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Direction of magnetic force between moving particles

  1. Dec 18, 2015 #1


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    [Split off from this thread]
    I think that the force is always repulsive. I think that an electron would have to travel faster than c for it to become attractive.
    Last edited by a moderator: Dec 19, 2015
  2. jcsd
  3. Dec 18, 2015 #2


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    Whether it is repulsive or attractive depends on whether the two charges are moving in the same or opposite direction. I made analogy with currents running on parallel, neihgboring wires though. I think the same effect can be observed for individual electrons.
    Last edited: Dec 18, 2015
  4. Dec 18, 2015 #3

    Vanadium 50

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    I believe that is correct. There is also an intrinsic dipole, but I don't think this changes the conclusion.
  5. Dec 19, 2015 #4
    Can you give more explanation to your answer?
  6. Dec 19, 2015 #5

    That effect is due to special relativity. Due to length contraction the electrons in one wire see more protons and fewer electrons in the other wire, hence an attraction between the wires. No protons, no attraction.

    An electron moving faster than light would appear to be a positron, which would appear to attract an electron. Richard Feynman thought that this actually was what a positron was.
  7. Dec 19, 2015 #6


    Staff: Mentor

    The electric force is repulsive. The magnetic force may be attractive. The only way for the net force to be attractive is for the magnetic force to be greater than the electric force. I believe that only happens if v>c.
  8. Dec 20, 2015 #7
    I am familiar with your idea about the wire, I have not met the idea of an ekectron moving faster than the speed of light being a positron
    I know of Feynman's view, I think he saw positrons as electrons travelling back in time, shown my the arrow on the positron lines in his diagrams
  9. Dec 20, 2015 #8
    If someone fires a bullet at you that travels faster than light. The bullet hits you, then you see the bullet flying through the air and be swallowed by the gun. So the bullet appears to be going backward in time.

    If an FLT electron in a magnetic field hits you, you then see it returning to its source curving as though it were positively charged.
  10. Dec 22, 2015 #9
    "The causality problems with superluminal travel come about as follows. Special relativity is based on the axiom that all observers have the same laws of physics, and these are converted from one observer to another by a well-defined procedure called Lorentz-transformation. This transformation from one observer to the other maintains lightcones, because the speed of light doesn’t change. The locations of objects relative to an observer can change when the observer changes velocity. But two observers at the same location with different velocities who look at an object inside the lightcone will agree on whether it is in the past or in the the future.

    Not so however with objects outside the lightcone. For these, what is in the future for one observer can be in the past of another observer. This means then that a particle that for one observer moves faster than light – ie to a point outside the lightcone – actually moves backwards in time for another observer! And since in special relativity all observers have equal rights, neither of them is wrong. So once you accept superluminal travel, you are forced to also accept travel back in time.

    At least that’s what the popular science books said. It’s nonsense of course because what does it mean for a particle to move backwards in time anyway? Nothing really. If you’d see a particle move faster than light to the left, you could as well say it moved backwards in time to the right. The particle doesn’t move in any particular direction on a curve in space-time because the particles’ curves have no orientation. Superluminal particle travel is logically perfectly possible as long as it leads to a consistent story that unfolds in time, and there is nothing preventing such a story. "

    ---Sabine Hossenfelder http://backreaction.blogspot.com/se...0-04:00&max-results=20&start=27&by-date=false
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