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Can magnetic field expand faster than light?

  1. Jan 24, 2009 #1
    When I was reading abuth EM oscilators, a question suddenly ran through my mind:
    “Is there a frequency, when the speed of a colapsing/forming magnetic field would exceed the speed of light?“
    Einstain said that nothing can go faster than light. Well, ok, but if the magnetic field (generated by EM oscilator, with AC current) was for example 1km vast (from it`s source), with frequency higher than 75 kHz, then the magnetic field would have to travel the trajectory of 4km in one period (it would have to expand to 1km distance and diminish back, then reverse polarity and to 1km and back) faster than 300000km per s. As long as this is, according to Einstein, impossible, could it be a reason why EM waves are generated at higher frequencies?
  2. jcsd
  3. Jan 24, 2009 #2


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    Changes in the field would still propogate at the speed of light. They would spread outward like ripples in a pond.
  4. Jan 24, 2009 #3
    So does it mean, that even if I turn the polarity of the field almost instantly, for a moment, in some distant point, the field would still have the same polariy and magnetic induction as before? What would happen to magnetic field of a toroidal permanent magnet rotating at a speed near the speed of light? To not exceed the speed of light the field lines would have to curve, how?
  5. Jan 24, 2009 #4


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    There is no physical "almost instantly" when the results move at the speed of light. No matter how fast you physically swap the polarity, it will be a snails pace when compared to the speed of light. At best you could, in principle, approach some arbitrary fraction of the speed of light.
  6. Jan 25, 2009 #5
    To add to what DavidC was telling you,

    Yes. And they do curve. From a rotating magnet, with the poles not aligned with the axis of rotation, as you imply, there are spiraling magnetic fields alternating in direction propagating outward at the speed of light (in vacuum conditions). The guys proficient in Cosmology can tell you about this.
  7. Jan 25, 2009 #6
    Thanks for your answers

    I know, but as in the example shown in the thread, if the field was vast enough, even this fraction of speed of light would be enough. In your reply to my thread, you have said:
    I was asking about some distant point of the field, to which "ripples" has not yet arrived. If the field was realy vast, and I turned off the field generator, would the field, in some very distant point, be able to, for example, attract some metal micrometeorite? (even though, it`s generator was turned off)
    If I understand this well, then the rotating permanent magnet would generate waves. would it? If yes would they be ordinary EM waves?
  8. Jan 25, 2009 #7
    There are some misconceptions that you seem to have.

    It will benefit you if you can stop thinking of electric and magnetic fields as isolated entities. While it is educationally pragmatic to isolate them, it is not always appropriate to do so. Particularly for considerations of 'light'. Maxwell's laws tell us that a time-varying E-field will generate a B-Field and time-varying B-field will generate an E-field.

    Magnetic fields do not "attract". They exert a force on charges moving through them. So yes, if the field at a point has not yet "realized" the generator is off, then it will still exert a force on a charge moving through it.

    Rotating Permanent Magnets do emit electromagnetic waves. So does accelerating anything that contains charge. The challenge is to accelerate them so intensely and coherently that the EM waves will have an amplitude observable above any background radiation. The EM waves generated by rotating a magnet would be "normal"; however, the analysis necessary to predict the frequency and amplitude of the waves would be different.
  9. Jan 25, 2009 #8
    As MackBlanch says, yes. The funemental frequency would be the rotational frequency of the magnet. DaveC's ripples are spiraling ripples. Since we know that the alternating magnetic fields point (mostly) in the circumferal directions, the alternating electric field would be parallel with the axis of rotation.
  10. Jan 25, 2009 #9


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    What do you consider to be "ordinary EM waves" as opposed to "extraordinary EM waves" or whatever?
  11. Jan 26, 2009 #10
    Thanks for replyes

    I was just asking, because as phrak said
    I taught, these waves might be somehow different... but As Mack said I was not considering, that changing M field generates E field. My fault.

    And what about momentum? How would the change of momentum of the object slowed/accelerated by the field (with generator just turned off) manifest in the field itself? I mean, according to law of conservation of momentum, the momentum of the field would have to change somehow. Does such a magnetic field (with generator just turned off)even have weight? (so we can talk about it`s momentum - or change in it)
  12. Jan 26, 2009 #11
    That's OK, Tominator. I remember asking the same sort of questions about moving magnets. There's nothing wrong with this.

