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When is field interaction instantaneous?

  1. Mar 13, 2006 #1
    I think I understand how signals can be sent by field disturbances, and that the disturbance is propogated at the speed of light.

    Now, I was wondering about the case where a stationary field is already set up and you place a test charge in that field. Does that charge move instantaneously or is there a time interval (equal to d/c, where d = distance to field origin) between the placement and the motion?

    My intuition tells me that the motion must be instantaneous because the field is already set up at that location. Am I correct?

    Thanks.
     
  2. jcsd
  3. Mar 13, 2006 #2

    jtbell

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    When you place a test charge in a pre-existing field, it "feels" the effect of the field instantly. Any time delay due to distance from the source of the field has already happened, when the source of the field was moved into position.

    If you now move the source charge, the field at the test charge does not change (and the test charge does not "respond") until after a time delay, of course.
     
  4. Mar 13, 2006 #3
    Yeah, that's what I figured. Thanks.

    But it blows my mind, you know? I mean, nothing is there!

    Thanks again.
     
  5. Mar 13, 2006 #4

    vanesch

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    Well, the FIELD is there. This is actually a big discussion people had at the end of the 19th century. They considered fields just as "a mathematical trick", like the field of stress in a solid, or the temperature field or something. You know, a mathematically nice entity to work with, but which has an underlying explanation in a particle view. That's why people where looking for the "material carrier" for electromagnetic fields, which were originally seen as a kind of stress in a strange gooe, permeating all of space ; they called it the "ether". But the more people studied the properties of this ether, the stranger it became.
    Now, it is accepted that fields are genuine physical entities, which are "there".

    As to your question of instantaneous effects: as said before, a test particle will immediately sense the local field. But the field will not change immediately all over the place if you move a charge somewhere else, because a "genuine" field obeys partial differential equations of hyperbolical type which include finite propagation speeds. This must be the case if relativity is correct.
     
  6. Mar 14, 2006 #5
    The subject of field propagation(and especially field "collapse") is not well understood.
    Assuming the "C" constraint, bizarre effects can be postualized.
    For example, take a superconducting magnet in outer-space and ramp it up for 1 minute. Then, turn it off.
    The magnetic field, though collapsing, exists for 1 minute even AFTER the source is turned-off.
    This is postulated but not proven.

    Such a "free-space" field effect should occur.
     
  7. Mar 14, 2006 #6
    Don't all fields have to be made up of particles?
     
  8. Mar 15, 2006 #7

    vanesch

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    Classically, no: fields are entities of their own. Quantum mechanically, there are two equivalent viewpoints. One can take the field as fundamental, and the particles as artefacts that result from the quantum states of the fields, or one can take the particles as fundamental, and the field as bookkeeping devices for the quantum behaviour of the particles - however, this last view has, as someone recently pointed out, a difficulty in non-inertial frames: the number of particles is not the same for all observers, which would mean that it is not an observer-independent concept.
    So the most standard view in quantum field theory is that ALL is fields, and what we have been calling particles are just some manifestations of the quantum properties of those fields in certain, approximate, conditions.

    Which brings us to the conclusion that if something is physical out there, according to classical, as well as quantum theory, it are fields.
     
  9. Mar 15, 2006 #8

    jtbell

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    It is not a postulate (assumed true without having been derived from anything more fundamental), but rather a consequence of Maxwell's equations for eletromagnetism.

    Replace "superconducting magnet" with "radio antenna". Are you suggesting that if we turn off the antenna, the radio signal should cease everywhere instantaneously? That would seriously violate Maxwell's equations, which have been extensively verified and form the basis for radio/TV broadcast engineering.
     
  10. Mar 15, 2006 #9
    I don't disagree at all. Perhaps you misunderstood my comments(or, I just poorely worded them)
    Radio waves would behave as you described, but a magnetic field is dependent on the source existing. When you turn off the electromagnet, the magnetic field collapses or otherwise "disappears"
    However, the interesting thing about this is that the magnetic field does not do this instantly, and I would suppose that it does so at a maximum rate of C.
    So, if one had a really large and powerful electromagnet, then turned it on for a few seconds then turned it off, it seems reasonable to assume that the magnetic field continues to exist, though briefly, without it's source.
    I believe that gravity works the same way.
    For example, I would suppose that if our sun suddenly popped-out of existance(I know it's not possible, just humor me here), that our Earth would continue to expierience a gravitational attraction for 9-minutes towards a mass that no longer exists.
     
  11. Mar 15, 2006 #10

    vanesch

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    You know, a radio antenna is nothing else but exactly that! Usually one works with the electric field (in dipole antennae) but you could just as well use a circular antenna. What you do is flip the current regularly (say, at 27 MHz) in that antenna. Each time you create a magnetic field, and then you change its polarity. This changing is what is generating radiowaves. So if you suddenly switch off your magnet, you send out a pulse of radiowaves.

    Yeah, you just sent out a radiowave...

    Well, gravity just as well as EM has a problem with that thought experiment, in that EM imposes conservation of charge. You cannot "make disappear in a puff of logic" some charge without violating the very equations giving the solution you're trying to talk about.

    So, as to the question: what would the maxwell equations predict if charge suddenly disappeared ? The anwer is that the Maxwell equations don't allow for charge to disappear.
    A similar thing happens in gravity. Mass-energy cannot "disappear", and as such, the same answer is given to the same question. The sun cannot disappear suddenly. It could go somewhere, suddenly (which would generate a gravity wave), but it couldn't disappear.

    cheers,
    Patrick.
     
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