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I understand that the electromagnetic force can be described as the exchange of virtual photons. I also understand that it's possible for virtual photons, unlike their real counterparts, to have mass (for various complicated reasons.)

I'm quite new to all this, so I'm finding it hard to believe that photon exchange is powerful enough to move macroscopic objects (or hold up a train), so I'm convinced I'm looking at it the wrong way.

So specifically:

  1. Are the virtual photons just a useful way to imagine what's actually happening, or are actual photons being exchanged?
  2. Can the force applied to magnetically attracted/repelled macroscopic objects be described purely in the QED sense of virtual particle exchange, or is it the result of something else we can point to, or is it voodoo from the metaphorical aether?
  3. Can force simply be spontaneously created by a field interacting with objects within it (as far as I can tell that's not what fields are/do)

And very generally:

  • Where does the force to move macroscopic objects originate (at the most fundamental level)?
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Why don't You include the "physical process(if any)" for electric charges, gravity, strong and weak nuclear forces in Your question? The "if any" is really fun. –  Georg Sep 23 '11 at 9:18
I'm tempted to answer "no, there is no physical process behind magnetic attraction" ;-) (just kidding, of course... it's not a bad question) –  David Z Sep 24 '11 at 4:52
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2 Answers 2

up vote 4 down vote accepted

I don't know where you get your intuition about how strong a virtual particle can push or pull. Why would virtual particles push any less than real ones?

  1. Virtual particle exchange is mathematically equivalent to other formulations of the theory. I don't know how to answer your question, because it is meaningless in logical positivism. If two ways of calculating something give the same answer, it is a meaningless question to ask which of the two is really going on underneath it all.
  2. The force between two magnets is due to virtual particle exchange, it is also due to the field, because the virtual particle exchange just gives the classical field. But you can also just say "two magnets feel a force" in the 17th century style, without saying anything more. There is no reason to reduce the force between magnets to some pushes and pulls of material objects, because material objects are made out of fields too (although of a different kind, with different statistics). Your intuition is backwards--- nonlocal pushing and pulling is the norm, not the exception.This is not voodoo because you can measure it, and anything you can measure is real.
  3. Classical fields are defined by the forces they create on objects, so a field can push and pull. The reason magnets attract is that they create a field which pushes them. If you think of a quantum field, then it is virtual particle exchange, but this is equivalent for magnets to just a field pushing and pulling.
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Lot of words, no answer, which is: we don't know –  Georg Sep 24 '11 at 7:44
@Georg: I don't know about "we", but I certainly know! –  Ron Maimon Sep 24 '11 at 7:54
Very much liking this answer, it's beyond the (very incomplete) model I have in my head. I've realised that the "physical process" wasn't what I'm really looking for. For example, understanding gravity used to be much the same for me - force seeming to come from nothing. The answer I found there was in general relativity - which explains the "force" as objects moving in locally straight lines in curved spacetime. Is there a similar answer (or potential answer) for magnetism? –  whalabi Sep 25 '11 at 20:45
@whalabi--- yes there are several geometrical pictures--- the simplest one is to think of an extra 4th dimension of space compactified into a tight circle, so that if you go around the extra dimension, you come back after a short length. If you make a particle "spiral" in the extra dimension. The locally straight spirals give the charged particle motion in an emergent electromagnetic field. Even without the extra dimension, the electromagnetic field is understood in a geometric way, but it is slightly more involved to explain in words because it changes phases of wavefunctions. –  Ron Maimon Sep 25 '11 at 21:00
Brilliant, that works for me, for now. Thanks for your time Ron and everyone else who has contributed. –  whalabi Sep 25 '11 at 21:32
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The physical process of Magnetic force in relativistic Quantum Field Theory is much easier to understand non-pertubatively versus the pertubative picture of virtual photons.

Just look at the phase change rates of the charge's field induced by the Magnetic field $B$ using the vector potential $A^\mu$. What you see is that the de Broglie wave length becomes shorter in the direction of the acceleration of the charge.

There's are pictures (11.5 and 11.6), specifically concerning your case, in my book in the chapter "The Lorentz Force derived from the Klein Gordon equation"


Regards, Hans.

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