    A magnetic field, in a region of space free of charge, and changing over distance will result in an electric field. As far as propagating magnetic fields go, they will always be associated with propagating electric fields. In the case of a spinning magnet, it is an induced electric field.

    But propagating fields are not the only fields allowed in free space, The magnetic field about a stationary magnet is one example, or course. Upclose to the spinning magnet, the fields don't look like propagating fields at all.

    I'm not sure what object you mean. If you stop the magnet from spinning, the electromagnetic waves will still propagate outward at the speed of light (it is light), and has momentum given by the averaged Poynting vector. Like any other electromagnetic wave, it is capable of transfering momentum.
  13. Jan 27, 2009 #12
    As it is mentioned above, I asked about speed of a change in realy vast magnetic field. DaveC said that
    Then, I was asking about some distant point of the field, to which "ripples" has not yet arrived. If the field was realy vast, and I turned off the field generator, would the field, in some very distant point, be able to, for example, attract some metal micrometeorite? (even though, it`s generator was turned off). This question was answered by Mack
    And then I asked
  14. Jan 27, 2009 #13
    I am going to restate my understanding of your question:

    If the magnetic field pushes on the charge moving through it, then the charge must push back on something because "every action has an equal and opposite reaction". How is the "push back" manifested?

    I am not confident in my answer, and I should probably let someone else answer, but I believe the "push back" manifests itself as an alteration in the field itself.
  15. Jan 28, 2009 #14
    so in principle. if I had two magnets in a void, X distance away from each other so the fields don't overlap...

    if one of those magnets were spinning the field would eventually become an elliptical pattern and push the 2nd magnet away?

    or did I misread something...?
    Last edited: Jan 28, 2009
  16. Jan 28, 2009 #15


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    Like gravity, the fields always overlap; they extend to infinity. (Unlike gravity though, the strength drops off as the cube rather than the square of the distance).
  17. Jan 28, 2009 #16
    Moonknight, What is it you read?
  18. Jan 29, 2009 #17
    I'm sure you're correct Dave, but with a changing magnetic field there is a re-enforcing electric field, as you've said. At distances that are large compared to the rotational period, there is a propagating wave whos strength reduces as the square of the distance.

    This same sort of fuzzy business comes up with say, a discrete capacitor, where the near electric fields are calculated in one way but the transmitted radiation another. How it all meshes together is a mess involving intractable and mysterious Bessel functions of some sort. Concerning the capacitor, Feynman takes so steps toward it in his "Lecture on Physics".
    Last edited: Jan 29, 2009
  19. Jan 29, 2009 #18
    From what I understand the only way to move faster than the speed of light is to literally stretch spacetime itself. And when I say stretch I mean the sort of stretching we might have seen with accelerating expansion.

    Since a magnetic field would exists within this structured, yet distorted structure - my answer would be "no". Its top speed would always be relative (and smaller) than the paradigm it exists within.

    Last edited: Jan 29, 2009
  20. Jan 29, 2009 #19
    In one of the threads, I have posted 2 questiones and I have forgot to separate them, that might have confused you, Moonknight, sorry.

    One was about a field of spinning permanent magnet (this was answered by phrak and Mack)
    The other was about the speed of a change in a magnetic field. Now, we are discussing a special case: If we turned off the generator of a vast magnetic field off, it would take some time for the change to get to some distant point of the field. The question was: Can such a field (with generator just turned off) affect some metal object (metorite,or...) at the distant point?(to which the change has not yet arrived)
    If yes, what would be the reaction of the field? (how would momentum conserve?)

    I taught, it might drain power from the magnetic field as a reaction.
    The reaction of the field can not affect the generator, because at that time it is offline.
    If the field affects the object moving through it, and the reaction does not affect the generator (because it was turned off), then this could be used as a propulsion system, if we manage to connect the object and the generator. The object would have to be large electro-magnet, which would be turned on in the same moment, the generator would be turned off. So the electro-magnet would "bounce off" the magnetic field without pushing on the generator.(which is at that moment offline)
    So my question is: Would the magnetic field itself (with generator just turned off) affect (for example) some metal object.
    Last edited: Jan 29, 2009
  21. Jan 30, 2009 #20
    sorry, I posted that without looking at the whole thread...

    It's not what I read, it's what I didn't read...

    thanks for clearing that up though
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