Frequently Asked Questions

The following are questions submitted to us. Helen Quinn, content provider for this web site, offers answers to the questions.

1. Why is so much energy produced when an atom is split or fused?
2. More about time dilation
3. How does a cyclotron work?
4. Have quarks been observed and isolated in the laboratory?
5. Is mass always conserved?
6. How can the exchange of a photon attract a proton and an electron, yet repel two electrons?
7. More about the speed of light.
8. I was looking for a SU3 chart of the quark model.
9. Why is the visual spectrum continuous if it is produced by electrons going from one quantum state to another within the atom?
10. Time, microphysical processes, and probability.

FAQ6: How can the exchange of a photon attract a proton and an electron, yet repel two electrons?

Question: Seems like it's reasonable to want to know how the exchange of a photon can attract a proton and an electron, yet repel two electrons. Where can I find the answer to this? It seems like the emission of a photon would kick back an electron, same as the capture of one. So I know I'm missing a vital idea.

Response

This is a very perceptive question. It gets at a point where most so-called explanations of the interaction between two charged particles gloss over important details. The force between the particles is not just from the exchange of photons, it also involves their Coulomb fields as well, and that is the part that gives the repulsion.

First imagine a situation with two charged objects that I've somehow nailed down so they cannot move for 100 years (and nothing else around). In this static situation no photons are present! Yet there is still a force between the charged particles due to their (static) Coulomb fields. (The Coulomb field is the electric field around a static charged particle.) So far so good, you probably actually knew that already.

Photons are changing electromagnetic fields (electromagnetic waves) and so in a totally static situation there are no photons, but there are static electric fields associated with each charged particle.

The attraction or repulsion between like or opposite-sign charges has to do with the way their Coulomb fields add to one another in one case and cancel in the other, and the fact that the energy stored in the field is proportional to the field strength squared. The force is in the direction that would reduce the energy stored in the sum of their two coulomb fields if the particles moved in the direction of that force. In this situation it is easy to understand why opposite-charged charges attract (because their fields cancel one another more and more the closer they are together). Similarly charged particles repel because their fields can only add. If they are close, you are adding large field to large field, while if they are far apart the field of one is small wherever the field of the other is large, so you get a smaller total field energy in that case.

Where do they photons come in? They are what happens as the particles move around.

What happens when I let one of the particles go, say I allow it to move a foot and then I magically nail it down again. It has to carry its coulomb field along with it. The coulomb field extends infinitely far out. But it cannot just change everywhere instantaneously as the particle moves. That would give effects that correspond to faster than light speed communication. At any instant what we must have is the new coulomb field (corresponding to the new location of the charge) plus a bunch of photons which mask the changes except in a local region close to the particle, and so far away it still looks like the old coulomb field. As time goes by these photons radiate away, till eventually we do see the new coulomb field.

In a situation where the particles are freely moving around you can see that this story looks quite complicated, and indeed if I try to follow it in this way it is very complex. The calculation based on Feynman diagrams that encodes electron-electron scattering or electron-positron scattering is not really about the repulsive or attractive forces, but more about how the two particles exchange energy and momentum as they pass by one another. The coulomb field effects are there as well, and we tend to sweep them under the rug when we talk about it because the calculation, done correctly, takes care of them, but its not so easy to describe how that is achieved. Indeed there are many variants of the formalism, all of which give the same final answer, but which look quite different when it comes down to how this result is achieved.

Similarly when talk about atoms we do no encode the force between the nucleus and the electrons in terms of photons; that part of the problem we actually solve exactly when we find the stable states for an electron in an atom, ignoring the possibility of radiation. The we treat the transitions between those states with radiation of a single photon by the Feynman-diagram-like language.