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We often hear that the electromagnetic wave "consists" of real photons while the electromagnetic field "consists" of virtual photons. Granted "virtual" means these particles don't exist other than as an easy way to describe the transfer of energy, momentum, and quantum numbers in interactions. It's also understood that real photons are not "little balls", but rather excitations of the quantum field of probabilities. With these disclaimers out of the way, here's my question.

When we see a trajectory of an electron deflected in the magnetic field, we can (with the above stated understanding) hypothetically describe this interaction as the electron absorbing and/or emitting virtual photons (interacting with the field created by other charges).

We also often hear that the electromagnetic wave involves alternating electric and magnetic fields. If an electron crosses the wave (which could be a low frequency), it seems that the electron would also be deflected. However, there are no virtual photons in the wave for the electron to absorb. And there is no field created by other charges for the electron to interact with. The charges that created the wave seem irrelevant, as they may be light years away. Thus no emitted virtual photons either.

All we have in the wave is real photons that cannot be fully absorbed by electrons. It seems that the only most probable process is the Compton scattering that is dramatically different from an electron simply being deflected in the magnetic or electric field with no real photons to scatter.

Is this really the case that the electromagnetic field of the wave is different in nature from the static field produced by local charges? For example, is the electron deflected differently in the static field compared to a low enough frequency wave? Is it true that the deflection in the static field produces no scattered photons while the deflection in the otherwise similar field of a slow wave results in a bunch of scattered photons? Or is this simply an error in reasoning and no such differences exist?

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  • $\begingroup$ In your views, a proton passing through one of the quadrupole at LHC would interact with the electrons in the coils. But that should then involve a delay equal to a light-round trip of the order of the meter. Considering the proton moves itself at the speed of light, it means the proton would miss of the order of a meter of magnet on entry, and should still feel the magnet a meter after moving out of it. Considering the magnets are a couple of meters long, I think you see the problem! $\endgroup$ – user154997 Oct 22 '17 at 9:41
  • $\begingroup$ @LucJ.Bourhis What is the right way to think of the case you described? $\endgroup$ – safesphere Oct 22 '17 at 18:01
  • $\begingroup$ Not sure. I just wanted to point out that the issue you have with electromagnetic waves far from charges is in fact the more general issue of how any macroscopic field emerges from QED. I only understand bits and pieces of it. $\endgroup$ – user154997 Oct 22 '17 at 23:50
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At the beginning, I’m referring only to the classical point of view and not to the quantum field theory (developed for the inner-atom interactions), for which I hope to see the answer from other people.

We often hear that the electromagnetic wave "consists" of real photons while the electromagnetic field "consists" of virtual photons.

What is an EM field? A field is something that exert a force. An EM field does not exert any force. What we have are electric fields and magnetic fields. Electric fields exert force between charged particles and magnetic fields exert force on the magnetic dipole moments of the subatomic particles. About the third case, the Lorentz force and the over induction processes see the following description of Lorentz force below.

What is a EM wave? Each photon is particle with an oscillating electric and an oscillating magnetic field component. If the sources - mostly electrons - get accelerated synchronous the emitted photons are in phase too and the resulting radio wave is really a wave with oscillating field components. The emission of photons from a thermic source one hardly can call a wave, nether you’ll be able to measure directly a wave characteristic from a thermic source.

Since the idea of field lines is the only model for electric and magnetic fields and the inner structure of these fields (field lines) is not developed, the only possibility how one can explain the interaction in these fields are virtual photons. Although, during the approach of an electron to the nucleus there are realized real photons and real photons are involved to excite electrons from the nucleus.

When we see a trajectory of an electron deflected in the magnetic field, we can (with the above stated understanding) hypothetically describe this interaction as the electron absorbing and/or emitting virtual photons (interacting with the field created by other charges).

Let me go into details. A moving - non-parallel to an external magnetic field - electron gets aligned with its magnetic dipole moment to the external field, by this gets deflected, that is an acceleration, emits a real photon (see Synchrotron radiation, by this gets disaligned, and so on until the electron exhausts its kinetic energy and comes to rest in the center of the spiral of its trajectory. This phenomenon is called the Lorentz force.

We also often hear that the electromagnetic wave involves alternating electric and magnetic fields. If an electron crosses the wave (which could be a low frequency), it seems that the electron would also be deflected.

If an electron is under the influence of EM radiation some of the photons of this radiation would interact with the electron. Simply the electron absorbs the photon and the electron gain energy. Or nothing happens because photons are indivisible particles from their emission until their absorption. (You will not be able to change the wavelength of any EM radiation without absorption and re-emitting processes.) So in general I agree with your statement below:

It seems that the only most probable process is the Compton scattering that is dramatically different from an electron simply being deflected in the magnetic or electric field with no real photons to scatter.

The next of your statements will have the answers above, if you are willing to rename “EM field” and “EM wave” by “EM radiation”. I’ve changed the expressions as follows:

Is his really the case that the EM radiation is different in nature from the static field produced by local charges? For example, is the moving electron deflected differently in the static magnetic field compared to a low enough frequency radio wave?

Yes. That is how th receiver of an antenna works.

Is it true that the deflection in the static magnetic field produces no scattered photons...

No. See the synchrotron radiation, the photons are not scattered, but simple emitted from the exhausting electron.

... while the deflection in the otherwise similar field of a low frequency radio wave results in a bunch of scattered photons?

The absorption of the photons from EM radiation is accompanied be the re-emission of photons of different wavelengths. The radio wave gets damped.

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