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Why do all elementary particles spin?

  1. Feb 1, 2009 #1
    Any ideas here? For that matter, why should the spin of each particle always be the same? Or can we do no better than to say they do, and that its spin is a fundamental property of what defines each particle?
  2. jcsd
  3. Feb 1, 2009 #2


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    It's a fundamental part of the definition of each particle species. It's a consequence of the postulates of QM and the rotational invariance of space. The rotational invariance is usually imposed by postulating that spacetime is Minkowski space (although this isn't necessary if we're only interested in the spin; we could consider the non-relativistic spacetime as well). This forces us to consider representations of the covering group of the Poincaré group on Hilbert spaces, and it turns out that it's natural to identify each irreducible representation with a possible particle species. ("Possible" because not all of them correspond to particles that exist in the real world). Within each irreducible representation, the spin is the same.

    Chapter 2 of Weinberg's QFT book describes this stuff pretty well, but it's not really easy. I still don't fully understand this stuff myself.
  4. Feb 2, 2009 #3
    Thanks for the references. Not sure I fully understand, but the general drift of your argument is comprehensible enough to give me something to reference.
  5. Feb 7, 2009 #4
    Figure that out, and you'll win a Nobel Prize. :smile:
  6. Feb 27, 2009 #5
    "Why do all elementary particles spin?"

    If only we knew. The property we call spin is derived from some very deep interaction with spacetime at the quantum level (for which we currently have no physical theory). By the time this interaction is transmitted all the way back up to our macroscopic version of spacetime (for which we do have a physical theory) we see an effect we call "spin".

    Spin is a reminder of just how much we don't know.
  7. Feb 28, 2009 #6


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    I'll refer y'all to the already ongoing thread on this subject.

    abbotsys: I thought Dirac had explained spin pretty well?
  8. Feb 28, 2009 #7
    The pion, otherwise called the pi meson, has no spin. The pion is the lightest particle with no spin. There are actually 3 pions, one with zero charge, and two with charge equal to + or - 1 electron (or proton) charge. Their masses are about 1/7 the mass of a proton. They are all unstable (radioactive), and decay into two photons (pi zero) or a muon and a neutrino (pi plus and pi minus).
  9. Feb 28, 2009 #8


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    but pion is no elementary particle..
  10. Feb 28, 2009 #9
    The spin is a symmetry property under rotation. The pion does have spin, it just happens to be zero, and it's quite an important property !
  11. Feb 28, 2009 #10
    . What about the pi zero, which is its own antiparticle?
  12. Feb 28, 2009 #11
    The pion isospin triplet has one given Lorentz spin of course, the same for the 3 members of the isotriplet. They are pseudoscalars. I don't understand the subtlety here.
  13. Mar 1, 2009 #12
    The spin is basically a "number" that labels the way the mathematical entities describing a particle (function, vector field,...) transform under a "rotation"..

    However to understand why half-integer spins are also possible one has to know a bit of group theory (the fact that we must use SU(2) as a rotational group and not the normal SO(3) )
  14. Mar 2, 2009 #13
    The lowest mass particle that has isospin I= 0 is the eta particle at mass = 547 MeV. Does the eta have spin, even if its isospin equals zero? Or is isospin = 0 equivalent to having no spin? Is the eta particle an elementary particle?
  15. Mar 2, 2009 #14


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    it is a pseudoscalar meson, isosinglet state.

    The physical eta meson is a mixture of the octet and singlet eta-meson.

    the singlet eta meson has quark-wavefunction:
    (1/sqrt 3) (u,u-bar + d,d-bar + s,sbar)

    The octed eta meson has quark-wavefunction:
    (1/sqrt 6) (u,u-bar + d,d-bar - 2s,sbar)

    For more info, there is a great chapter about it in the Particle Data group.

    So no, the eta is a meson, a composite particle.

    And don't mix isospin with spin....
  16. Mar 7, 2009 #15
    are all these things figured out in bubble chambers?and by spin are physicists refering to the spiraling patterns left in bubble chambers? lots of questions because im uneducated
  17. Mar 7, 2009 #16


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    NO. LOL.

    The reason for why particle make spirals is that charge particles spiral in electric and magnetic fields, it is called the Lorentz Force.

    Spin is related to transformation properties under rotation groups.
  18. Mar 8, 2009 #17
    dunno half of what you just said but hmm ok. thanks
  19. Mar 8, 2009 #18


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  20. Mar 9, 2009 #19
    spin is the angular momentum when the particle interacts with other matter.
  21. Mar 9, 2009 #20


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    no not accurate, spin is the rotation transformation property. An non interacting particle also have spin.
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