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Force carriers

  1. Mar 20, 2006 #1
    Can someone kindly explain to me in how can a particle carry a force?

    my interest would be strong force and gravity (discriminating against the other ones for no obvious reason :) )

    by what process a particle ("force carrier") reaching an object make that object move in space closer to some other object?

    i hope my question is stated coherently, thanx a lot for possible answer.
  2. jcsd
  3. Mar 20, 2006 #2


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    The particle is the force. In quantum mechanics, the energies are quantized, into quanta, such as the photon, or the Z particle.

    I don't understand the question.
  4. Mar 21, 2006 #3
    I'm not sure I get your question, but I'll try. My guess is that any 'movement' you speak of is due to a conservation of energy and the tendency of processes to move toward stability/ lower energies.

    Strong Force: Pions mediate the strong force; quarks have color and the color force is mediated by Gluons.
  5. Mar 21, 2006 #4


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    You essentially mean, by "throwing stuff at eachother", you could eventually understand a REPULSIVE force, but not an attractive one, because you'd be transmitting momentum along with the movement of the mediator, which is of course directed AWAY from the source, right ?

    Answer: you are right that this cannot be done by a REAL particle. But quantum theory is very strange, and you can have VIRTUAL particles. These can be pictured as intermediate states which are not observable, and where the particle carries IMAGINARY mass. We say that the virtual particle is OFF-SHELL: doesn't respect the relativistic relationship between energy, momentum and mass, which is: m^2 c^4 = - p^2 c^2 + E^2
    (which reduces, for p = 0 (particle at rest) to the famous E = mc^2). A particle that satisfies the above relationship with its rest mass m, is said to be a REAL particle. But in quantum field theory, you can also have intermediate (but not final) particle states where the momentum, and the energy are such, that the m^2 comes out negative (so that m is an imaginary number). THESE exchanges can induce ATTRACTION.

  6. Mar 21, 2006 #5
    so far what i get from the replies:

    -particle is the force (i must say i do not understand this, i know that we do not know what force is, we can only describe its effects. )

    -imaginary particles (i think my question is not that advanced, but i understand what you wrote)

    Allow me another attempt to pose the question:

    this is how i picture it, [strong force]: a proton1 and proton 2 in space. A pion oscilating between them. By what mechanism/method/process/means the oscilating pion knows the distance between the two protons, so it call pull one back if it gets too far, and how does it "pull" the proton? (what mechanism/means/process it uses to act on the proton that it moves closer to the other one?

    i bet you can see how confused is must be my notion of gravity if we assume gravitons. SO, how (what process/means) the graviton when it reaches some object make it move in space attractivelly to another object?

    IF you have some intuitive explanation of what im trying to picture?, or do i picture it wrong...?

    If this picture is totally wrong, could you explain in this way how the pion mediates the force? and how the force is "carried" by the particle?

    (i was never confused this way before i took particle physics intro), "ignorance is bliss" :rolleyes:
  7. Mar 21, 2006 #6
    I'll take quatum electrodynamics as a demonstration. Two electrons in the vacinity of one another will repel. What the standard model says is that the two exchange a virtual photon (virtual meaning it cannot be measured, loosely), propogating from one electron to the other, it doesn't matter which. The photon carries momentum away from the electron it has just left so in order for the momentum to be conserved this electron must alter its momentum, for this conservation the electron must travel away from the direction of propogation of the photon. The other electron absorbs the photon, gaining momentum, so its momentum will be altered, moving in the direction of propogation of the photon.

    It's similar with other particle interactions, the main point is about momentum exchange.

    The pion doesn't mediate the strong force by the way, that's an out-dated model (it's from an old model called Yukowa theory). The gluon mediates the strong interaction between the quarks that compose hadrons.
    Last edited: Mar 21, 2006
  8. Mar 21, 2006 #7
    thats very much what i was looking for, thanx a lot. But how that would work for attractive forces? Momentum conservation?

    Are gluons emitted from protons/neutrons? or are they just popping out from space?

    I think we do not know why virtual photons and gluons get emitted?

    thank you for clarification
  9. Mar 23, 2006 #8

    Where can I find a paper or website that says this?
  10. Mar 23, 2006 #9
    The current theory of the strong interaction is Quantum Chromodynamics (QCD) and the paritcle responsible for its mediation is a massless gauge boson know as the gluons, which is exchanged between the quarks that compose hadrons.

    The pion model is from Yukowa theory, a model of the strong force where a massive scalar particle mediates the field between hadrons. Its range is equal to the compton wavelength of this boson.
    Last edited: Mar 23, 2006
  11. Mar 23, 2006 #10


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  12. Mar 23, 2006 #11
    ty jtbell and perturbation.
  13. Mar 27, 2006 #12
    The particle exchange picture is, in a precise sense, an approximation to what is happening.
    (1) Loosely stated, this approximation applies best when the dimensionless coupling strengths associated with any given interaction are less than 1 (so that perturbation theory is applicable).
    (2) The observed interaction (force) between two particles involves an indefinite number of ``exchanged particles".

    Anyway, the largest contribution to the force between 2 particles comes from a qauntum mechanical interference of 1-particle and 0-particle exchange (and so on as more and more particle exchanges are considered). It is true that momentum is conserved at each interaction point (delivery and reception), but it is not like a game of marbles; in the example of electrodynamics, it's the effect of an ``virtual" photon's wavefunction when overlapping with a ``real" electron's wavefunction. When the probability amplitude for 0-particle, 1-particle, 2-particle, 3-particle, etc. exchange is calculated, the the electron's wavefunction is shifted in position toward or away from the other charged particle (depending on the charge of that particle). Note: A classical potential, like the electrostatic potential, is actually a ``coherent state" of an indefinite number of photons.

    To address a later post, the pion exchange model is not outdated; it is a particle in what is called an ``effective (quantum) field theory". At low enough energies, the pion exchange model is a good approximation to the effect of the strong interaction among quarks. In fact, going from QCD (quarks and gluons) to hadrons (like protons and neutrons) is a complicated business, and effective field theories, are useful.

  14. Mar 27, 2006 #13
    Yes, I just meant it's outdated in view of the standard model of quantum chromodynamic interactions. It's still used as an approximation in the SM, but I know what I meant.
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