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Confused about electrodymagnetism

  1. Oct 5, 2012 #1
    I`m not a physics expert and have loads of questions about electro magnetism, both classical and quantum.

    What is a field made of ? in classical it is essentially nothing but how does the force from the charge propagate through empty space. In quantum it says that its a cloud of virtual photons but how do these photons "know" how to stay within the boundary of the field.
    When a charged object enters a field how does the object that mediates the field know that an object is there that it can react with? Classical offers no explanation(i think) and quantum says the photons are fired at each other. How do the photons from around the field know that a charged particle has entered it if no information has been conveyed to them? how does the example of throwing a ball between 2 boats explain attraction? How are the virtual photons generated? what about conservation of energy? Where does the energy come from that keeps generating these photons? I have loads more questions. Any help would be appreciated. tks
  2. jcsd
  3. Oct 5, 2012 #2

    Simon Bridge

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    Welcome to PF;
    It looks like you have a lot of study to do. A rigorous treatment would be beyond the scope of these forums but we may be able to give you an idea to go on with.
    Well I don't think anybody is an "expert" in quantum mechanics ... but some people do do it for a living.

    Looking through your questions, it looks like you are having trouble keeping the different models straight. You should realize that you are talking about different ways of describing reality here.
    The classical EM field is a consequence of Maxwel's equations.
    It is a kind of metaphore we use to relate the motions of charges in one part of space with those of another.

    There are philosophical problems with this - you have mentioned the main one: "how does the force get from one place to another?" ... this is the "action at a distance" problem. Classical theory does not answer this problem - "them's just the rules". (You should ask yourself why action at a distance is such a problem anyway: how is it that you come to expect interactions to be local and how close together do two things have to get before you are happy with their interactions? Perhaps all interactions are "at a distance" anyway - just that some of the distances are too small to see?)

    Field theory attempts to get around this by having a particle mediate the interaction. There is no mystery about how material objects get from A to B after all, or that they can make things happen when they get there.

    In the Field Theory, the EM-field metaphore is no longer needed. We refer to an EM-interaction mediated by photons, and described by Feynman diagrams, instead. Field Theory supplants classical electromagnetism.

    The Field of Field Theory is not "made up of" photons, and it is not the same as the EM-Field. The Field of FT is statistical in nature ... and that makes it hard to think about.

    It doesn't. All a charge knows is that it has encountered a photon.

    That would be broadly correct - the classical model allows for action to happen at a distance ... it seems that charged objects can recognize each other over large distances and react accordingly.

    At this level it is unusual to use forces at all - we'd normally refer to energies and potentials. A charge will follow the local gradient of the EM-potential "downhill". This way a charge need know as much about EM physics as the ball on a slope needs to know about gravity.

    No it doesn't - that would imply there is someone doing the aiming. A better picture would be for photons to be fired off at random in all directions from something like an electron.

    They don't know anything about the charge until it gets close.

    Depends what you mean by "information".
    Try it and see. Though that would be for repulsion iirc. Texts using the thrown vs swapped balls analogy (and that is all it is) are usually pretty good at describing this. Hint: Law of conservation of momentum.
    They just are - those are the rules.

    You may want to look up "self energy of the electron" to get a better idea about what these models are trying to describe. Photons are being released and absorbed all the time in this model.

    energy can come from the object's kinetic energy for example ... releasing a photon slows it down.
    but we have to be careful here because Field theory is relativistic ... and an electron that is stationary wrt the observer, releases a photon, it is no longer stationary: photon and electron head off in opposite directions by conservation of momentum: how are we to account for conservation of energy? That what you mean?

    There are two major positions here.
    1. conservation of energy can be violated provided the time-scale is sufficiently small. If the energy violation is ΔE then it is OK as long as it lasts ##\Delta t \leq \hbar/\Delta E## ... a result that agrees well with the observed ranges for the various interactions.
    2. conservations laws are strictly observed - the "virtual" particles are just that - virtual. They are an artifact of the way the calculations are carried out. (Look up "perturbation theory"). There are lots of situations where we do some intermediate steps in our calculations without the intervening steps having any physical, literal, reality. Long division, for example.

    Wikipedia takes position (1), here's an example of position (2).
    Bear in mind that these are only models, not reality, and that physics, as with all science, is a work in progress: there are no ultimate answers here (The Guide not withstanding.).

