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B Do photons carry magnetic force?

  1. Feb 19, 2017 #1
    I understand that in the standard model, photons are the carriers of the EM force. Does that apply to a normal bar magnet? I.e., when you sprinkle iron filings near a bar magnet and the filings align along the field lines, does that mean that photons are being exchanged between the magnet and the filings?
     
  2. jcsd
  3. Feb 19, 2017 #2

    Student100

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    I don't think this question is at all easy to answer at a basic level. A static magnetic field can be thought of as being "made up of" virtual photons. Try http://www.math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html for more on virtual particles.
     
  4. Feb 20, 2017 #3
    Thanks for the link - a very interesting article, though not for the faint of heart :-)
    However, it did raise another question - if virtual photons can transfer momentum, why do we need "real" photons? I.e., what is the difference between a real photon and a virtual photon? And, of course, what is a field made of, and how do electrons know they are approaching each other and thus need to shoot out virtual photons to change each other's momentum?
    And finally, when a high school senior asks "how does a magnet work?", how would you answer them?
     
  5. Feb 21, 2017 #4
    A field is a mathematical object which assigns a value to every point in space or spacetime. The value can be a number, or it can be a vector or tensor depending on the kind of field. The electromagnetic field is a matrix at each point, but it can conveniently be rewritten as two vectors (for the electric field and the magnetic field). It's just pure math. There isn't any sort of "aether" that the field rests in.

    AFAIK, the EM field does not have to have quantized parameters (like angular momentum), but any measurements of the field will give quantized values. So, we imagine that the field is made up of lumps called photons. But the thing is, not all aspects of the EM field can assigned to the lumps. So, we invent something called virtual photons which don't follow all the rules of photons. (In particular, they can have energy that is not proportional to the momentum.) But they do follow other rules, such as some interaction cross sections, so it makes sense to treat them like particles in some respects. Both particles and virtual particles are somewhat artificial things, when the most fundamental object is the field. But the thing is, you can't measure the field directly. You can only measure the particles, and the effects that virtual particles have on particles.

    When two electrons approach each other, they don't fire off virtual photons. Rather, they interact with the EM field. And the part of the field they interact with hasn't been associated with any real photons, so we say it interacted with virtual photons. Actually, it is the electron field that interacts with the electromagnetic field.
     
  6. Feb 21, 2017 #5
    How do magnets work?
    Every atom has a bunch of electrons. Each electron has an electric field and a magnetic field around it. The electric field is completely canceled out by the positive charges in the nucleus. In most materials, the magnetic field of each electron is canceled by the magnetic field of other nearby electrons which are pointing in random different directions. But in iron and some other materials, some of the electrons like to line up with each other, and the fields add together, creating a macroscopic magnetic field.
    [Obviously, there's more to the story, but I wouldn't get into it unless they know how to ask the right questions.]
     
  7. Feb 21, 2017 #6
  8. Feb 21, 2017 #7
    Khashishi - thanks so much for your kind responses.

    Weirdo - omg, like I wasn't already confused enough...
     
  9. Feb 22, 2017 #8
    I'm sorry :cry: It was a general comment, not addressed directly to you. Explanations via virtual particles are quite good and easy to grasp (that's why they are widely used in pop-sci books), but you always have to remember that they are not completely "true" (whatever that word means).
     
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