Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/17/84; site bcsaic.UUCP Path: utzoo!linus!decvax!bellcore!petrus!sabre!zeta!epsilon!gamma!ulysses!mhuxr!mhuxt!houxm!vax135!cornell!uw-beaver!ssc-vax!bcsaic!michaelm From: michaelm@bcsaic.UUCP (michael b maxwell) Newsgroups: net.physics Subject: Quantum electrodynamics and Feynman diagrams Message-ID: <326@bcsaic.UUCP> Date: Tue, 8-Oct-85 12:51:23 EDT Article-I.D.: bcsaic.326 Posted: Tue Oct 8 12:51:23 1985 Date-Received: Fri, 11-Oct-85 07:03:08 EDT Organization: Boeing Computer Services AI Center, Seattle Lines: 97 Since the funny gravity comments have died down, I decided to inject something more grave... First, the disclaimer: I'm not a physicist, and my knowledge of physics is limited to three semesters in college and what I've read in places like Scientific American. So- My question concerns the QED explanation of interactions between charged particles. In the articles I've read that attempt to explain this to laymen like me, they often include a Feynman diagram of a scattering interaction between two electrons, like the following: e- e- \ / \ / \ / \ / \ / |~~~~~~~~~~~~~~~~| / \ / \ / \ / \ / \ / \ The explanation runs something like the following. The two electrons are travelling along, exchange a virtual photon, and are scattered. The photon therefore acts as the carrier of the electromagnetic force between the two charged particles, and there is no action at a distance. Voila. When I first read this, I tried to expand on the explanation in my mind. My reasoning went somewhat as follows. The electrons are constantly emitting and re-absorbing virtual photons (as well as other virtual particles; e- and e+; anything else?). Each virtual photon carries with it a bit of momentum away from its e- parent; if it is absorbed by the other e-, it transfers that momentum to that e-. The time it can "remain" (exist?) apart from its e- source is dependent on the uncertainty principle--the more energetic the photon, the larger its equivalent momentum, hence the shorter time it is "allowed" to rob that momentum from the parent e- before it is absorbed by the other e- or re-absorbed by its parent. So (I thought), there is an explanation for why the strength of the interaction decreases with distance: energetic photons can't "last" long enough to get very far from their parent. I then considered the direction of the interaction; the two e-s repel each other. Simple enough, I thought; the photon has momentum in the direction of its travel, which is away from its parent (to the right, say, in the above diagram); so by conservation of momentum, the parent e- moves in the opposite direction (to the left, in this case). And when the photon is absorbed by the other e-, it gains the momentum of the photon in the direction of the photon's travel, i.e. away from the other e- (to the right in this example). Hence the net result is that the two e-s are repelled, as observed. Then came the crunch. How do you explain attractive forces? I.e. what happens when you replace one of the e-s with a positive particle? It seems to me that there ought to be an attactive force, but I can't figure how exchanging a photon can result in a force of attraction! Putting it naively, when a virtual photon arrives at an e-, how does it "know" whether it came from an e- or an e+? I've asked this question of several people who know more physics than I do, and I'm not satisfied with (or maybe I don't understand) their replies. Some samples: (1) This is the wrong way to visualize forces between two particles; rather like trying to explain the results of a slit experiment with a particle model rather than a wave model. Along these same lines is the explanation that scattering is always a repulsive phenomenon: shoot an electron towards positively charged nuclei, and they are always scattered away from the nucleus. My reaction: then what does the exchange of photons have to do with em forces between particles? Furthermore, what does the exchange of mesons have to do with the attractive forces between nucleons? (In this case, I'm even more puzzled, since if I understand rightly, mesons *always* induce an attractive force.) (2) A variation of (1): When you have two charged particles attracting each other, a standing wave of virtual (?) photons is set up between the particles. Interactions by means of standing waves can't be described by the simple model of exchange of virutal photons. If (2) is right, then is scattering always a matter of repulsion? If so, is there some way of visualizing how the standing waves set up a force of attraction between two differently charged particles in a non- scattering environment? I.e. what is the difference between the standing waves set up between differently charged particles, and the waves between particles with the same charge? What am I missing? Is there something "funny" about talking about the momentum of photons? I've assumed that a photon has momentum in the direction of its travel; is this wrong? Or do virtual photons not have a direction of travel, in some sense? -- Mike Maxwell Boeing Artificial Intelligence Center ..uw-beaver!{uw-june,ssc-vax}!bcsaic!michaelm