    I can imagine.

    I am conscious that the replies I have given are going to be inadequate - and, in some sense, downright wrong. (I brace myself for people saying what a mess I'm talking.)

    None of the answers here are likely to be very satisfying anyway - you should realize that whole books are written on the themes you have introduced. The best anyone can do is point, hopefully, in the direction of more understanding. The amount of mileage you get out of all this depends entirely on your willingness to learn.

    Have fun.
    Last edited: Oct 5, 2012
  4. Oct 6, 2012 #3


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    Although I'm a theoretical physicist, I couldn't disagree more with this kind of explanation, particularly when given to a beginner, who looks more for qualitative answers.

    Maxwell's equations are a mathematical description of electromagnetic phenomena. They are the most successful of such descriptions as far as one can get with classical (i.e., nonquantum) physics. However, the em. field is not a consequence of Maxwell's equations but the opposite is true. These equations are the result of some centuries of experimental research about electromagnetic phenomena.

    According to our understanding, as far as we restrict ourselves to classical physics, the electromagnetic field is a physical object as are solid bodies, liquids and gases making up our macroscopic world. We realize this field as forces acting on bodies, which carry an electric charge. The electric charge is an elementary property of matter, and also cannot be explained by more simple entities.

    As it turned out also optics, dealing with the phenomenon "light", is explained in terms of the electromagnetic field. It also became clear that there are any other kinds of electromagnetic waves, like radio waves or X rays, etc. which all are nothing else than electromagnetic fields oscillating with different frequencies.

    Not going into details, I just mention that this classical picture of electromagnetic fields, interacting with charged matter, providing the electromagnetic interaction between charged matter and induced themselves by electric charges and currents, is not complete. It does not work whenever the very smallest charged particles, the elementary particles, atoms, molecules etc. are involved. Here one needs the quantized version of Maxwell's electrodynamics to describe the phenomena correctly. Today, there's no limitation for this enhanced description known. To the contrary, this socalled quantum electrodynamics (QED) is the most accurate theory in physics ever, agreeing with observables like the electromagnetic moment of the electron up to 12 significant decimal digits!

    Classical Maxwell theory gives a very detailed answer to this question, and philosophical problems are not physics problems and thus better left to philosophers who might write some big books about them. Usually they don't help the natural scientists at all to understand nature, but that's another thing.

    According to Maxwell theory there is no action at a distance, but the electromagnetic interaction of charges is due to the local interaction of one charge with the electromagnetic field caused by all other charges and currents at the place of this charge. Maxwell theory is a classical local and causal relativistic field theory (as QED is a local and microcausal relativistic field theory). All interactions are thus described as local in space and time.

    This is the usual misleading handwaving given in popular science book in an attempt to explain quantum field theory to the layman. This is not too good a description and has not much to do with how quantum field theory really works. It's much better to say that the interaction is mediated by the fields. That's true as well for the quantum theory as the classical theory.
  5. Oct 6, 2012 #4


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    Simon, this is complete hoo-ha. If the classical electromagnetic field is a "metaphor(e)", what are Maxwell's Equations, a "parable"?
    At least you acknowledge it. Which leaves open the question of why you think such misinformation would be of benefit to anyone, especially a beginner.
  6. Oct 6, 2012 #5
  7. Oct 6, 2012 #6

    Simon Bridge

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    I did say "kinda". :(
    The interaction with a field is what usually gets called "spooky action at a distance" isn't it? After all, "where does the field come from?" was the question. It comes from something a long way away. "How does it get there" was the jist of another question.

    Anyway - it is a fair point. I thought I had modified this position a bit lower down but maybe I just set up more confusion?

    Thank you.

    One of the reasons I answer these questions is because I know a lot of people who are better at these explanations than me will also answer too. The resulting discussion deepens my understanding and improves my descriptive abilities for the future. It also benefits OP to see a diversity of opinion and how scientists resolve disputes.

    However - this only happens if people will demonstrate what they feel would be a more correct or useful answer to the same questions instead of just telling me I got it wrong. C'mon, how does that help anyone - least of all the OP?

