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EM force carriers

  1. Jan 28, 2013 #1
    I got confused now while thinking of electro-magnetic force.
    Magnetic force is caused by electron spins.
    Electric current is a directed flow of electrons.
    The carriers of the electro-magnetic force are photons, but they don't cause any of the forces I wrote upwards. Why are they the carriers, and not the electrons? Why is the force called electro-magnetic?
  2. jcsd
  3. Jan 28, 2013 #2


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    Charges lead to forces, but that forces are transmitted via electromagnetic fields (in a field-picture) or photons (in a particle-picture).
    The distinction between "magnetic" and "electric" is arbitrary and depends on the reference frame. It is "the electromagnetic interaction".
  4. Jan 28, 2013 #3


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    You are mixing together several disciplines at several different levels, so it is, indeed, confusing.

    In classical electrodynamics, the source of electric field is the electric charge. Source of magnetic field is the electric current. There are no spins in classical electrodynamics. You can have magnetic dipoles, which is what spins ultimately are, but in classical electrodynamics they are represented as tiny current loops.

    So why is it an electromagnetic force and not just electric and magnetic? Well, suppose you placed an electron. It's not moving, so it produces only electric field. Now you start moving. Electron is now moving relative to you, so it produces magnetic field. How did it know to produce magnetic field when you started moving? Well, it doesn't. Electric and magnetic fields are aspect of the same electromagnetic field that manifest slightly differently depending on your choice of reference frame.

    This understanding is what led to discovery of Special Relativity, which ultimately explained electrodynamics as a relativistic theory. When described in terms of Special Relativity, one usually talks about 4-vector potential as the source of the electromagnetic forces. The source of the 4-vector potential is the 4-current, which includes the information for both static and moving charges for whichever frame is relevant.

    This is where we jump off into Quantum Mechanics. In Quantum Mechanics, the charged particles travel as waves, whose behavior is influenced by the 4-potential, giving you electromagnetic forces. The spin is a quantum phenomenon. Particles are point objects, and yet, some of them behave almost as if they were tiny current loops. They have angular momentum and they have a magnetic moment. Working together, they can produce quite significant magnetic forces, such as in the magnet.

    Now, so far, in all of the above, the field is just the field. You can have electromagnetic waves, which is light or radio waves, but there is no concept of "photon" in any of that. The field itself is the force carrier. When you apply principles of Quantum Mechanics to the electromagnetic field itself, however, you end up with Quantum Electrodynamics. In it, the field itself is represented as particle exchange. These particles are the photons. The significance of such an interpretation comes from the fact that when you make measurements on the electromagnetic wave, it behaves as if its energy is quantized. You can only absorb or emit a discrete number of these quanta. In other words, only an integer number of photons can be emitted or absorbed.

    Of course, superposition makes this distinction moot at any time except measurement. The system can be in a superposition state of having and not having emitted a photon, which is really no different in any way from having emitted half of a photon. And that "half of a photon" can be absorbed by something else, which will receive "half" of the energy, itself going to a superposition. But when you make a measurement, you'll find either a state of whole photon emitted or no photon emitted.

    What about force carriers, then? Well, it's not just electromagnetic waves that are quantized, it's the field. The electric and magnetic fields between particles are now described as photon exchange. Now, these photons are virtual, which gives them many odd properties. But they essentially behave the same way. Any interaction can be recorded as superposition of all possible exchanges of integer number of virtual photons. Hence photon is named as the force carrier particle.

    There are many mathematical advantages of such a model. Strictly speaking, you do not have to quantize your interaction field. The problem is, it's almost impossible to do any real computations if you don't. And in terms of interpretation, it doesn't matter. The same reality is being described either way. So it ultimately doesn't matter if you want to think of virtual photons carrying interaction force as underlying reality or just a mathematical trick to make our lives easier. There is no real distinction in terms of any observation you could make.
  5. Jan 28, 2013 #4
    Thanks for explaining this, but I didn't understand it well, yet. For example:
    There are a positively and negatively charged particles with same size, and the distance between them at a certain moment is the size of these particles. Now, the particles start to attract each other until they collide. Before the collision, while they are approaching each other because of the EM force, what are the field carriers, photons, doing? Do they cause the particles to attract each other? If yes, how?
  6. Jan 28, 2013 #5


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    What do you mean with size here? All known elementary particles are point-like, or so small that no size could be measured yet.

