Light and Particles

[Bob Angstrom]

Can light react with particles other than electrons and, if so, how?

[Ken G]

In the classical picture, light interacts with anything with charge. It just likes electrons because they have such low mass that they are easy to push around and exchange energy with. But since light also involves energy, it may interact gravitationally with anything, if we really want to get technical, and at high enough energy the other forces are all the same thing anyway, so things get complicated. But the first part is probably the answer you are looking for.

[Bob Angstrom]

Ken,

Thanks for the answer but I should have been more specific with my question.
I have been reading some theories that describe light as an energy exchange among receptive electrons but the theories don’t mention the involvement of any other particles. I was wondering if other particles could emit or absorb photons or do they just bend and reflect?

[swansont]

Nuclei can absorb and emit photons, but have fairly high energy states. Atomic electrons are the most common because that's what's there. (technically the bound system absorbs the photon. Free electrons scatter photons but can't absorb them) But if you formed e.g. muonium, then the muon would absorb and emit the photons.

[Ken G]

Note also that, in effect, a pure vacuum can absorb photons, if two photons of high enough energy collide. The vacuum can convert the photons into other particles (two by two, usually). Basically, you have quantities that need to be conserved, including charge, momentum, and energy and other quanta, and as long as you conserve them all there'll be some chance of just about anything happening.

[Fortis]

Thanks to QFT, photons effectively can interact in a whole bunch of ways that differ from your standard EM. For example, one of the simplest Feynman diagrams for the photon propagator can include a fermion loop, such as an electron positron pair being created, which then disappear again to form the original photon. These leptons, in turn, can now interact with another particle (e.g. a neutrino) via the weak interaction.

After a while things can get very complicated.

[DrChinese]

I was wondering if other particles could emit or absorb photons ... ?

The nucleus is composed of quarks bound into protons and neutrons. The attraction between the nucleus and its bound electrons is mediated by virtual photons. So there is a quick way to see that the answer is yes.

On the other hand, neutrinos are essentially unaffected by light.

[Bob Angstrom]

Thanks much for everyon's input. These are the answers I was looking for.

[TheBlackCat]

Don't forget the radio waves can alter the spin of the nucleus, and likewise can emit radio waves when their spin returns to the ground state.

[Ken G]

It sounds like TheBlackCat has in mind the all-important 21 cm line of neutral H, which is about the only way to see cold atomic H in emission. But if so, there's a nitpick-- it is actually the spin of the electron that flips, because the electron is easier to flip. So it's another electron interaction, but one that happens due to spin-spin interactions with the nucleus, so the nucleus does play a key role.

[TheBlackCat]

Actually, I am thinking of Nuclear Magnetic Resonance (NMR or MRI). In that case it is the nucleus whose spin changes, as the name suggests.

[Ken G]

Ah, OK. Then you can use any frequency of photon you want, it depends on the magnetic field you use. But no doubt you are saying that in normal applications, it is radio wavelengths that are used. Presumably because they penetrate tissue and have a long enough wavelength to get the job done?

[TheBlackCat]

No, there is a charecteristic frequency of EM radiation that a will alter the spin of a specific nucleus in a specific magnetic field, and for reasonably-powerful magnetic fields (i.e. massive superconducting magnets) the frequency of that electromagnetic radiation in the radio end of the spectrum