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Force Carriers

  1. Jan 29, 2005 #1
    How do you observe a carrier particle? How do you know that particles carry the strong force or weak force?
     
  2. jcsd
  3. Jan 29, 2005 #2
    Virtual particles are the carrier particles, I believe they don't exist long enough to be detected (Heisenberg's Uncertainty Principle), and forces carriers is an altenative model to force fields.

    They exist in order to conserve various quantum properties in reactions.

    For example a weak force interaction.

    An neutron ---> proton + electron + electron anti-neutrino.

    However one of the quarks involved changes from down to up, a lower energy state, and a Bose particle, W is released. This W particle will decay to electron + antineutrino afterwards.
    It is a good model to explain such interactions.

    As for actual observation, I am not sure, but I think they don't exist long enough to be directly observed.
     
  4. Jan 30, 2005 #3
    uhhh what about photons? Virtual photons (or virtual anything) do pop on and off but these are usually spontaneous vacuum fluctuations. Photons have infinite range, but virtual photons have very very short lifetimes. So in this case, magnets should not work because the photons "die out" before it reaches an object which experiences its magnetic force.

    The weak interaction is indeed a short-range force because its bosons are extremely humongous and decay very quickly. Gluons, photons and the weak gauge bosons have been observed in accelerators.

    By seeing what they react with? Gluons usually get together with stable quarks after the collision and stay that way. If a particle causes another known particle to change its flavour (usually can be seen when there is a sudden dissapearance or appearence of ionization tracks), then it can be inferred that it is a weak gauge boson. Of course there may be other reasons when ionization tracks suddenly appear or dissapear.

    I may be wrong on this one though.
     
    Last edited: Jan 30, 2005
  5. Jan 30, 2005 #4
    Guys, beware with what you are saying here. Virtual particles fill up the vaccuum in field theories and they exist as long as you talk about a vaccuum. So they are always there. However (and this is where you are a bit off) virtual particles can become real for a very short time, if enough energy is available in order to give them a "valid" reason to exist. This indeed works via the Heisenberg-uncertainty principle and E=mc². However not all force carriers are virtual and not all virtual particles are force carriers. For example in QED, an electron can suddenly appear and then dissappear shortly after. This initially virtual electron became real for a very short while and then returned to its "virtual" state of existence. The energy comes from interactions between charged particles that take place inside this vaccuum. The real charged particles indeed interact via the exchange of virtual photons, that is correct.

    Force carriers (gluons, photons, etc) can certainly be detected directly when they are real. If they are virtual, they can only be detected indirectly. I mean, they can only be seen via their influence on real matter-particles (ie : via the interactions that they mediate between such real matter particles)

    Read my journal for more info

    regards
    marln
     
  6. Jan 31, 2005 #5
    ^ hey marlon, so are you saying that virtual force carriers exist all the time, and where's there's an interaction, the virtual force carriers get the energy from the interaction to become "real"? Then after this boson is exchanged, it becomes virtual again?
     
  7. Jan 31, 2005 #6
    No, that is not what i meant. The vaccuum is filled up with virtual particles all the time. When enough energy is available, these virtual particles can become real for a very short while.

    When for example two charged particles interact via the exchange of virtual photons, these photons are virtual during the entire interaction. Virtual in this context really means that these photons cannot be the end-product of some interaction, they are merely an intermediate stage of the interaction.

    When such an interaction is taking place in the vaccuum, there is energy from this interaction that can be used to make on of the constituent virtual particles of the vaccuum real. This explains why in QED, an electron can suddenly appear and the disappear at a random location.

    regards
    marlon
     
  8. Jan 31, 2005 #7
    hmmm, so do these particles normally get the energy from an interaction??

    I don't quite understand this part, can you explain it to me again?

    Thanks alot.....
     