    You think you can give better answers to the questions in post #1 then please don't keep them a secret.
  8. Oct 7, 2012 #7


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    No! The very idea of fields is to avoid "spooky action at a distance", which cannot be because of relativity. There can be no causal signal propagation faster than the speed of light in vacuum. The interaction between objects is mediated by the field which itself is a dynamic entity moving according to relativistic field equations. These interactions are thus always local in space and time. There's no spooky action at adistance whatsoever involved in this picture. It was not by chance that the relativistic structure of spacetime has been found through the analysis of Maxwell's field equations of electromagnetism!
  9. Oct 7, 2012 #8

    Of quanta.

    As probability and you shouldn't classically insist that objects have fixed properties in spacetime before a measurement is carried out.

    Their distribution depends on initial parameters and the solution of the Schroedinger's equation.

    By exchange of virtual photons(this is only a model, not what really happens, which seems to be unclear).

    The model in QFT is mathematical and i can't supply the answer, but there is one.

    This is only a model, nobody is insisting that virtual particles are real and measurable. It's an useful approximation.

    Your questions are hard for one reason - you are asking for a classical-like explanation for events of quantum nature. That's why the CI is the most prominent one, it's sort of like the easy solution that keeps most everyone happy( an useful bridge between what is observed and what seems to underlie it)
  10. Oct 7, 2012 #9

    Simon Bridge

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    Cool. Remains only to answer OPs questions then ;)
  11. Oct 7, 2012 #10
    quantum electrodynamics is based on the mediation of photons between electrons,positron or say leptons.this is very accurate theory so we really believe in it.the idea of action at a distance is avoided by virtual photons which mediate the interaction and attraction and repulsion can be explained in a simple calculation using scattering matrix and taking the non relativistic form which can also be explained qualitatively.see here
  12. Oct 7, 2012 #11
    Interesting! It looks to me that the author there tries to make attraction by means of photons plausible thanks to going back in time (what is the effect of a photon transmitted in the negative t direction? [...] thereby resulting in the opposite momentum transfer. ") ...

    To my regret, my definition of "time" doesn't permit such mathematical creativeness. Is there perhaps a different way to describe the same theory (giving the same results but possibly called a "different" theory) and in which the standard meaning of "time" is maintained? Or alternatively, can anyone explain how to map such a mathematical time to common physical "time" (= measure of progress of physical processes)?
  13. Oct 7, 2012 #12
    Thanks Guys. I`m slightly more confused than I was before I asked the question but at least I now know the work that has to be done to find this out. One more question though. Has physics found a reason as to why there are only two charges. Or for that matter, 3 dimension. Or, gravity being only attractive.(surely there could be theoretically speaking, equations that describe a repulsive gravity. but then anthropic principle comes in i think but its only a suggestion)
  14. Oct 7, 2012 #13

    Simon Bridge

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    Have you read this: http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html yet?
    It covers most of what you asked and should clarify my own response somewhat.

    Also try: http://vega.org.uk/video/subseries/8
    Feynman explaining QED etc. in an accessable manner.
    (There have been advances since these lectures but they'll help your ideas along.)

    Physics does not do "why" questions so I'm afraid so that would be a "no".

    There has been a lot of work on how it is that we come to be in a Universe with these kinds of properties - largely speculative afaik.

    eg. we live in 3+1 dimensions (string theory not withstanding) where the +1 is time. There is an argument where you consider the possible phase-space of every conceivable arrangement of dimensions and think about what physics would look like in those kinds of Universes ... like, what would it mean to have two time-like dimensions and five space-like?
    (Bottom of this wikipedia page.)
    How we end up with this configuration is something cosmological models of the early Universe have to deal with.... so read about the Big Bang for some ideas I guess.

    The phase-space approach can be very useful in working out these sorts of questions.
    What would electromagnetism look like if there were three charges? The nuclear forces?

    But I think you are correct that the anthropic principle will come in here - it is basically a philosophical questions after all. In the end: thems the rules (as near as we can make out).
    Last edited: Oct 7, 2012
  15. Oct 7, 2012 #14

    Simon Bridge

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    You want the Universe to conform to your definitions?
    Feynman has a famous reply. ;)
    (perhaps a bit unkind - it's from the first in the series in the link in the last post - he also says that "the rules are so skrewy that nobody will accept them" : it takes years of training just to stop people running away.)

