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Magnetic relativity

  1. Jun 14, 2009 #1
    From what little I understand of relativity theory, I have gathered that gravity is not considered anymore to be a "force", per se, but rather the intrinsic curvature of space-time itself. The body in free fall towards the Earth experiences the phenomenon of weightlessness in his descent, because no "force" is really pushing or pulling it down, it travels its own version of a straight line over a "curved space-time" (unless I misunderstand completely). From what else I've gathered throughout my studies, it appears to me that the other fundamental forces are still, in contrast, considered to be a "force" the way that word is intuitively thought to mean.

    Consider, then, a metallic man, whose body is attracted by the force of a magnet. Would this metallic man then not feel this sensation of weightlessness in his free-fall towards the magnet? Or would he feel a force, either pushing him towards or pulling him into the magnet. Certainly, we, non-magnetic people feel the force of a magnet pushing, but only when it makes physical contact with us, but what of the magnets themselves? Does a force act upon them or do they follow their normal course in a curved space-time?
  2. jcsd
  3. Jun 14, 2009 #2


    Staff: Mentor

    Well, a uniform magnetic field exerts no net force on a magnetized object, but it does exert a torque. There is no inertial frame in which a torque would disappear, so I think that would still be a real force. But the question you ask is interesting.
  4. Jun 14, 2009 #3


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    Science Advisor

    Assuming that the magnetic force acts uniformly on his entire body, no, this "magnetic man" would not feel any force. I am not sure what you mean by "Certainly, we, non-magnetic people feel the force of a magnet pushing". Certainly we feel the weight (gravitational) holding a magnet in our hand. Do you mean holding one pole of a magnet near an opposite pole and feeling the force? That is because we are not allowing the magnet to move with the force. If we did not hold it, say just held it in our open palm (greased liberally to remove friction!) the magnet would move in our palm without our feeling any force (except, again, the weight of the magnet). This really has nothing to do with relativity; it is true classically.
  5. Jun 14, 2009 #4
    I am newbie to GR also. I had the impression that ONLY gravitational force curves the space-time around it, other forces act the usual way. Does that make sense?
  6. Jun 14, 2009 #5
    Magnetic and electrical attraction is a force. Just like if I push you. And no he will not feel "weightless"....that only occurs with an acceleration "g" in gravitational free fall...a special case of acceleration.

    This question seems confused....If there is no mass nearby, then the guy will be weightless when stationary without any magnet nearby, and any acceleration due to a magnet brought near him will feel similar to gravity, but generally to a different degree..."equivalence principle"....

    Is this what the poster is getting at??

    Seems like the magnetic properties of the metallic man should count...different metals will have different attraction and different acceleration right...even if the same mass??.... ferrites, magnetite and lodestone,etc....
  7. Jun 14, 2009 #6
    So far, that's the best understanding available. It's a good distinction to remember, but "the usual way" is only understood vaguely...other forces act in their unique way, for example
    F = qE + qv x B but precisely WHY that works is still pretty much a mystery....
  8. Jun 14, 2009 #7
    haha I spend some time to come up with an alternative to write instead of "the usual way", couldn't really find one.
  9. Jun 14, 2009 #8
    When they magnetically levitate a frog in a strong magnetic field gradient, the frog is weightless because the diamagnetic force perfectly cancels out the force of gravity.

    http://www.hfml.ru.nl/levitate.html" [Broken]
    Last edited by a moderator: May 4, 2017
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