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Project due tommorrow <-

  1. Feb 22, 2004 #1
    project due tommorrow plz help!!!<---------

    hello, i am doing a project on the standard model for my school, i have two questions,

    1)mesons are made of a quark and an anitquark, why dont they annihalate each other?

    2)what is residual strong force all about, if each proton and neutron are colour neutral then why does the nucleus stick together?

    3)also what are neutrinos, and i mean all of them tau electron and muon, and what do they do and how are they made

    4)what is the point in spin? i have read many websites about this but none of them explain how different spins make different particles act towards each other.
  2. jcsd
  3. Feb 22, 2004 #2
    Re: project due tommorrow plz help!!!<---------

    for a quark antiquark to annihilate, they have to be the same quark. for example, a red up quark can annihilate with an antired up quark. an anti up quark cannot annihilate with a down quark, so there is no annihilation.

    it is like the van der Waal's force between atoms. just because the nucleons have no overall color, does not mean that there is no force between them. their consituents have color, and so can interact.

    neutrinos usually come out of beta decay, although there are other reactions that can produce netrinos as well.

    every particle is labelled by a set of quantum numbers, one for each symmetry. the symmetries are spacetime symmetry and gauge symmetry. charge (or hypercharge), color, and weak isospin label the gauge symmetries, and spin labels how the particle respects the spacetime symmetry

    in other words, spin tells you how the particle acts under rotations and boosts.
  4. Feb 22, 2004 #3


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    There are some mesons made up of a quark and its anti-quark. These have very short lifetimes. Example pi-zero has a lifetime of 8.4x10-17 seconds, in contrast to pi-plus (or minus) which has a lifetime of 2.6x10-8 seconds.
  5. Feb 23, 2004 #4
    alrite, thanx alot guys, but now i have one more question if you dont mind.

    How does elecromagnetic force get 'mediated' by a photon, are charged atoms just constantly emmitting photons in all directions? and why when a photon hits an atom with an opposite charge is it attracted?
  6. Feb 23, 2004 #5


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    'How does elecromagnetic force get 'mediated' by a photon'

    There are several answers to this, all varying in complexity. The simple answer is all forces (we think) in nature need a mediating 'virtual' particle, to carry various quantities amongst interacting particles in order for our eqns to make sense. Typically, the governing quantum quantity (the amplitute or the sqrt root of the probability) diverges badly if there was no such *thing*.
    The actual form of this, requires a little review of quantum electro dynamics and feynmann diagrams (plenty of tutorials on the net).

    ' are charged atoms just constantly emmitting photons in all directions? '

    It depends on the system you are studying and what you mean by 'charged atoms'. Loosely speaking, yes, *excited* atoms are not stable and like to return to ground state energy. Usually they will emit a photon (but there are many other rxns possible) and return to ground state. Be careful about 'all directions', sometimes one direction is prefered depending on the kinematics. 'Charged atoms' might mean a configuration that has already lost or gained an electron, there too it might emit a photon, but it also really wants to return to a stable configuration, so it 'typically' wants to emit or absorb an electron so as to return to the energetically favored config.

    'and why when a photon hits an atom with an opposite charge is it attracted?'

    A photon does not have a charge. This question needs to be reformulated to make sense.
  7. Feb 23, 2004 #6
    i think you misunderstand what i am asking, what happens when a photon mediates a electromagnetic interaction, are the photons exchanging between the paricles, if so, how do these photons know in which directing to shoot off.
  8. Feb 23, 2004 #7


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    "i think you misunderstand what i am asking, what happens when a photon mediates a electromagnetic interaction, are the photons exchanging between the paricles, if so, how do these photons know in which directing to shoot off."

    In ANY EM interaction, exactly one photon (to first order of simplicity) will exchange between the interacting incoming particles. The consequence of this might change the nature of those particles and turn them into something else (or they could stay the same, depends what you are studying).

    In EM the photon will exchange momentum between the two particles. You can think of it naively as one gets pushed, and the other gets pulled. But thats a little wrong too.

    The problem is, these things are NOT particles like you are thinking of them. They are quantum mechanical *things* that are both like a wave and a particle.. In this context, more precisely a field.

    So its not really like billiard balls, shooting off in different directions. Now you might ask, why does the photon know (seemingly in advance) that its supposed to be attractive for like charged particles and repulsive the otherway around?

    The answer to that is a little involved, simply put, I'll have to get into writing down wave functions, explaining what plane waves are, and doing some quantum mechanics, and I don't know if thats going to help you at this stage (it might confuse)

    Suffice it to say, it comes out right, these virtual particles (the photon in this case) wiggles throughout space in just the right way so that the two interacting particles (which are also fields and wiggling throughout space) respond correctly.

    Why doesn't the two particles just interact without a mediating force carrying particle? B/c nature doesn't work that way.. Or theoretically there is no way to reconcile special relativity, quantum mechanics and electromagnetism without such a picture.

    I hope this helps, I know im being vague and elusive, but if you want me to get more involved I can.. The answer unfortunately is tricky for a beginner, and as usual can get harder and harder as it gets more precise.

    Note: Be careful when you look up Feynmann diagrams. These diagrams are NOT to be taken seriously, in so far as thinking of them as paths in classical space. They are pictoral calculational gadgets that allow us to write things simply.
  9. Feb 25, 2004 #8


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    The photons do not "know" what direction to shoot off. At any instant in time, virtual photons are coming into and out of existence around a given electron, and there is a given probability that they will couple in a photon exchange.

    As to precisely how the photon exchange affects the particles, I do not think we really know - we cannot make any real observations, so it would be pure conjecture, and common intuitive mechanistics is a bad idea on the quantum scale. All we do know is that the photons change the states of the particles in a certain way, and that mathematically, it works, and is very efficient in describing alot of apparently different things and makes very accurate predictions.
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