    How about:
    ... the second heading from the top.
    Last edited: Oct 7, 2012
  16. Oct 7, 2012 #15


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    I think we must extend our definition of time to include closed timelike curves, since we have this same fruitless discussion on virtual particles once a week.
  17. Oct 8, 2012 #16
    That looks much, much better. Thanks! :smile:

    I interpret Feynman diagrams as a mere calculation trick, just as McIrvin and, I think, Feynman himself originally.

    So, while Brown explains it with "back in time" processes of virtual photons like pingpong balls, McIrvin explains it as due to interference of virtual photons like waves ("wave functions"). If I correctly understand it, he merely uses the "backward in time trick" for a calculation of the wave function. and he remarks:

    "As long as I remember what's really going on, this trick is formally OK and saves a lot of trouble"

    Why? :confused:
    Please indicate where in the above physics FAQ explanation, inclusion of "closed timelike curves" in physical "time" is required.
    Last edited: Oct 8, 2012
  18. Oct 8, 2012 #17


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    I suspect that if Bill were Foghorn Leghorn, he would say, "That's a joke, I say, that's a joke, son!" in his booming voice, and give you a hearty slap on the back. :biggrin: (Of course, he's free to correct me if necessary. :blushing:)
    Last edited: Oct 8, 2012
  19. Oct 8, 2012 #18

    Simon Bridge

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    Feynman pretty much describes everything we do in physics as a trick of calculation. Artithmatic is a trick of calculation, says he.

    ... and explains why you use the reverse time trick too.

    Both authors do it, one casually assigns more "reality" to the model than may be warrented.

    However, it may be a reasonable thing to do since, to a photon, no time passes, the particles are in the same place ... so one could argue the other way that there is a real ambiguity about which particle did the emitting and which the receiving. (Indeed both could be the same particle... but that is a different interaction again.) If you swap the roles over, then you've swapped cause and effect ... that's all the "backwards in time" means. Massive observers would still experience things happening forward.

    (McIrvin's model of the interaction seems to have more physics in it though.)

    Easy there - just because you happen to like the explanation does not mean it is true. All that "we just do the math backward" stuff could just be to stop people who think like you from running away? Or to keep the explanation in topic... McIrvin makes a lot of fudges on that page in the name of keeping things simple.
  20. Oct 9, 2012 #19
    Funny enough, in an earlier version of my reply I gave an example of a misunderstanding based on such a concept, but next left it out as it was not necessary to mention it, since I was asking a question (and next you provided a satisfying answer). Some people "explain" "spooky action at a distance" by saying that for a photon no time passes so that the whole universe can be reached "in no time" (I will not link to one of these sites as it likely would be categorized as "crank site" by the Mentors).

    A photon is not a reference system, and no valid or useful reference can be set up in which a photon is in rest.
    If you want to discuss or debate that, please go to https://www.physicsforums.com/showthread.php?t=235132 (indeed, the topic of observation at c is in the domain of the relativity forum and that thread is still open).

    Note that I came to physicsforums because I would like to make sense of the predictions of QM, in the same way that I already understand SR and GR (no hocus-pokus). Consequently I have every reason to "run away" when people try to sell me ideas that do not make sense to me, and to say "thank you" when they refer to existing sensible explanations. And please don't confuse that with me trying to forbid you to think differently.
    At first sight those admitted fudges of McIrvin are very innocent compared to what appears to be a super-fudge of Brown. McIrvin suggests that the trick (not "the model"!) is not OK if one doesn't remember "what's really going on".
    Ah OK! No serious intention to link electrostatics to black holes and wormholes. :biggrin:
    Last edited: Oct 9, 2012
  21. Oct 9, 2012 #20

    Simon Bridge

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    Yeah I've seen those things ... that way lies a minefield.
    Good grief - well, clearly the case not only can be argued, but is indeed being argued.

    Not at all ... it is just that the same charge (hokus pokus) can be applied to GR/SR descriptions, and has been, and in much the same way as you have objected to a possible way that time could work.

    There is a trick to getting your mind around GR which you appear to have "got" - the similar trick in QM is to realize that everything is statistical... so "opposite charges attract" on average: there is a chance they could repulse or just do nothing.

    When you try to apply that to GR, all those deterministic things are only true on average, you get Field Theory and modern particle physics. But you lose a lot of the no-nonsense aspect of GR ... in a way it is what Bohr and Einstein spent all that ink arguing about isn't it?
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