons in the question will be much better Aug 12, 2023 at 13:15

3 Answers

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

• Perhaps this changes the question to "Why do photons interact with each other so much less than with matter?" Aug 12, 2023 at 2:33
• @mmesser314 it is an experimental fact that is beautifully modeled with quantum electrodynamics , because of the coupling constants involved in the Feynman diagrams of the calculation. Aug 12, 2023 at 3:35
• I never heard of this! Do the photons have to be high energy? Is the interaction "actually" between virtual $e^-~e^+$ pairs that the photons generate for short time periods? Aug 12, 2023 at 5:47
• This may be a semantic issue, but wouldn't say that light interacts with light for two reasons. First, this photon-photon scattering is a quantum effect, classically they don't interact. Second, by using the same reasoning, you can probably conclude that everything interacts at high-enough loops. I think it is useful to preserve a distinction between interacting (exists as an interaction vertex) and scattering, although this is outside the scope of the question asked.
– JGBM
Aug 12, 2023 at 15:26
• @RC_23 Yes, the energy of the photons in the center-of-momentum frame has to be of order the electron mass or higher to yield a significant scattering probability. The photons are interacting via an electron loop.
– Sten
Aug 12, 2023 at 18:37

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.

• from the article......If we have to summarize our present understanding, it seems to us that current investigation concerning the highest energy photons from cosmic sources has not found clear and significant evidence for deviations and need for new physics Aug 12, 2023 at 13:00

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.

• +1, but I usually consider superposition of waves to be what happens when things don't interact with each other. The waves pass through each other and continue unchanged. Interference comes from adding up (with phase) how much is present. Aug 13, 2023 at 0:35