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Relationship between electron and quark charge

  1. Feb 21, 2012 #1
    Dear Physics Forum,

    Is there any model where the electron and quark share a fundamental relationship, maybe in some string theory? I ask because it seems too much of a coincidence that two completely
    independent particles would have a simple charge relationship of 1/3:1 .

    Thanks, Mark
  2. jcsd
  3. Feb 21, 2012 #2


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    Cancellation of anomalies requires that the total charge in each generation of fermions must add up to zero. (Since they come in three colors, quarks must be counted with a weight of three.) Thus, up + down + electron + neutrino = 3(2/3) + 3(-1/3) + (-1) + 0 ≡ 0.
  4. Feb 21, 2012 #3
    Thanks for your response. I think I understand your statement - that theory requires the total charge in each generation to = zero, but I still don't understand why in fact (in an experimental sense) this is so. Why do the proton and electron have identical but opposite charges despite being made of very different things? It seems to coincidental to be a coincidence - meaning maybe they are related to each other in some more fundamental way.
  5. Feb 21, 2012 #4
    Grand unified theories can offer an explanation of this, by positing that quarks and leptons are the same at high energies but become different due to symmetry breaking at low energies. But relationships between these particles survive at low energies including simple ratios between their charges.
  6. Feb 22, 2012 #5


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    Sure enough, many GUTs do contain constraints that relate the charge of different particles. And for a time, people thought that pursuing GUTs would turn out to be fruitful. Unfortunately they also contain a prediction about the rate at which protons decay, which experiment has failed to detect. While GUTs are an appealing idea, they fail to describe the world we live in, and are not at present a serious alternative to the Standard Model.
  7. Feb 22, 2012 #6


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    Maybe you look at it the wrong way round. Charges are more fundamental than elementary particles. So there are different combinations of charges. An electron is some value of electric charge in combination with zero of baryonic charge, while a quark is a non-zero combination of both charges.
  8. Feb 22, 2012 #7
    That is intriguing. I'd like to push on it a little. I agree that charge must be more fundamental than either quarks or leptons, I think that is what I'm trying to understand - if charge is more fundamental, then how are quarks related to leptons? The idea that BillK and The Duck referred to - that there are no leptons or baryons at high energies, and these particles result from symmetry breaking at low temperatures seems a satisyfing explanation.

    There must be some issue at hand regarding charge being a component of both leptons and quarks, given the attempts to explain this through grand unified theories.
  9. Feb 22, 2012 #8
    Indeed , I would like to know that as well. A more in depth answer over why two fundamentally different particles share similar property of having a 'charge density' .
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