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Particle charges?

  1. Oct 21, 2011 #1
    Hello, im Pauline!, this is my very first post, im new to this, ive only been teaching my self this, so please dont be mean! my question is this: ive noticed some particles have a fraction of a charge for example, a up quark has a 2/3 charge and a down quark has -1/3 charge. how can there be a fraction of a charge? what happend to it being either there is, or theres not a charge?
     
    Last edited: Oct 21, 2011
  2. jcsd
  3. Oct 21, 2011 #2
    Can you stand a really long post Pauline?

    I could link you to it at another place, but that will take the fun away. It's all a matter of matter, and how physicists developed their understanding of it. And it's also a lot of quotes to which I don't have the sources anymore. I wrote this first when I didn't have any net I think, and for my own pleasure.

    "Using magnetic fields, though, we have managed to trap a small amount of anti-matter. Indeed, in 1995, scientists at the CERN accelerator in Switzerland made nine anti-hydrogen atoms. How long would it take to make three grams (the mass of a penny) of anti-hydrogen? CERN makes about ten million anti-protons in a second. If CERN could keep generating anti-protons at that rate non-stop, they could make three grams in about six billion years. Fermilab could do the job in a tenth the time, since they make 100 million anti-protons per second. Still, six hundred million years is a long time to make three grams of mass.

    Thus, we don't make much mass in particle accelerators, because it takes too much energy. The lights of Chicago may not actually dim when they run the Chicago's big accelerator at Fermilab, but the accelerator is a "significant drain" on the electricity grid, says Koji Mukai of NASA'S Goddard's Space Center. Consider how much energy is in a kilogram (2.2 lbs) of water. If we could convert that mass into the equivalent energy, we'd have enough energy to drive a car for about 100,000 years without stopping, say CERN scientists."

    "The mass of a proton is about 938 MeV. It consists of three quarks, each of which have a mass on the order of 3 MeV (more or less, not very accurate.) There is a huge discrepancy between 938 and 9. The remainder of the mass of the proton is the potential and kinetic energy of the gluons holding the whole thing together. The correct vision of a proton is a little subatomic gluonic lightning storm, buffeting three nearly insignificant quarks. "

    'Virtual particles'? what is virtual particles, and gluons? And leptons?
    And do they have anything to do with matter?

    Let's take a look. First of all, when discussing those things we have a principle. It's a really important one too. It's called Heisenberg's uncertainty principle and states that it won't, never ever, be possible to measure a simultaneous definite value for both the position and momentum of a particle. That idea have since then been widened to count in a lot of other properties/combinations too. Here's how he looked at his idea.

    "For example, there is a passage (Heisenberg, 1927, p. 197), where he discusses the idea that, behind our observational data, there might still exist a hidden reality in which quantum systems have definite values for position and momentum, unaffected by the uncertainty relations. He emphatically dismisses this conception as an unfruitful and meaningless speculation, because, as he says, the aim of physics is only to describe observable data.

    Similarly, in the Chicago Lectures (Heisenberg 1930, p. 11), he warns against the fact that the human language permits the utterance of statements which have no empirical content at all, but nevertheless produce a picture in our imagination. He notes, "One should be especially careful in using the words ‘reality’, ‘actually’, etc., since these words very often lead to statements of the type just mentioned." So, Heisenberg also endorsed an interpretation of his relations as rejecting a reality in which particles have simultaneous definite values for position and momentum." So now you know why I so often put 'reality' in brackets. I have that same feeling as he seemed to have. Reality is a really weird idea :)

    This principle is what allows 'virtual particles' to influence 'real particles'. So how many fundamental 'real particles' do we have that make up matter? Twelve, as far as I understand.

    "In the modern theory, known as the Standard Model there are 12 fundamental matter particle types and their corresponding antiparticles. The matter particles divide into two classes: quarks and leptons. There are six particles of each class and six corresponding antiparticles. In addition, there are gluons, photons, and W and Z bosons, the force carrier particles that are responsible for strong, electromagnetic, and weak interactions respectively.

