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Q & A: magnetic action at a distance

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Electromagnetism
Q:
Thank you for this service. I think the main confusion I and others have about magnets is more fundamental. I understand how I can make a ball move when I push it. I understand how I can pull on a rope and move a sled. I even understand how I make every object in my car jolt forward when I slam on the brakes. Magnets and Large Planets move things towards them _from a distance_ without touching them. I think that is what is confusing. I understand that Einstein explained how gravity works by describing it as inducing an acceleration so that, like the objects in my car, they all "fall" forward. But that affects EVERY object and magnets ONLY affect iron or other magnets. So, how do magnets pull things towards them from a distance? What's the mechanism?
- Gary (age 43)
Berkeley, CA
A:
You're getting at one of the ways that the world most mystifies people- the apparent existence of forces at a distance. It turns out that there really is something carrying the force between the magnets- a magnetic field. The field isn't visible and can't be felt by non-magnetic objects, so it seems like there's nothing there, but it really is there. It can be measured with all sorts of instruments, including a little compass.

So the question becomes whether saying there's a magnetic field is just renaming the fact of action at a distance or whether the field really should be seen as an ingredient of the physical world. The main symptom of the field being real is that it takes a while to propagate from one place to another.

Say you turn on an electromagnet. How soon will it start pulling on a distant magnet? If it started immediately, you might say there was action at a distance, and the field was a mere name.  It doesn't. The other magnet doesn't feel anything until a little time has elapsed. The field propagates at the speed of light. Admittedly the delay times are short for distances on earth, but they're easily measurable with common lab instruments.

Mike W.

p.s. I've marked your question as stand-alone, rather than leaving it at the tail end of a long thread.

(published on 12/10/2011)

Follow-Up #1: carrier of magnetic field

Q:
Thank you very much. So are there particles that propagate this magnetic field? Are they electrons? (I believe I read that this field originates in the North Pole and ends at the South Pole, is that true? And, if all that is correct, do scientists understand how the magnetic field creates motion in objects? (electron exchange causing movement? what?)
- Gary (age 43)
Berkeley, CA
A:
The quantum ingredients of magnetic fields are photons, the same type of excitations that carry light or radio waves. The way that these fields exert forces on matter is indeed understood, along with the way matter gives rise to the fields. The classical description goes back to Faraday, Maxwell, Lorentz, etc. The quantum description, quantum electrodynamics, was developed by Feynman, Schwinger, and Tomanaga. The theories are precise and mathematically self-consistent.

With regard to the Earth's magnetic field, the external field does loop around from one pole to the other. Magnetic field lines don't end, however. (Technically, divergence of B=0) They loop back around through the Earth.

Mike W.

(published on 12/13/11)

Follow-up on this answer.