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Educational Challenge

  1. Jan 21, 2010 #1
    I have a challenge for anyone gifted enough to communicate with a stupid layman in terms that I can understand. I'm struggling in my attempt to understand the concept of "spin" with regard to fundamental particles.
    1.) How can a fundamental particle that has no spatial dimension have an axis to rotate around?
    2.) If the term "spin" does not actually refer to physical "rotation", what does it mean?
    3.) How is this quality (whatever it is) measured?
    For anyone up to the challenge, please keep in mind that I am ignorant beyond belief, cognitively impaired and mathematically impotent.
    Any takers?
  2. jcsd
  3. Jan 22, 2010 #2


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    1 & 2) The spin of leptons, quarks, and gauge bosons are angular momentum that do not arise from any rotation. Historically, angular momentum was first observed in the rotation of large classical objects. This led to the belief, now no longer believed, that rotation was required for an object to have angular momentum. Even classically, it was known that the electromagnetic field could have angular momentum without rotation.
    3) Spin is measured in many ways, but they wold be difficult to explain to a
    "cognitively impaired".
  4. Jan 22, 2010 #3
    The word spin comes from the historical development of qm. Basically the particles behave as if they had a B field generating from spinning. The knowledge that they could not actually be "spinning" came later but the name spin stuck. Somebody else want to fill something about the experiment where spin was first discovered? I have to go to class.
  5. Jan 22, 2010 #4


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    There are two kinds of angular momentum: spin and rotation. An electron has spin. A pool ball with top-spin on it has rotation. As a conceptually simple, but completely impractical example, we could hypothetically do an experiment where we place a pool ball floating at rest in outer space without any rotation, and then bounce an electron off of it. If we could measure the resulting state of the pool ball with enough precision (which of course we really can't), we could find that the pool ball was rotating slightly. Since angular momentum is conserved, the conclusion is that some was transferred between the electron and the ball, presumably because the electron reversed its direction of rotation. When the transfer happens, we will always observe that it's the same amount, so we conclude that the spin of an electron is always a fixed value. This is phrased in terms of an impractical experiment, but we really do observe this kind of thing in practical experiments.
  6. Jan 22, 2010 #5


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    1) is a bit more subtle. You can see spin by imposing an external magnetic field, so your ideal "point" gets a direction to be labelled as "z". And the same happens with the relativistic thing, "helicity" (can anyone propose a laymen way to distinguish between spin, chirality and helicity?), it is considered by labelling as "axis" the one of the trajectory of the particle. This is specially important because it implies that zero-mass particles, which travel at lightspeed, have the same helicity in all the reference frames.

    Did we win your challenge?
  7. Jan 22, 2010 #6
    Here is a subtle non-quantum-mechanical experiment. I can give you a sealed black box, say 10 cm on a side, and ask you to tell me whether it has "spin". Unless you pick it up, you will not realize that there is a spinning gyroscope it. I will then ask you whether it has a fundamental spacial direction (meaning axis of gyroscope). I will then ask you whether it has a magnetic field, and you can use a compass needle to verify that there is none. Now, the box could be put into a magnetic field. If the gyroscope has metallic (lead) rotor, it will try to align parallel to or anti-parallel to the magnetic field (because the eddy currents in the rotor produce a torque). So a very unexceptional black box has very exceptional properties, including spin and an apparent magnetic moment (but without a real magnetic dipole field). How can that be? Well, it is. Let it be so.
    Bob S

    [added] Assume that the gyro is completely non-magnetic and electrically uncharged.
    Last edited: Jan 22, 2010
  8. Jan 22, 2010 #7
    Very nice and palatable explanation Bob. As Richard Feynman said, "If you don't like it, then go to another Universe!"

  9. Jan 22, 2010 #8
    No feedback from OP?
  10. Jan 23, 2010 #9
    Thank you very much for your efforts. My level of ignorance is truly saddening, so the challenge was, perhaps, not fair to begin with. Despite that, your explanations (especially Bob's) helped. Unfortunately, it also clarified my primary difficulty in understanding...
    Bob's description involved a mystery box with the internal gyro. While that helps me imagine the "rotation-less" spin, it still requires the box to "be there". Yet, the point particle in question has NO spatial dimension. That's my real challenge... How can something with NO spatial dimension do ANYTHING? Do you professional physicists view the "point particle" as something that is really "there", or is it simply a mathematical model? Does this concept reflect a material reality???
    A greater challenge in explanation, I'll admit. Any takers now?
  11. Jan 23, 2010 #10
  12. Jan 23, 2010 #11
    But, even if I hide the box in a Zhang optical cloak, it's still THERE. Invisible, yes, but still tangible... still with spatial dimension... still existent. Nice try, but...
  13. Jan 23, 2010 #12


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    With the shift to quantum mechanics came a shift in expectation. In retrospect: before QM it seemed attainable to formulate a picture of microscopic physics that is intelligible in terms of things that are tangible on human scale. A picture that is visceral, intuitive.

    With quantum physics, all of that had to be abandoned.

    It seems to me that at present just a single expectation remains, all others have been abandoned, relinguished. My expectation (be it justified or not) is that the Universe has properties that are consistent over time. My expectation is that the Universe in general is self-consistent. My expectation is that there are underlying symmetries that act as structural elements.