    These force carriers are also fundamental particles. All we know is that quarks and leptons are smaller than 10^-19 meters in radius. As far as we can tell, they have no internal structure or even any size. It is possible that future evidence will, once again, show this understanding to be an illusion and demonstrate that there is substructure within the particles that we now view as fundamental."

    "Leptons are spin-1⁄2 particles. The spin-statistics theorem thus implies that they are fermions and thus that they are subject to the Pauli exclusion principle; no two leptons of the same species can be in exactly the same state at the same time. Furthermore, it means that a lepton can have only two possible spin states, namely up or down.#

    And

    "Leptons have various intrinsic properties, including electric charge, spin, and mass. Unlike quarks however, leptons are not subject to the strong interaction, but they are subject to the other three fundamental interactions: gravitation, electromagnetism (excluding neutrinos, which are electrically neutral), and the weak interaction.

    For every lepton flavor there is a corresponding type of antiparticle, known as antilepton, that differs from the lepton only in that some of its properties have equal magnitude but opposite sign. However, according to certain theories, neutrinos may be their own antiparticle, but it is not currently known whether this is the case or not.

    The first charged lepton, the electron, was theorized in the mid-19th century by several scientists and was discovered in 1897 by J. J. Thomson. The next lepton to be observed was the muon, discovered by Carl D. Anderson in 1936, but it was erroneously classified as a meson at the time. After investigation, it was realized that the muon did not have the expected properties of a meson, but rather behaved like an electron, only with higher mass.

    It took until 1947 for the concept of "leptons" as a family of particle to be proposed. The first neutrino, the electron neutrino, was proposed by Wolfgang Pauli in 1930 to explain certain characteristics of beta decay. It was first observed in the Cowan–Reines neutrino experiment conducted by Clyde Cowan and Frederick Reines in 1956.The muon neutrino was discovered in 1962 by Leon M. Lederman, Melvin Schwartz and Jack Steinberger, and the tau discovered between 1974 and 1977 by Martin Lewis Perl and his colleagues from the Stanford Linear Accelerator Center and Lawrence Berkeley National Laboratory. The tau neutrino remained elusive until July 2000, when the DONUT collaboration from Fermilab announced its discovery.

    Leptons are an important part of the Standard Model. Electrons are one of the components of atoms, alongside protons and neutrons. Exotic atoms with muons and tauons instead of electrons can also be synthesized, as well as lepton–antilepton particles such as positronium."

    So what happened to those protons and neutrons then? Wasn't they the ones making up the atoms nucleus? (And how many atoms do we have? Ninety two, naturally occurring). After all, isn't all matter made out of atoms, in where we find protons and neutrons?

    Well, according to the standard theory those protons and neutrons in 'reality' are made out of quarks. The proton has two 'up quarks' and one 'down quark', the neutron has two 'down quarks' and one 'up quark'. The proton also carries an electrical charge, which mean that at least some of the quarks should be 'electrically charged'. As the neutron, as it sounds, is 'neutral', having no charge, also is built of the same quarks as the proton there had to be something else making them differ, and that was how you combined those quarks.

    Before the discovery of quarks all charges was thought to be multiples of the proton charge but finding that it was made of quarks the protons charge had to be split up. The standard model describe three basic amounts for a charge. + 2/3, −1/3, and −1. When it comes to electrons they are similar to the muon and the tau, having the same electrical charge and acting similarly, although the electron having a different mass and that the muon and tau could decay into other particles, whereas the electron was stable and unchanging.

    So .. "Everything around us is made of matter particles. These occur in two basic types called quarks and leptons. Each group consists of six particles, which are related in pairs, or ‘generations’. The lightest and most stable particles make up the first generation, whereas the heavier and less stable particles belong to the second and third generations. All stable matter in the Universe is made from particles that belong to the first generation; any heavier particles quickly decay to the next most stable level.

    The six quarks are paired in the three generations – the 'up quark' and the 'down quark' form the first generation, followed by the 'charm quark' and 'strange quark', then the 'top quark' and 'bottom quark'. The six leptons are similarly arranged in three generations – the 'electron' and the 'electron-neutrino', the 'muon' and the 'muon-neutrino', and the 'tau' and the 'tau-neutrino'. The electron, the muon and the tau all have an electric charge and a mass, whereas the neutrinos are electrically neutral with very little mass."
     
