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My understanding of Quantum Fields [Please correct me if I'm wrong]

  1. Mar 7, 2014 #1
    I believe I understand the basics of some fields, but I am unsure about the correctness of all of my knowledge.

    - All particles are excitations of their field (If it's a fermion) and the field they mediate (If it's a boson)
    - Electron repulsion is created by the exhcnage of virtual photons that exist for short time periods.
    - Strong nuclear force is solely based on the exhcnage of muons between hadrons and gluons between quarks.
    - Weak nuclear force is esentially beta decay and the boson of it is the -W boson emitted from the decaying up quark in a proton, which then may decay into an antineutrino and electron.
    - The Higgs field gives fermions their mass, although my understanding is as basic as the cocktail party analogy.
    - Particles can "borrow" energy from the uncertainty principle, although my understanding is quite limited.
    - Electron-Positron pairs can appear out of nowhere in a vaccuum and this is the cuase of the Casimir effect.

    My questions are:
    Why does strong nuclear force ignore leptons?
    Why does the Higgs field ignore the Photon and neutrino?
    Why does the whole "Zero Point Field" energy thing happen with the electron-positron pairs appearing and then annhilating?
    And why do particles attract, I believe I understand how they repel, but can anybody explain the attraction?
  2. jcsd
  3. Mar 7, 2014 #2


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    hi themadquark! :smile:
    sorry, but no you don't understand how particles repel

    they do not repel by one of them emitting a virtual particle in the direction of the other, so that the first particle receives a kick-back, and the second one receives a shove

    the virtual particles you see in Feynman diagrams are only a mathematical trick, designed to help you calculate various coefficients of the hamiltonian

    the clue's in the name … "virtual" is the opposite of real", and these virtual particles don't (and can't) exist :wink:
  4. Mar 7, 2014 #3


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    When you look at a Feynman diagram, realize that the lines you see are not classical trajectories of particles. You're thinking that a photon that moves left to right must also push left to right, and therefore repel. Not so, the momentum exchange can go either way. Attraction/repulsion are represented by exactly the same diagram. It's the interaction at the vertices, namely the sign of the charges, that determine whether the result is an attraction or a repulsion.

    The strong force is based on gluon exchange, which binds quarks to form hadrons. At a lower energy one can describe the residual force between hadrons as due to pion (not muon) exchange.

    Beta decay mediated by W exchange is one aspect of the weak force. Other weak "neutral current" interactions are mediated by the Z boson.

    The vacuum state is an eigenstate of the Hamiltonian. That means it is time-independent. It does not fluctuate. Virtual particles do not "pop in and out of existence." The correct way to say it is that the vacuum state is a superposition of states with varying number of virtual particles.

    On a truly fundamental basis, no one knows. On a technical level, it's because the strong force affects particles possessing color charge, and leptons have zero color charge.

    Again, no one really knows. But the Higgs theory is intentionally designed this way - to give mass to the W and Z bosons and not the photon. It breaks the gauge symmetries associated with the weak forces and leaves the electromagnetic gauge symmetry unbroken.
  5. Mar 7, 2014 #4
    I saw this on a website: "The best way to approach this concept, I believe, is to forget you ever saw the word “particle” in the term. A virtual particle is not a particle at all. It refers precisely to a disturbance in a field that is not a particle. A particle is a nice, regular ripple in a field, one that can travel smoothly and effortlessly through space, like a clear tone of a bell moving through the air. A “virtual particle”, generally, is a disturbance in a field that will never be found on its own, but instead is something that is caused by the presence of other particles, often of other fields." would this be correct? I am reading a book that is sometimes a bit too vague on complicated subjects... I also read that this supposed photon exchange was actually two electrons creating disturbances in the electromagnetic field which pushed eachother apart. Nonetheless, I'm glad to know I was wrong, and that the book was. Guess that's the reason for forums.
  6. Mar 8, 2014 #5


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    hi themadquark! :smile:
    i suspect this is describing the alternative use of "virtual particle" …

    one of a pair of particles created by "borrowing" energy from nowhere and which (by the uncertainty principle) can exist only for a very short time before decaying into real particles (and "returning" the "borrowed" energy to nowhere)

    these pairs of virtual particles have a location, and have identifiable "daughter" particles

    feynman diagram virtual particles do not come in pairs, have no location, do not produce anything, and "exist" (and are a "disturbance") only in the minds of mathematicians :wink:
  7. Mar 8, 2014 #6


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    No, no, no, no, NO. This is total misinformation. Say what you like about virtual particles, but energy is an exactly, locally conserved quantity, and cannot be "borrowed"! This is a popsci idea and does not have a place on PF.
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