    In quantum field theory, you can describe the attraction as exchange of virtual photons. This is just a description - physics cannot answer "how", it can just provide models which give the correct predictions for experiments.
  7. Jan 28, 2013 #6
    magnetic force, macroscopically, is actually a relativistic effect resulting from electrostatics.

    and yes, electric current is indeed caused by electromagnetic fields. you assert an electromagnetic field over a piece of conductor. charges flow until there is a pileup of enough electrons on one side to repel further electron movement towards that side. You allow the flow to continue by completing the circuit.

    the photon picture is not so useful when talking about forces. the photon picture is more useful when talking about molecular spectroscopy because without it you cannot explain things like rotational spectra selection rules. however it is useless when talking about currents and magnets.
  8. Jan 29, 2013 #7
    Well I believe it becomes useful when someone has reached certain level of knowledge and then he asks why the electrical phenomenon in a wire is almoust instant compared to the slow movement of actual electron and here comes the picture of the moving electron creating an em field and photon as a carrier of that field which in turn excites further electrons and keeps the current flow at a almoust near to "c" speed in non vacuum conditions hence the fast effects of current through a wire.
    Is there any other way of telling why does electric current is fast compared to the particles that make up one?

    @Chemist@ They are written together as electro-magnetic because they actually are bound , you said electric current is directed flow of electrons which is correct but then again imagine that directed flow without photons of the Em field , you do realize that it would be a totally different picture.
  9. Jan 29, 2013 #8
    Okay, so virtual photons are emitted between the particles in electric force. What about the magnetic force at microscopic scale? It is caused by electron spins, but where are the photons here?
  10. Jan 29, 2013 #9


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    This is incorrect. I can get magnetic force from moving charges. Where is the "spin" there"?

    I think that you are trying to get a lesson in QED via hand-waving argument. While this may be possible to some extent, it is impossible to have this explained accurately enough at this level.

  11. Jan 29, 2013 #10
    Well I'm not the top expert on this case but I'll try as best as I can.
    In both cases we can speak of photons both in the EM field and in a magnetic field , like the one caused by a ordinary ferrite magnet or any other.
    But the distinction between the "real" photons and virtual ones is because the real ones in the em field carry energy , an actual energy that you can measure like a radio wave or induced EMF in a winding or coil.
    The magnetic field from a magnet which isn't an electromagnet but a permanent one has virtual photons in it's magnetic field because the field just is like a phenomenon but it does no actual work , otherwise we could extract energy from it in the case of real photons because they arise only when there is a moving charge.
    But the magnet doesn't poses any time varying field the field of it just is a property of it's electrons like you said.
    So even though the photons are in both cases in the case of magnetic field they don't do any actual work as there is no work done by the magnet either.By the way this is the point were many of the perpetual motion activists mix up things and think of magnets like energy sources WHICH THEY ARE NOT.

    Zapper was faster while I was writing my answer so that now complicates the question further.
    @ZapperZ but isn't, that a moving charge such as electric in a conductor creates magnetic field because of the flow of electrons but in a permanent magnet the field is created from the electron spin.?So his not incorrect , he just didn't tell the whole picture, merely a part of it.
    Last edited: Jan 29, 2013
  12. Jan 29, 2013 #11
    So, the electrons exhibit some properties (charge and spin), that trigger the EM field to cause virtual photons to appear and they produce the force. The electric force is composed of attractive and repulsion forces. As these are caused by virtual photons, how do they differ in behavior in these two cases?
  13. Jan 29, 2013 #12


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    That is just a sign in the coupling constants of photons to charged particles - the sign of those charges.
  14. Jan 29, 2013 #13
    But what causes that difference?
  15. Jan 29, 2013 #14


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    That is beyond the scope of physics. Physics cannot answer "why" our laws of physics are like that. It can just give some ways to describe them, and to predict the result of experiments. Quantum electrodynamics can do this very well.
  16. Jan 29, 2013 #15
    Just to add to mfb , physics is like paper and ink , physics doesn't deal with the meaning of words written on that paper, just the ink and paper.
  17. Jan 29, 2013 #16
    I should expect something like this. It could be maybe all random, as our universe should be one among infinitive.
    Okay, I want to thank everyone for explaining these things to me. You are a friendly community and I am looking forward for more discussions.
  18. Jan 29, 2013 #17
    Thank you for your good feedback on this forum but I must point out that all is not that random at all I think people got into this misunderstanding because in particle physics for example in particle accelerators they never know the outcome precisely that's why they use the collected data to get the actual numbers and types of particles that came out of a certain two particle, mostly proton i guess collision. , also there is no evidence on infinite number of universes,and the "multiverse" theory as much as I know is not widely accepted .
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