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Particle spin

  1. Dec 5, 2011 #1
    Why do all particles spin? What is the force that causes this spin?
  2. jcsd
  3. Dec 5, 2011 #2


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    http://en.wikipedia.org/wiki/Spin_(physics [Broken])

    The above may help. Spin is an intrinsic property of elementary particles. There is no force involved to get it going.
    Last edited by a moderator: May 5, 2017
  4. Dec 5, 2011 #3
    Would it not be helpful in understanding quantum physics to know why paricles spin and what forces are at work to start the spin and to maintain it?
  5. Dec 5, 2011 #4
    Hi Charles,
    "why" questions in physics often have no answer....yet....

    and yours is no exception. It's like asking: "Why the electron?" or "why does electron mass have the value it does?"

    We don't know why particles in this universe exhibit the observed characteristics we measure. We don't even know why the elementary particles we observe
    exist, nor the four fundamental forces. What we observe about particles is summarized in the Standard Model of Particle Physics. We don't even know enough about gravity to have included it there yet. The model helps explain observations, that is, experimental results, but not their fundamental origins.

    See some aspects of spin here:

    And even more fundamental question is "why are we able to explain so many phenomena with man made mathematical models"...and spin is an example.
  6. Dec 5, 2011 #5
    Spin is just added into quantum mechanics as an ad hoc hypothesis, to explain certain experimental results. Quantum field theory correctly describes what spin is. It is something you get when you marry quantum mechanics with special relativity
  7. Dec 5, 2011 #6
    Are there any fundemental facts that we know for sure? Is all quantum physics conjecture?
  8. Dec 5, 2011 #7
    Yes, we know that spin is a natural consequence of any Lorentz invariant quantum theory (relativistic quantum mechanics). But in non-relativistic quantum mechanics, it is put in by hand so to say
  9. Dec 6, 2011 #8


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    That is not true. Also in the nonrelativistic case one can derive the spin magnetic moment and Landé factor g=2. See e.g. "Nonrel. Particles and Wave equations" of Levy-Leblond.
  10. Dec 6, 2011 #9
    I don't think one can derive a multi-component wave function in non-rel. QM. And deriving a Landé factor of g=2 in non-rel QM already relies on the assumption that you have a spin 1/2 particle. That is, it realies on the assumption that the interacting part of the Hamiltionian is [itex]H_{int}=\frac{ge}{2mc}\bf{S}\cdot\bf{B}[/itex] where [itex]\bf{S}[/itex] is the spin operator and [itex]\bf{B}[/itex] is the magnetic field the electron is in. Yes, you can calculate that g=2 in non-rel. QM, but like I said, you first have to assume the electron already has the property of spin, while Dirac showed that spin a natural consequence of writing a first order, Lorentz invariant Shrodinger like Equation for the electron.
  11. Dec 6, 2011 #10


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    As mathman said, there are no forces involved. They don't ever start to spin. Spin is simply one of the numbers that distinguishes between the different particle species that can exist in a spacetime such that "space" is isotropic.

    In QM, the assumption that space is isotropic takes the form "there's a group homomorphism from SU(2) into the set of linear operators on the Hilbert space". SU(2) is the covering group of the rotation group SO(3), and SO(3) is a subgroup of both the Galilean group and the Poincaré group. For this reason, I would say that spin is just as much a part of non-relativistic QM as of relativistic QM. It's made a part of the theory by an assumption about the existence of a representation of the symmetry group of the spacetime we have chosen to consider.
  12. Dec 6, 2011 #11

    A. Neumaier

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    Most of the stuff taught in a typical quantum physics textbook consists of fundamental facts that we know for sure, and not of conjecture.

    Conjecture starts at two different boundaries of the vast field of quantum theory:
    - when we are trying to understand the underlying philosophy, and
    - when we go to smaller scales (structure of elementary particles; Planck scale) where the theory is not well developed enough and experiments are difficult or currently impossible.
  13. Dec 6, 2011 #12


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    Electron spin is a consequence of underlying symmetry of flat space-time, Galilean in the case of non-(specially) relativistic physics, or Minkowskian in case of (specially) relativistic physics.

    Definitely spin is not an (ad-hoc or not) hypothesis, it's a consequence.
  14. Dec 6, 2011 #13


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    Facts are obtained from experiments. Experiments are used to refine theories. Theories, such as Quantum Mechanics, are judged by their ability to successfully predict the outcomes of experiments. So there is a fundamental relationship between theory and experiment (i.e. facts).

    Quantum physics is generally considered to be one of the most successful scientific theories ever created. That is because it has made so many specific predictions which have been experimentally verified. So if you imagine that scientists just make haphazard guesses or ignore deeper questions, you would be mistaken. Hard questions are asked every day, but the answers are not always forthcoming. As already mentioned by others, there does not appear to be an answer to many "Why" questions.
  15. Dec 6, 2011 #14
    Are elemental particles subject to entropy? Are there dead electrons "no spin".
  16. Dec 6, 2011 #15


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    There are spin-0 particles, but there are no spin-0 electrons because having spin 1/2 is part of the identity of an electron, along with its mass and charge. If it doesn't have spin 1/2, it can't be an electron, by definition. There are no spin-0 particles with the same mass and charge as an electron, as far as we know.
  17. Dec 6, 2011 #16
    PFMentor- thanks
    So are electrons that existed soon after big bang still around still acting as electrons?
  18. Dec 6, 2011 #17


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    In general yes, although there are reactions in which electrons may be created or destroyed.
  19. Dec 6, 2011 #18
    Out of nothing? Really? Like what?
  20. Dec 6, 2011 #19


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    He didn't say "out of nothing." :smile:
  21. Dec 6, 2011 #20


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    Not out of nothing, out of energy. The equation e=mc^2 allows us to equate an amount of energy with the mass of a particle. If we observe any event that creates an electron we will see that the electron is created and energy is removed from the event equal to the mass of the electron.
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