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Squashed
2007-Jan-04, 02:45 PM
I have been wondering about magnetic field lines (http://helios.gsfc.nasa.gov/magfield.html) lately.

I have read that magnetism is mediated by photons which sounds okay but if this is true then why do the photons move in a curved path (http://www.manep.ch/img/photo/challenges/emfields/maglines.jpg)?

Photons are supposedly charge-less and the only force that can deflect the photon's trajectory, mid-flight, would be gravity but then I think about magnets and magnetic fields and the inference that magnetism is mediated by photons and so I wonder why a magnet-photon would follow such a non-linear path.

Are there photons that carry a charge (I know there are particles which carry a charge)?

tusenfem
2007-Jan-04, 03:07 PM
First of all, these are (if I am correct) virtual photons, not the ones you can see.

Second, why would they travel along curved path? The fieldl ines only describe the direction of the magnetic field and the number of field lines per square cm (e.g.) describes the strength of the magnetic field. It has nothing to do with the mediator of the magnetic field.

AFAIK photons cannot carry charge, I think they have the wrong spin for that, but my QM and elementary particle physics is a little rusty

hhEb09'1
2007-Jan-04, 03:17 PM
I have read that magnetism is mediated by photons which sounds okay but if this is true then why do the photons move in a curved path (http://www.manep.ch/img/photo/challenges/emfields/maglines.jpg)?Those lines of magnetic field shown in the illustrations are not the path of the photons. Sometimes, the answer is just simple. :)

Squashed
2007-Jan-04, 03:21 PM
First of all, these are (if I am correct) virtual photons, not the ones you can see.

Second, why would they travel along curved path? The fieldl ines only describe the direction of the magnetic field and the number of field lines per square cm (e.g.) describes the strength of the magnetic field. It has nothing to do with the mediator of the magnetic field.

AFAIK photons cannot carry charge, I think they have the wrong spin for that, but my QM and elementary particle physics is a little rusty

If virtual photons do transmit the force or create the field then do these virtual photons also possess momentum as regular photons do?

Whatever creates the field lines must travel a curved path or else the total momentum of the magnet would be constantly decreasing because the momentum of the photons would carry away a permanent magnets' momentum.

Momentum is the result of mass times velocity (and photons do have momentum) and so if momentum is being lost then so is mass.

The photon's path must be a closed loop starting with the magnet and ending with the magnet or else the magnet would be continously radiating mass away from itself via photons.

hhEb09'1
2007-Jan-04, 03:25 PM
the magnet would be continously radiating mass away from itself via photons.Ay (http://www-tech.mit.edu/Shakespeare/hamlet/hamlet.3.1.html), there's the rub (http://www.sciencebuddies.org/mentoring/project_ideas/Elec_p017.shtml)

Are they shuffling off the mortal coil (http://en.wikipedia.org/wiki/Coil)? :)

publius
2007-Jan-04, 03:57 PM
Squashed,

Good ol' classical EM is still the best way to think of macroscopic EM effects. The only time you need to go quantum field theory is for very small, and very high energy EM effects, like the dance of subatomic particles.

The QED picture of charges "shooting little balls" back and forth seems comforting at first -- there's no mysterious "field", no action at distance and all that stuff. But it turns out the QED picture is far weirder and far less comforting than the classical picture.

The photons we know and love when we think of light and other EM radiation is just that the quanta of a radiation field. Those are "real photons". The mediators of the forces are very different animals, and don't behave anything like real photons.

In the quantum field picture, *very roughly*, a field is sort of a "sea of virtual particles". The vaccuum is excited with energy, the energy density of the classical field picture roughly, and that energy is manifesting as these virtual photons popping in and out of existence which can locally interact with the charges.

A magnetic field has virtual photons that act differently somehow than the electric field photons, and cause the sideways, velocity dependent magnetic force.

THe magnet is not radiating away energy or momentum. It made a static field, and the QED picture of a static field is these virtual particles jumping around.

-Richard

Jeff Root
2007-Jan-04, 06:07 PM
My level of understanding is roughly the same as everyone else
who has posted so far. We each have slightly different parts of
the picture. You can put them together like a jigsaw puzzle.

As far as I have been able to discern, magnetic field lines do
not actually exist. They are a visualization tool similar to
lines of elevation on a topographic map, or, to make a closer
analogy, like lines showing the direction of surface water flow
on a hydrological map. What seems to give the notion of actual
"lines" some kind of reality is that iron particles form lines
in magnetic fields. The formation of lines is a natural result
of opposite magnetic poles attracting and like poles repelling.
Each particle attracts neighboring particles at opposite ends
(even if the particles are spherical) and does not attract near
the middle. The particles segregate into rows. The exact
locations of those rows is of no significance. The locations
are simply the result of chance positioning. However, the
spacing between rows (or lines) is an indicator of the field
strength. The stronger the field, the closer together the rows
of particles.

Photons, whether physical or virtual, are electrically neutral
and magnetically neutral, even though they consist entirely of
electric and magnetic fields! They have no electric or magnetic
charge, so they are not affected by electric or magnetic fields,
and do not follow curved paths.



If virtual photons do transmit the force or create the field then do
these virtual photons also possess momentum as regular photons do?
This is an outstandingly good question! Perhaps one reason I
think it such a good question is that I don't know the answer.

Because they are massless, photons follow different rules for
momentum and energy than massive particles follow.

Massive particles:

momentum = mass * velocity
kinetic energy = 1/2 * mass * (velocity squared)

Massless particles:

energy = Planck's constant * frequency
momentum = Planck's constant * frequency / speed of light
momentum = Planck's constant / wavelength

No energy is expended by a magnet exerting a force.

Magnetic fields are generated by the motion of electric charges.
Generally that means the orbiting of electrons in atoms. Most
atoms and molecules have no overall magnetic field because the
field generated by one electron cancels the field generated by
another electron. In ferromagnetic materials, though, the
configurations of the electrons in the atoms are right so that
the fields add together, at least in part, rather than cancelling
completely, so the magnetic field can extend well away from the
individual electrons.

Going back to virtual photons... They have only a few properties
of their own, but those properties are pretty weird. They have
whiskers dyed green, but always wear so large a fan that they
cannot be seen. Or something like that. I've read one very good
article about virtual particles, by Yuval Ne'eman. He also wrote
a book titled "The Particle Hunters" which he told me goes into
more detail, but I haven't read it-- partly because it was a
rather expensive book, and obscure enough not to be found in
secondhand bookstores. Perhaps secondhand copies can be found
on e-Bay. I have no experience with buying things online.

-- Jeff, in Minneapolis