# Interaction of photons with charged particles

1. Aug 29, 2014

### PeterPeter

Can someone please tell me the name of the theory describing the interaction of photons with charged particles eg electrons, protons?

Can you also suggest a good introductory web page describing that theory at First Year University level for self study? Better still, a good VIDEO lecture on YouTube.

Thanks.

2. Aug 29, 2014

### ShayanJ

3. Aug 30, 2014

### PeterPeter

Thanks. I'm trying to understand why light reacts with charged particles much more than with neutral atoms.

4. Aug 30, 2014

### ShayanJ

First, of course photons interact with neutral atoms, because they interact with atoms' electrons and protons!
Second, photons have no interaction with neutral elementary particles like themselves. But they can have interaction with composite particles composed of charged particles because it has interaction with charged constituents.
But why photons interact with only charged elementary particles?
I guess that's something more fundamental than QED. It has something to do with gauge invariance and gauge theories but I don't know enough to explain. I hope people having enough knowledge will answer the question.

5. Aug 30, 2014

### mal4mac

Quantum electrodynamics is a quantum theory of the electromagnetic force.

Imagine two unmoving electrons, QED theory visualizes the electrostatic force between them as arising from the exchange of quanta in the form of virtual photons:

http://hyperphysics.phy-astr.gsu.edu/hbase/forces/qed.html

Now imagine two unmoving neutrons - there is no electrostatic force, nothing happens, QED has nothing to say about the matter, no exchange of virtual photons is involved.

A photon can interact with a neutral particle, including itself; read up on "pair production":

https://en.wikipedia.org/wiki/Pair_production

This is uncommon in your everyday life, of course, because it requires very high energy.

Last edited: Aug 30, 2014
6. Aug 30, 2014

### PeterPeter

Does all this mean that a plasma is opaque to light of all wave lengths?

7. Aug 30, 2014

### Matterwave

I am fuzzy on this myself, but I believe light can weakly interact with neutral elementary particles. For example, a neutrino (even if it is of Majorana nature, meaning no magnetic moments) may interact with a photon...but of course very very weakly.

8. Aug 31, 2014

### mal4mac

I'm also fuzzy on this stuff. But isn't physics itself fuzzy here? My instant reaction was that a neutrino can't have a magnetic moment because it's a fundamental particle, i.e., not composed of quarks like a neutron. I found this thread, where tom.stoer appears to back me and also appears to be not-fuzzy:

9. Aug 31, 2014

### ShayanJ

Well, I'm only an undergrad and so I'm not that much exposed to experimental data. I think that's where your belief comes from. Can you give some papers about that or explain observations which led you to think like that?
Thanks

10. Aug 31, 2014

### mal4mac

Perhaps, but to begin understanding this stuff, using the full machinery of QED, requires total commitment to physics, at University level, for about four years. (I'd probably need a year or two of revision to get back to that level myself!) At a first year level I'd remain content with a popular science level of description, like:

"Instead of being locked up in distinct energy levels, the free electrons can take on any kinetic energy. So any photon that passes through the plasma can be scattered or absorbed. Plasmas are therefore opaque because light cannot pass through them freely---it bounces off the free electrons. Plasmas are like a very dense fog.

Examples of opaque plasmas: Surface of Sun: looks solid, you can’t see into Sun.
Candle flame: try looking through it, it’s opaque." www.ucolick.org/~faber/ay5/lecture_notes/lecture11-12_doc.doc

Rather than trying to understand the full machinery of QED *now*, you should be mastering electromagnetism (Maxwell's equations), Quantum physics (Schrodinger's equation...), and the mathematical methods needed to solve these equations. Then you can master QED - as I said, four years work! (At least.)

11. Aug 31, 2014

### Orodruin

Staff Emeritus
Dirac neutrinos can (and probably do) have a magnetic moment. The problem is that the standard model value is far below experimentally accessible values. Also Majorana neutrinos could possibly interact with photons through transition magnetic moments.

12. Aug 31, 2014

### Orodruin

Staff Emeritus
On the oroginal topic: I do not see why it should be confusing that the quantized theory of light interacts with electric charge when this is already a property of the classical theory of electromagnetism. First thing to do is to study classical electromagnetism, which should be much more accessible to a beginning undergrad than QED.

13. Aug 31, 2014

### sophiecentaur

I agree totally. There is a bit of a fashion to assume that everything is best approached with the most up to date theory. No one would be thinking QED when playing tennis or pool so it's clearly not the most suitable for all situations.
Classical EM theory deals with most situations - even radiation (photon) pressure - so make sure you get familiar with it.

14. Aug 31, 2014

### Matterwave

My statement is certainly not based on experimental data, the coupling would be way too small to test for.

I believe one arrives at these couplings based on the CPT theorem...but I am too fuzzy on this matter to provide much help here...