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So if spin isn't really spin

  1. Sep 28, 2011 #1
    So if spin isn't really "spin"...

    Then how do magnets work?

    I really don't know much about spin, except that it's not really the particles spinning. I've read a few articles on them but I haven't yet learned the math necessary to understand what's going on there (although I've heard that nobody really gets it).

    I know that magnetism arrives from the movement of electrical charges. I've been told that magnetism arrive from many electrons all "spinning" in the same direction, amplifying the magnetic effect, thus making a magnet as we know it.

    But the electrons aren't REALLY spinning! So why should magnetism arise from them at all?

    I hope I haven't opened too big a can of worms...
  2. jcsd
  3. Sep 28, 2011 #2
    Re: So if spin isn't really "spin"...

    Rather than go through lengthy explaination on the properties of angular momentum, magnetic dipoles etc YOU may find this link useful


    It explains the spin porperties in relatively simple terms and has hyper links on any term used to provide more detail Hope this helps.
  4. Sep 29, 2011 #3
    Re: So if spin isn't really "spin"...

    Just as electrons possess intrinsic angular momentum ("spin"), electrons also possess an intrinsic magnetic dipole (which is aligned with their spin).
  5. Sep 29, 2011 #4
    Re: So if spin isn't really "spin"...

    I've looked at the link, but I'm confused about one thing: "intrinsic angular momentum."
    When I think "momentum," I think things with velocity vectors moving around. But if there is some momentum that is intrinsic... Where does that leave us? I'm just having a hard time thinking about what it means for such a thing to exist.
  6. Sep 29, 2011 #5
    Re: So if spin isn't really "spin"...

    All particles can be thought of as a particle or as a wave, as thy both have characteristics that can be described as a particle and a wave. The Heisenburg uncertainty principle taught us that one cannot visualize a particle as both a wave or particle at the same time. It also taught us that when we examine a particle we alter its position and sometimes energy state. When you try to describe an electron by its wave function you need to determine its instrinsic angular motion. Most of quantum mechanics deal with the wave function of a particle, where you seldom see that discussed in macro world sciences. It is discussed but they usually describe particles as particles rather than waves.
    Electrons are both a particle and a wave therefore it always has angular momentum
    particles also never stay still for that matter.
    In quantum mathematics instrinsic angular momentum can also be termed as "Spin quantum number" http://en.wikipedia.org/wiki/Intrinsic_angular_momentum

    List of angular momentum quantum numbers

    Intrinsic (or spin) angular momentum quantum number, or simply spin quantum number
    orbital angular momentum quantum number (azimuthal quantum number)
    magnetic quantum number, related to the orbital momentum quantum number
    total angular momentum quantum number

    these are all needed to to describe the various wave, motion behavior characteristics of particles. The energy state of a particles also effect these characteristics.

    Keep in mind I am still studying quantum mechanics myself so others may have corrections to my answer.
    Last edited: Sep 29, 2011
  7. Sep 29, 2011 #6
    Re: So if spin isn't really "spin"...

    well they exhibit that characteristic....but WHY they do is unknown....as is why they have the charge or the mass they exhibit....

    Then how do magnets work?

    decent discussion here:

    A magnetic material is one where the dipoles are mostly aligned in the same direction.
    But an electromagnet magnetism arise from the current flow and resulting electromagnmetic field without any particular alignment of any core material.
  8. Sep 29, 2011 #7
    Re: So if spin isn't really "spin"...

    I have a question about "spin" as well. The terminology confuses me.

    The way I see it -- "spin" implies that some property of the electron is a function of time, and this function has a period (returns to its original state at a certain frequency). To me, this is the semantic meaning of the word rotation, which is implied by "spin".

    Is this correct?

    And if not, then why the heck is it called spin? If it's just an intrinsic property of which two different states give two different measurements, then why not call it moose, or happiness, or color?
  9. Sep 29, 2011 #8
    Re: So if spin isn't really "spin"...

    I think historically people used to think that the particles actually were spinning, but then they wised up and they were stuck with the name "spin." From how I'm understanding it, it helps to think of it as if it is spinning, although there's no reason to think that that's ACTUALLY happening.
  10. Sep 29, 2011 #9
    Re: So if spin isn't really "spin"...

    So then does EVERYTHING with spin have a magnetic property? They don't interact electromagnetically but they still have spin.

    If spin is about magnetism, then what does it even MEAN for neutrinos to have it?
  11. Sep 29, 2011 #10


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    Staff: Mentor

    Re: So if spin isn't really "spin"...

