jhc@mtung.UUCP (Jonathan Clark) (10/16/85)
<> In response to an earlier posting of mine, responding to a question about the mechanism whereby two electrons repel each other and any photons that might be involved, rimey@dali.berkeley.EDU (Ken Rimey) writes: >No, the force between two charged particles does not involve real photons. >Hold two identically charged pith balls near each other. You will not >detect any light or radio waves between them. I suspect that the author has forgotten that photons are the name that we give to the particles that mediate the electromagnetic force. They are not limited to that part of the spectrum which we interpret as radio and light. Call 'photons' 'gauge particles' if it makes more sense. So yes, the Coulomb force *is* mediated by the gauge particles of the electromagnetic force, viz. photons. >You say that virtual particles exist only for short times, and therefore >shouldn't be able to travel large distances. Indeed, this is the reason >that the strong nuclear force has a finite range. But moving clocks run >slow, and clocks on photons don't run at all. This is why the force >between charged particles can be felt even at large distances. Doesn't this have more to do with gluons (all right, strong force gauge particles) being massive and therefore being unstable? Now turning to the complementary question about why unlike forces attract, well that's the way the universe works. As to the *mechanism*, when an (isolated) electron and a positron are attracted to each other, they are losing (electromagnetic) potential energy, and thus are emitting photons (thus adhering to the law of conservation of energy). If the electron moves left (towards the positron), then the photon(s) go right (away from same), thereby conserving momentum. Questions like 'how does the photon "know" whether it just got absorbed by an electron or a positron' are very close to meaningless. An energy and momentum transfer took place, that's all. If the question was 'how does the electron know that there is a positron out there' then remember that the electron is emitting and reabsorbing photons (virtual ones) all the time, and because it exists in the EM field of the other, it is (statistically) more likely to emit the said photons in such a manner that it moves closer to the positron. Again, this is the way the universe works. Of course, it may work differently tomorrow when someone discovers how to generate a probability field or a new GUTS theory crops up. Comment to diehard QMers out there: it was quite painful to write the last couple of paragraphs. I beg your indulgence as to the use of both language and concepts (like "exist"!). -- Jonathan Clark [NAC]!mtung!jhc My walk has become rather more silly lately.
sra@oddjob.UUCP (Scott R. Anderson) (10/17/85)
In article <613@mtung.UUCP> jhc@mtung.UUCP (Jonathan Clark) writes: >rimey@dali.berkeley.EDU (Ken Rimey) writes: >>You say that virtual particles exist only for short times, and therefore >>shouldn't be able to travel large distances. Indeed, this is the reason >>that the strong nuclear force has a finite range. But moving clocks run >>slow, and clocks on photons don't run at all. This is why the force >>between charged particles can be felt even at large distances. > >Doesn't this have more to do with gluons (all right, strong >force gauge particles) being massive and therefore being >unstable? Massive, yes, unstable, no. As mentioned often in this group, the Heisenberg Uncertainty Principle places limits on how long a virtual particle can exist. If the particle is a massless spin-zero particle like the photon, the probability of interacting with it at a distance r from its source goes like 1/r, the familiar electromagnetic potential. Yukawa was the first to show that a virtual spin-zero particle with mass m (e.g. the pion) has an exponentially small probability of being at distance r, 1 - exp(-r/R) r where R = h-bar/mc. This is the reason that a force mediated by such a particle is so short-ranged (remind me to tell net.internat that any extended ascii set should include h-bar). Note that the more massive the particle is, the smaller the range R is. >Now turning to the complementary question about why unlike >forces attract, well that's the way the universe works. >If the question was 'how does the electron know >that there is a positron out there' then remember that the >electron is emitting and reabsorbing photons (virtual ones) >all the time, and because it exists in the EM field of the >other, it is (statistically) more likely to emit the said >photons in such a manner that it moves closer to the >positron. I'm afraid that you are mixing your classical with your quantum-field- theoretical here. If you accept the existence of the EM field, then, as you say, an electron reduces its potential energy in this field by transferring the energy to the field, i.e. creating photons. But no virtual particles are necessary! The latter are a *replacement* for the concept of the field. It is difficult to think of them in classical particle terms, though, because their primary characteristic is the momentum they carry (not to be confused with "the direction they are travelling"; this has no meaning). This momentum can be both pointing away from *and* pointing towards the source particle. What is unusual is that like-charged and oppositely-charged particles pick up one or the other, exclusively. >My walk has become rather more silly lately. Mine's random! Scott Anderson ihnp4!oddjob!kaos!sra
michaelm@bcsaic.UUCP (michael b maxwell) (10/21/85)
In article <1002@oddjob.UUCP> sra@oddjob.UUCP (Scott R. Anderson) writes: >concept of the field. It is difficult to think of [virtual particles] in >classical particle terms, though, because their primary characteristic is the >momentum they carry (not to be confused with "the direction they are >travelling"; this has no meaning). This momentum can be both pointing >away from *and* pointing towards the source particle. What is unusual >is that like-charged and oppositely-charged particles pick up one or >the other, exclusively. Now this is what I was hoping would come up in regards to my original posting (asking how the exchange of virtual photons "explained" repulsion between like-charged particles, and repulsion between unlike-charged particles, if you came in late...). I can understand that a virtual particle would not have a direction of travel, and therefore that the momentum of a virtual particle might not have anything to do with direction of travel. I have a more difficult time understanding how one photon can have a momentum pointing away from and towards the source particle at the same time--BTW, if they don't have a direction of travel, how can there be a source particle? But what I'd really like to know is, is there some sort of intuitive explanation for why like- or unlike-charged particles "pick up" a photon with one or the other momentum exclusively? -- Mike Maxwell Boeing Artificial Intelligence Center ..uw-beaver!{uw-june,ssc-vax}!bcsaic!michaelm