I have a question regarding the interactions of electromagnetic fields.

Say you have two superconducting electromagnets A and B. The properties of the magnets are known such that you can tell precisely how quickly it takes each one to produce it's full strength magnetic field and how long it takes for that field to no longer be present at the magnet when it is turned off. They are connected by a non-conductive rod at opposite ends at a distance equal to (time it takes for the field to no longer be present)x(speed of light). A________B

A magnetic field is induced in magnet A. A))))))))B Using a very precise computer, you shut off magnet A's field.

For a very brief period of time there should be no field present directly at magnet A, but the field is still propagating through space. A__))))))B

Now if I turn on magnet B at that moment, producing a field with opposite polarity, magnet B should be repelled by that field. A__))))((B --->

My question is, if magnet B is pushing off of the field magnet A produced. But that field is no longer present in magnet A itself. What prevents the rod they are attached too from moving in a single direction, as there wouldn't be an opposite force acting on magnet A to cancel it out?


1 Answer 1


In the described scenario, a magnetic pulse travels outward from the first magnet. The pulse would travel in all directions, so there is no net force on magnet A. Eventually the pulse reaches magnet B which is turned on just before the pulse gets there. The pulse pushes on magnet B. You are proposing that magnet A and magnet B are connected by a long (presumably nonmagnetic) rod so that the push is given to both magnets.

The next thing to ask yourself is "What happens to the pulse?". Let's say it is reflected backwards by its interaction with magnet B. Whatever momentum it carries, plus an amount equal and opposite to the reflected pulse's momentum, is transferred to magnet B.

Now let's say that magnet A is turned off just before the reflected pulse gets back to it, so the pulse simply proceeds in the backwards direction.

Step back and look at the whole system after that point in time. There are *two pulses moving through space: the first pulse, which started out symmetrical but is now missing the part that was reflected by magnet B, giving the first pulse a net momentum in the backward direction; and the second pulse, which travels asymmetrically in the backward direction and thus has a net momentum in the backward direction. So, after all the pulse emission and reflection, there is field momentum going backwards and magnet B has momentum going forward.

Bottom line: Yes, there is a net push of the magnet(s) in the direction from A to B, but it is not a reactionless drive because the momentum in the electromagnetic pulses moving in the opposite direction precisely balances the momentum of the magnets moving in the A->B direction.


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