# Can someone explain electromagnetism?

1. Jun 19, 2012

### iced199

I know its mediating boson is the photon, but i don't understand how that works. Wikipedia is no help, so i hope you guys can. How can the particle of light (even though it is oscillating electric and magnetic waves) which is neutral in charge, affect anything with an electric charge? Say, explain how photons are used to bind electrons to the nucleus of an atom? I am not advanced in math, so can you please explain with concepts?

2. Jun 20, 2012

### electroweak

Charged particles (like electrons) can emit and absorb photons (electrons "couple to photons"). Imagine an electron emitting a photon and another electron absorbing it. Recall from relativity theory that even though photons are massless they carry momentum. Hence the first electron will recoil from emitting the photon and the second electron will recoil from absorbing it; this is simple conservation of momentum at each event (emission and absorption). Since we, the experimenters, don't see the photon (it doesn't propagate into our eyes), the process looks like two electrons repelling each other. At low energies (everyday energies) the combined effect of many such interactions reproduces the familiar Coulomb potential and inverse-square force law from classical EM.

The explanation behind "opposites attract" is slightly more complicated, but the idea of momentum transfer again plays a role. If you ever learn Quantum Field Theory (QFT), you will discover that the "opposites attract" rule has to do with the fact that photons have spin 1. On the other hand. gravitons have spin 2 and so gravity always attracts.

It is perfectly fine that photons have no electric charge. Momentum transfer has little to do with charge. In fact if photons were charged, we would run into problems with charge conservation: to emit a photon an electron would have to create new charge out of nothing. Since charge is conserved, photons must be chargeless. Charge tells us about couplings. If a particle (like an electron) is electrically charged, it couples to photons. If a particle (like a neutrino) is not electrically charged, it does not couple to photons. Since photons are uncharged, they do not couple to themselves; this really simplifies things in QED -- the quantum theory of EM. (On the other hand, QCD -- the theory of quarks -- is mediated by gluons which couple to "colored" particles. It turns out that gluons themselves are colored, so we get gluon-to-gluon couplings -- gluons emitting and absorbing other gluons -- and this effect makes QCD difficult to deal with.)

3. Jun 20, 2012

### iced199

Thank you. So it is all momentum transfer then? OK, then how is magnetism carried out by photons? Are magnetic fields created by photons transferring momentum between poles again?

4. Jun 20, 2012

### electroweak

Magnetism is more complicated since magnetic poles aren't typically taken to be particles in the same sense that electrons are. Keep in mind that physical magnets are ultimately just systems of charged particles in motion. I've never done this computation, but I suspect that the sum of many of the usual electron-photon interactions reproduces classical magnetism in the low-energy limit given that the electrons are organized in a magnet.

QFT is a useful tool for performing calculations about all sorts of processes. The general idea is to input the momenta of the incoming and outgoing particles, do some tricky math, and get back the probability that this particular interaction (with these momenta) occurs. In everyday situations, the most probable quantum interactions are those that look like their classical counterparts, which is why the sum of many interactions appears classical.

5. Jun 20, 2012

### michaelc187

Besides the fundamental laws of existance, electromagnitism is generally divided into static and dynamic approaches... meaning dynamic movement emitting the characteristic of excellerating electrons in the surrounding area and static attracting the electrons in the surrounding area.