# Quantum Electrodynamics

I was wondering if anyone could give a simple explanation of how light interacts with matter. From what I have read in QED, electrons will repel each other because of their ability to emit and absorb photons. However, I do not observe any visible light between objects around me, repelling each other when I place them on a desk. They also don't appear to heat up, or emit IR light, from just being placed beside each other either. What type of light is being emitted by this matter constantly?

Extending this idea, what happens to a single electron when placed in a box which allows no light to be input, but removes all light emitted? Does it simply lose energy in the form of light until the electron no longer exists?

In addition, does anyone know of any good visualizations of photon waves interacting with atoms? I have heard of an analogy of an atom being similar to a cork in a pond, while the waves are like photons coming to excite the material. Does this idea sound correct, and if so, does it fit well with the idea of visualizing oscillating probability densities of electrons and how they interact with photons? It would be nice to be able to visualize these processes and understand how they fit with tight binding / Hubbard models, etc.

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–  DJBunk Apr 5 '13 at 17:52
"I do not observe any visible light between objects around me" ...how would you observe them if they only interact with each other and not with you? "repelling each other when I place them on a desk"... try magnets. For QED, the second question might be: "How does the two point function of an electron with momentum $p=(m_e,\vec 0)$ in an otherwise empty universe differ (intuitively because of correction from its own field around it) from the (free) Feynman propagator $S_F$?" –  NikolajK Apr 5 '13 at 18:20
There are at least three separate questions here - I recommend splitting up this post. –  Chris White Apr 6 '13 at 0:28
Thank you for the ideas and answers everyone, @DJBunk , I will think more about the ideas given in the website - it is quite useful in helping understand QED with minimal QM background. I am a chemical engineer so some of the higher level physics terminology evades me. –  chase Apr 7 '13 at 18:54
@NickKidman - Thank you for answer, I will look into Feynman propagators. As for asking how I would observe them - I realize that I can't see the photons of them interacting with each other, but shouldn't they be interacting with my eye since it is matter? I think Rhodopsin is essentially what is used to interact with photons of visible light. This would be constantly excited if visible light were used to transmit information regarding force. So, it is probably much higher energies than visible, and I think according to the website, much smaller time intervals than possible to be observed. –  chase Apr 7 '13 at 18:59

However, I do not observe any visible light between objects around me, repelling each other when I place them on a desk. They also don't appear to heat up, or emit IR light, from just being placed beside each other either. What type of light is being emitted by this matter constantly?

The key point is that those photons are not real photons. They are called virtual (off-shell) photons because they cannot be detected. And they are just a tool to calculate the interaction between particles, in fact, the don't obey the energy/momentum conservation laws ($E^2\neq p^2+m^2$).

In this image, two electrons interact, exchanging a photon (wavy line). Since the photon is an internal line, we can't measure it. This is just one of the possible (infinite) ways two electrons can interact. See this image:

Also note that two normal objects won't interact much, but they will emit IR radiation due to blackbody effect (something totally different).

Extending this idea, what happens to a single electron when placed in a box which allows no light to be input, but removes all light emitted? Does it simply lose energy in the form of light until the electron no longer exists?

I don't know how an electron in a box can emit light.

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Thanks for the answer @jinawee , as for the light I was speaking of: I was mainly referring to the virtual photon in which you have shown in your Feynman diagrams. It appears that the electron changes direction by emitting a photon. Therefore if the electron in a box is moving within the box without interacting with the sides of the box, it would have to emit energy as a photon to do so. So I was just having a thought experiment of if this continued, then at some point the electron may emit all of its energy as photons. –  chase Dec 20 '13 at 1:26