# QED Theory

1. Sep 6, 2014

### Quarlep

I looked QED theory but I dont understand anything can someone explain me What is a QED theory ?

2. Sep 6, 2014

### ChrisVer

Your question is very general- you should specify it more to get a more specific answer...however I'd reply that QED is quantum electrodynamics. It's the theory which describes the electromagnetic interactions of matter from a quantum field theory point of view. What else do you want to know?

3. Sep 6, 2014

### Quarlep

Can it explain whats charge or why same chargea are repel and different charges are attraced

4. Sep 6, 2014

### bhobba

Yes.

A deep analysis shows its basis is what called gauge symmetry, without going into the details of what that is.

Thanks
Bill

5. Sep 6, 2014

### ChrisVer

From what I've seen so far it doesn't explain why particles have the charges they do...but the charges are taken conventionally as such (you have the freedom to choose whichever gauge transformation of U[1] gauge group $e^{i \lambda Q}$ and the charges you'll have for your matter field will be $\lambda Q$ )... However there are constraints on the all particle charges to avoid some gauge anomalies.
But yes, it can explain the repulsion and attraction between same and opposite charges, as well as give you the Coulomb potential in the low-energy limit.

6. Sep 6, 2014

### bhobba

That's true. It doesn't explain why particles have the charges they do - but it does explain why in general charge exists.

Its required to preserve local gauge symmetry:
http://quantummechanics.ucsd.edu/ph130a/130_notes/node296.html

Thanks
Bill

7. Sep 6, 2014

### mal4mac

Nice simple explanation of gauge symmetry here:

http://www.nobelprize.org/nobel_prizes/physics/laureates/1999/press.html

An electric field E can be expressed as a potential V. The very simplest transformation of V is to add a constant to it. But V can be calculated from any zero point, since only differences in potential are of significance. That the zero point can be moved in this way is perceived as a symmetry in the theory, gauge symmetry...

8. Sep 6, 2014

### Quarlep

Thank you very much

9. Sep 8, 2014