    Because we physicists like to mess with people's minds. :biggrin:

    Remember, we're the folks who also use the word "color" to represent the property of quarks that is associated with the strong interaction in a similar way that charge is associated with the elecromagnetic interaction; and the word "flavor" to distinguish between different types of quarks (up, down, etc.) or leptons (electron, mu, tau). (I've been waiting for years for Ben & Jerry to pick up on that one. :cool:)

    Also, the intrinsic angular momentum of an electron really is angular momentum, in the sense that it contributes to the total angular momentum of a macroscopic system, and can affect the macroscopic rotation of an object under the right circumstances. Look up the Einstein - de Haas effect.
  12. Sep 29, 2011 #11
    Re: So if spin isn't really "spin"...

    It is called spin because the generators of the group of spin transformations obey the same algebra as those for angular momentum (SU(2)). So, the analogy to something spinning around is purely mathematical.
  13. Sep 29, 2011 #12
    Re: So if spin isn't really "spin"...

    It's called a "spin" because it follows the same form of mathematics that the angular momentum of an electron has orbiting an atom. (this is completely analogous to a planent like earth "orbiting" the sun and possessing some resultant angular momentum, but then also having angular momentum due to its spin. Both forms of angular momentum follow the same mathematics in classic mechanics yet are interpreted differently depending on naming conventions. Well, in quantum mechanics spin and angular momentum differ more than just their names, however, they follow the same mathematical pattern as each other in much the same way orbits and spins do in classical mechanics. Further, the "spin" number in QM also denotes the magnetic moment of an electron, much like a spin could create in classic mechanics/electromagnetic theory, so using the naming convention "spin" makes sense for those two reasons and possibly more I don't know about.)

    Unfortuantely nhmllr, I can not answer your question. It's possible that nobody knows the answer to this, but I don't know (I don't know what relativistic QM says about this or other upper level theories like quantum electrodynamics, etc. After all, the spin is only derivable from relativistic quantum mechanics which is usually saved for graduate school.) I certainly know I didn't come across it as an undergraduate. However, I simply posted to make it clear to you that nobody has answered your question yet, just to confirm that you didn't miss anything in anybody's explanations. Somebody who knows more might come and post something soon though.

    Edit** I typed this up before the last two posters posted.
    Last edited: Sep 29, 2011
  14. Sep 29, 2011 #13
    Re: So if spin isn't really "spin"...

    Where did you learn that? That's definitely not the meaning of the Heisenberg uncertainty principle.
  15. Sep 29, 2011 #14
    Re: So if spin isn't really "spin"...

    This is great! This is exactly what I've wanted to know for years. Why don't schools just say that instead of leaving everyone to wonder why electrons are just randomly rotating like tops?
  16. Sep 29, 2011 #15
    Re: So if spin isn't really "spin"...

    Oh wow, color's already taken huh? I guess "moose" will have to suffice ;)

    EDIT: If I ever become some great particle physicist, I swear I'm naming the first undiscovered property "moose".
  17. Sep 29, 2011 #16
    Re: So if spin isn't really "spin"...

    I don't know your level of education, but I guess one usually learns this in introductory quantum mechanics courses.
  18. Sep 29, 2011 #17
    Re: So if spin isn't really "spin"...

    I hate to disappoint you, but there is already something called "moose model" in particle physics! ;)
  19. Sep 29, 2011 #18
    Re: So if spin isn't really "spin"...

    4th year chemical engineer. I've been taking QM for three weeks now. We just finished the time independent Schrodinger equation and have now begun the derivation of the Uncertainty Principle (well, at least for a particle in a box).

    I should really say its Quantum Chemistry, not QM. We use https://www.amazon.com/Quantum-Chem...505/ref=sr_1_1?ie=UTF8&qid=1317334127&sr=8-1".

    It really doesn't go into a lot of the deep math about it -- which is why I'm trying to learn more on here, because I find the subject interesting.

    (Btw, "I don't know your level of education, but..." usually comes across as "You seem kind of ignorant, but..." even if you didn't mean it that way).
    Last edited by a moderator: May 5, 2017
  20. Sep 29, 2011 #19
    Re: So if spin isn't really "spin"...

    Holy crap! I thought you were kidding, but I just Googled it, and indeed there is a moose model!
  21. Sep 29, 2011 #20
    Re: So if spin isn't really "spin"...

    Ah, I see. If it is a decent course, they should teach you more about spin eventually!

    Sorry, it was definitely not meant that way! I just wondered what kind of education you had, because it would've been odd if you were for example a physics major who never heard about the mathematical aspects of spin ;)
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