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An experiment on magnetism and momentum?

  1. Feb 18, 2008 #1
    Say we have two electromagnet A and B a light year apart from each other. Someone turn on A and a magnetic field is created and propagate at c toward B.

    It takes a year for the mag field from A to arrive B, B is turned on just before A's field arrive. B's field made to be opposite from A and is then repelled by the A's field.

    But since information can not travel faster than c, when B start accelerating A would not know. It takes a year for A to notice B's field.

    Ordinarily, if A's magnetic field remains unchanged when B's field arrive, A would experience a repulsion force and accelerate to the other direction in comply with conservation of momentum.

    But what if A change it's magnetic field just before B's field arrive? Then when B's field arrive A would feel an attraction force!! This makes both A and B feel a force to the same direction resulting in a serious violation to Newton's third law.

    This could be used to build a reactionless drive:
    step 1: A turn on, mag field out
    step 2: just before A's mag field arrive, B turn on with a opposite field direction from A so B gets repelled. And B's mag field travel to A too.
    step 3: just before B's mag field reach A, A reverse current direction to reverse field direction=> A is attracted to B when the field arrive.
    ........... get the picture?

    as long as A & B switch field direction exactly out of phase, all the force acting on them are of the same direction.
  2. jcsd
  3. Feb 18, 2008 #2

    i dont know... and what about the intermediate states?? the two fields will interact in some spacetime point in the middle... and the strenghts vary in function of the relative distances...

  4. Feb 18, 2008 #3
    I don't know the detailed explanation why this system does not violate conservation of momentum. Probably it has something to do with the fact, that electromagnetic field has it's own momentum, so the magnets+field momentum is conserved.
    A similar "reactionless drive" can be achieved simply by turning on a lamp on the "vehicle": each foton with energy E also carries a small momentum E/c, so any source of electro-magnetic waves can be used for propulsion.
  5. Feb 18, 2008 #4
    Sorry, double post.
  6. Feb 19, 2008 #5
    I don't think this kind of momentum can be compared to the momentum of photons, since the force is actually lorenz force here which can be many magnitude larger than photon's force depends of field strength and current.

    Also, the distance can be 3 meter with magnetic fields of the two solenoids oscillate at 100Mhz.
  7. Feb 20, 2008 #6
    My dear friend, photons are the smallest quantity of electromagnetic field in excistence, so the momentum of photons *is* very significant here. It is simply a matter of having enough of them.

    The concept fails because this momentum depends on the *square* of the field - hence, it is independent of the polarity, and is determined by the propagation vector of the electromagnetic wave. Changeing the current in the loop does not change the propagation of the wave, it only puts it out of phase by a half wave.
  8. Jul 6, 2010 #7
    no one has actually said why this wouldn't work? At least, not a plausible complete explanation? I've heard something related to this concerning electro hydrodynamics? I dunno maybe.

    So, why would the original question NOT violate conservation of momentum??
  9. Jul 10, 2010 #8
    In Step 1 the magnetic field of A varies in time, and is therefore accompanied by an electric field. The time-varying electric field in turn induces a magnetic field further out in space. (This is how A's magnetic field propagates out into space.) The momentum density of the electromagnetic field is proportional to E X B. Thus as the fields of A propagate out into space they engender momentum. Etc. I am confident that momentum is conserved throughout a cycle of your system when the momentum of ALL things is considered.
  10. Jul 11, 2010 #9
    This is the correct answer.

    In fact, Maxwell deduced the existence of electromagnetic momentum because it was necessary to explain this very problem.
    The magnetic field behaves exactly as does an ordinary object with energy and momentum.

    You can find his original treatise here if you're interested.
    Last edited by a moderator: Apr 25, 2017
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