    Mathematical structures are self-consistent. I mean, we call it math when it has the property of being self-consistent, that is the necessary and sufficient criterium. My hunch is that mathematics offers the capability of modeling physics because mathematical structures are self-consistent, and they can embody symmetries.

    I am much more comfortable with classical physics than with quantum physics. When I think in terms of classical physics I just do what physicists did a century ago, I visualize a world that is real, and really there. I don't mind that, it allows me to think fast, it's a good tool.

    In quantum physics I think there is a similar attitude. Use visualisations, use heuristics, give the brain something to work with, give the brain tools that it's good at handling. And if you keep making progress you must be doing something right.

    But at the end of the day I am aware of the ramifications of quantum physics. It seems to be thoroughly unfruitful to go into questions of 'what is real?', 'what is reality?'.
  14. Jan 23, 2010 #13
    "But at the end of the day I am aware of the ramifications of quantum physics. It seems to be thoroughly unfruitful to go into questions of 'what is real?', 'what is reality?'."

    Perhaps Cleonis's response encapsulates my real question. I recognize the necessity of mathematical formalism to express consistencies of mechanistic relationships between matter and energy. Unfortunately, I can not pretend to be remotely capable of understanding that mathematical language. Yet, if I have captured even the most general ideas described by this mathematical formalism, I find myself questioning the "material" nature of matter, and the kinetic activity of energy. I'm in wayyy over my head here, so I have no choice but to turn to people like yourselves.
    I sympathize with those that would prefer to "just calculate". I understand that such conjecture is "the way that madness lies". But surely you must think about such things. Within the ranks of professional physicists... those most familiar with the subject matter, and with the greatest understanding of the logical/philosophical ramifications... would you say that there is a consensus opinion as to whether Standard Theory, Super Symmetry, Membrane Theory or any other discipline that I am unaware of, describe a Universe that is fundamentally "material" in nature? Or, conversely, would it be more accurate to suggest that, based on current knowledge, the fundamental essence of existence is simply information?
    Please. I would really appreciate some feed back from qualified professionals.
  15. Jan 24, 2010 #14
    This is a pretty reasonable question and I find the answers so far to be unsatisfying. So I'm going to try to give some physical reasoning.

    The magnetic moment shows up experimentally in the Stern Gerlach experiment. But how do we know that it's associated with a spinning charge and not just an intrinsic magnetic moment? One clue I would think is the absorption spectrum of monatomic Hydrogen. I think I am correct in saying that there is a microwave line proportional to an applied external field. This corresponds to the atomic magnet flipping over. But what makes it special is that the magnet can't just simply flip over to align itself with the external field, it has to spin around the field axes in a precessing motion while it slowly realigns itself. It is this precession frequency that actually corresponds to the applied microwave frequency. This points to the gyroscopic nature of the magnet, and suggests that the magnetic moment is indeed related to a spinning charge.

    I think it's also true that the absorption line doesn't completely disappear when the external field goes to zero, but remains evident as a kind of energy-level splitting based on the relative spins of the proton and the electron.

    I don't think this explanation explains why the spin is half-integer, but I think a possible reason is the fact that if a particle has ordinary orbital-type spin, the quantization should break down into three values: +1, 0, and -1; whereas in the Stern-Gerlach experiment there are only two. Similarly, if the orbital spin has value 2, then it breaks down into 5 possible values along the z-axis. So I think the even-number values for the z spin component point to the idea of half-integer spin.

    I don't know if there are Stern-Gerlach type experiments with ordinary atoms where the orbital spin splits the beam into three trajectories.
  16. Jan 24, 2010 #15


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    I believe that has been done, although I can't give you a reference off the top of my head.

    Feeble Wonk, there isn't really any consensus about many things in physics. The true nature of what we now consider "point particles" is one of those things; for example, string theory proposes that the particles are not actually points but actually 1-dimensional extended objects. But there is no experiment currently conceivable that can discern whether that's correct or not.

    Physics really describes how the universe behaves, not what it is. So things like the fundamental nature of matter are, so to speak, out of our jurisdiction. We know through experimental observation that fundamental particles act like they have angular momentum, and it's also been observed that they are, as far as our current experiments can tell, pointlike. The experiments don't lie. It may seem like a logical contradiction, but it's only a contradiction if you make certain assumptions (such as: angular momentum only arises from rotational motion), and at some point, you have to accept the experimental results at face value and discard any assumptions that don't fit, no matter how much sense those assumptions may seem to make. (This is just how Einstein came up with his theory of relativity.)
  17. Jan 25, 2010 #16

    Andy Resnick

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    I wonder if you are confusing the model and the phenomenon. Experimental data are consistent with the ideas that an electron is a point particle, and that the electron has "intrinsic" angular momentum. Protons, by contrast, do not support an interpretation that they are point particles. The idea of 'intrinsic' angular momentum was invoked to explain how certain absorption bands change in the presence of magnetic fields.

    It's not a good idea to then turn around and state electrons 'are' point particles, and that spin 'means' particles spin about like an ice skater- those are human-invented ideas to describe and predict physical behavior.
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