  9. Feb 1, 2005 #8
    Look i urge you to read my journal. i have written several texts now on these topics. I suggest entries like "on confined species" or "string theory part 1" or "pions and gluons"

    regards
    marlon

    and yes, when some interaction (eg between charged particles) is going on, it is not that difficult to envision the fact that the vaccuum (filled with virtual particles) will be influenced by this interaction, right ? In QFT, this is expressed by the socalled vaccuum-polarization. This is the self-energy of the photon-propagator and it really expresses that the vaccuum has been replaced by some "virtual" dielectric that has an influence on interactions going on in this vaccuum.

    regards
    marlon
     
  10. Feb 1, 2005 #9
    :rofl: Brand new guy, so just nod:)
     
  11. Feb 1, 2005 #10

    Wellcome to the jungle...

    regards
    marlon

    err PF that is
     
  12. Feb 2, 2005 #11
    Hi miso,

    Maybe a little clarification in other words will help you out : Like I said virtual particles are an intermediate stage of an interaction between elementary particles in QFT. They are a tool to describe these interactions, yet their existence can be proven with experiments : see the Casimir-effect.

    The QFT-vaccuum is not empty because the lowest possible energy cannot be zero (you know, because of Heisenberg uncertainty : zero energy implies infinite spread of position). thus via E=mc², the vaccuum is filled with these socalled virtual particles. It was Dirac who came up with the idea that the vacuum was filled with virtual positron and electron pairs (the reason that they were pairs has to do with conservation laws like that of electrical charge). You can break up such a pair and make the particles real when you have enough energy coming from some interaction between two charged particles that were placed inside the vaccuum.

    Indeed, virtual particles and their fields (remember that position is not a well defined quantity in QFT ) do not exist for ever. In theory this can be proven by the fact that the socalled number-operator of such particles does NOT commute with the hamiltonian (it is not a conserved quantity). Therefore we can call these virtual particles : vacuum-fluctuations.

    Problem is that if the vaccuum has a gazzillion virtual particles (suppose each such particle is represented by one LOWEST energy quantum of a harmonic oscillator), the energy of this vaccuum (the zero-point energy) becomes infinite. This will lead to difficulties with curvature of space time and this effect also predicts a very large cosmological constant. This is however against observations. A possible way out is to renormalize this positive infinity by saying that each such virtual electron and positron has a supersymmetric counterpart with opposite energy. This leads to the situation that the positive infinity is "eliminated" (it is not really gone , though) by a negative infinity coming from the supersymmetry.

    regards
    marlon

    ps : i have added a nice article in my journal about the link between vacuum fluctuations and dark energy. just goto the "on virtual particles and dark energy entry"
     
    Last edited: Feb 3, 2005
  13. Feb 3, 2005 #12
    Problem is that if the vaccuum has a gazzillion virtual particles (suppose each such particle is represented by one LOWEST energy quantum of a harmonic oscillator), the energy of this vaccuum (the zero-point energy) becomes infinite. This will lead to difficulties with curvature of space time and this effect also predicts a very large cosmological constant

    So what if ZP energy is infinite and gravity is the difference between the attractive forces of two or more bodies, how do we know what the true gravitational force of one body really is, or what the true gravitational constant is; when all we are really measuring is the quantities above an unknown base quantity.
     
  14. Feb 3, 2005 #13

    What do you mean : the true gravitational force. Are you forgetting about General Relativity. And what is this "true gravitational constant" ???

    marlon

    ps : here is a nice article on vacuum-energy-density and dark energy-density. The first cannot exceed the latter and this article describes the measured experimental data :C Beck and M Mackey 2004 arXiv.org/abs/astro-ph/0406504
     
    Last edited: Feb 3, 2005
  15. Feb 4, 2005 #14

    Astronuc

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    Staff: Mentor

    I believe it is safe to say that 'virtual' particles do not just exist (in a vacuum of course) prior to or after a particular interaction between 'real' particles. Hence they exist for a very short time (< 1E-6 sec, perhaps much less). In other words, they are not waiting around to interact with 'real' particles.
     
  16. Mar 10, 2005 #15
    What do you mean : the true gravitational force. Are you forgetting about General Relativity. And what is this "true gravitational constant" ???

    In one of the general interest science articles published recently, it was a reported that someone had proposed that;

    What we observe as gravity is always the relationship between two masses, therefore we have no way of observing the base level; the currently accepted level being purely theoretical.

    According to the article someone is proposing that gravity is much stronger than current theory sugests; but I have not kept a note of the details.
     
    Last edited by a moderator: Mar 10, 2005
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