  4. Oct 21, 2011 #3
    Quarks are never free. They always come in pairs (meson) or triples (baryon) that end up with total charge that are integer multiples of e. The other elementary particles, the leptons, always carrier integer charge.
     
  5. Oct 21, 2011 #4
    Wow! not only you answered my question to such detail, you also put it in words i can understand! this must have took you forever! thank you!
     
  6. Oct 21, 2011 #5

    DaveC426913

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    no... 35 minutes. :wink:
     
  7. Oct 21, 2011 #6
    Glad you liked it Pauline. It is a little long, but considering its matter of gravity.
    It's surprisingly short.

    ahem :)
     
  8. Oct 22, 2011 #7

    arivero

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    Also, in some theories, a part of the electric charge is equal to one half of the sum of "lepton number" and "barion number". In particular, quarks have lepton number=0 and barion number=1/3, because three of them make a barion.

    So, for a quark, an electric charge of 1/6 comes from its barion number, for electrons and neutrinos a electric chargue of -1/2 comes from the lepton number. If you substract these charges, everything becomes more symmetric: electron and neutrino have a charge -1/2 and +1/2 (that becomes -1 and 0 when you add the -1/2) and up and down have charges +1/2 and -1/2 (that becomes +2/3 and -1/3 when you add 1/6).

    (It is possible to be a bit fuzzy with the conventions here; you could also choose (B-L)/2 instead of (B+L)/2 and then put lepton number of the electron = 1 instead of -1)
     
  9. Oct 22, 2011 #8
    Another perspective on the original question: why shouldn't there be a fraction of a charge? A lot of progress in physics has required that people abandon assumptions that they didn't know they were assuming.

    The charges of the quarks sound funny partly because the charge on the electron was made into the basic unit of charge. If for some reason the electron's charge had been quantified as "-3" (rather than as "-1"), then the up quark would have a charge of "+2" and the down quark would have a charge of "-1".

    There is surely a deeper reason for the pattern of charges that exist in the standard model, though we don't know what it is. In fact there are lots of patterns in the standard model (e.g. "accidental global symmetries" and "anomaly cancellations") which must have a deeper explanation. I've never studied it, but can't the pattern of charges be explained by supposing that a single generation of the standard model derives from a single spinor representation of SO(10)?
     
  10. Oct 23, 2011 #9
    Yes, this stems from electroweak symmetry breaking. It's a slightly simplified view, because the weak force only acts on particles with left-handed chirality, so the weak charge numbers for right-handed particles are different - though they add up to the same in electric charge terms. Basically

    Q = I3W + YW/2​

    where Q is the electric charge, I3W is the "weak isospin", and YW the "weak hypercharge".

    For left-handed particles, I3W = +1/2 for uL and νL, and -1/2 for dL and eL. YW is +1/3 for the quarks and -1 for the leptons. So, for example, the electric charge on eL is (-1/2) + (-1)/2 = -1. Here, the baryon and lepton numbers simply equal the weak hypercharges, with the sign reversed in the case of the leptons.

    For right-handed particles, I3W = 0 for everything so YW is +4/3 for uR, -2/3 for dR, -2 for eR and 0 for νR. Bizarre, but that's how it is.

    Note that for the neutrinos, the weak isospin and hypercharge of νL combine to give a net electric charge of zero, and νR has no charges of any kind so its only (known!) interaction is with the Higgs field.
     
  11. Oct 23, 2011 #10

    arivero

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    Adrian, that was the point of my answer; that if instead of Weinberg Salam, you upgrade to something with R and L isospins and Baryon minus Lepton, then the values are not bizarre anymore.
     
  12. Oct 24, 2011 #11
    Yes, there could indeed be a "super-weak" force that only acts on right-handed particles, but whose symmetry has also been broken by a hotter-scale super-Higgs mechanism, with the YW U(1) symmetry that we take as the starting point for WS actually being a combination of an I3SW and a YSW!
     
  13. Oct 25, 2011 #12

    naima

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    Hi all
    Are charge always things with numeric values ? I think of the color charges.
    I also read charges were operators (look at central charge on wiki).
    Could you clarify ?
     
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