Spin

Discussion in 'Physics & Math' started by The God, Mar 9, 2017.

  1. Farsight

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    • This is serious miseducation in the false guise of scholarship. Physics is not advanced by abuse of citations to mean something different than the author meant.
    Check out the Einstein-de Haas effect, which "demonstrates that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics". Some people will tell you that electron spin is not of the same nature as macroscopic spin, but the hard scientific evidence says otherwise. Something that I think is well worth reading is Hans Ohanian’s 1984 paper what is spin? He says “since the naïve mechanical picture of spin proved untenable, physicists were left with the concept of spin minus its physical basis, like the grin of the Cheshire cat”. He goes on to say Pauli pontificated that spin is an essentially quantum-mechanical property, and that the lack of a concrete picture was a satisfactory state of affairs. He then quotes from Pauli’s 1955 essay Exclusion Principle Lorentz Group And Reflection Of Space-time And Charge: “After a brief period of spiritual and human confusion caused by a provisional restriction to ‘Anschaulichkeit’, a general agreement was reached following the substitution of abstract mathematical symbols, as for instance psi, for concrete pictures. Especially the concrete picture of rotation has been replaced by mathematical characteristics of the representations of rotations in three-dimensional space”. I think that's astonishing. Particularly because Ohanian says this: “the means for filling the gap have been at hand since 1939, when Belinfante established that the spin could be regarded as due to a circulating flow of energy”. Belinfante’s paper was seventy seven years ago.

    I'm not sure what you mean. Electrons and positrons get their spin ½ spin when they're created in pair production.

    Again I'm not sure what you mean. If the motion is different the spin is different.

    It's because the electron is a Dirac spinor, and a Dirac spinor is a bispinor. For an analogy, think of a disk standing on its edge. Now make it spin like a wheel. It could be spinning clockwise or anticlockwise. Lets' say it's spinning clockwise, but now lets' walk round the back of it, and lo, it's spinning anticlockwise. Now keep the wheel-like spin, and spin it like a coin as well. Which way is it spinning? It's hard to say, because it's spinning on two axes. But note that you could have spun that "coin" with your left hand or with your right hand, so there's two different ways to make this disk spin on two axes.
     
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  3. The God Valued Senior Member

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    Since you are supporting the view that spin of an electron is what it is classically, not metaphysical of Rpenner (even I don't agree with his casual unscientific refrain)..The points restated are as follows..

    1. Where is the need for electron positron to acquire half spin? They can jolly well stay spinless. Conservation law cannot explain the cause of original spin.

    2. Do all electrons have same spin magnitude?

    3. Classically treating electrons as spherical, there will be infinite many rotation axis. So why only two chosen? Well I can understand we want only two stable energy states, but then what is the reference for spin axis?
     
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  5. exchemist Valued Senior Member

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    Be careful, here. Farsight is a notorious proponent of his own eccentric theories of physics. If you listen to him you are quite likely to get confused.
     
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  7. rpenner Fully Wired Staff Member

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    "Same nature" here means only that it is subject to the same conservation law, not that it is physically similar to a classical rotating body.

    Ohanian, Hans C. "What is spin?." American Journal of Physics 54.6 (1986): 500-505.
    http://iopscience.iop.org/article/10.1088/1367-2630/15/3/033026/meta cites the above paper to support the proposition that eqn. 2.25 reflects the “intrinsic nature of spin, which does not involve energy transport”
    Likewise, https://www.researchgate.net/profil...h_the_spin/links/54b7d6f00cf28faced608b9a.pdf shows that the spin probability current doesn't relate to motion of the particle, in section III.

    Pauli would have likely formed this opinion before Wigner's 1939 classification of unitary representations of the Poincaré group, which proves it.

    Both Ohanian and Farsight confuse the topic of Pauli's quote which is not the intrinsic spin of particles, but the mathematical treatment of rotation as the special orthogonal group in 3 dimensions, SO(3), while the full 6-parameter Lorentz group is SO(3,1) in three dimensions of space and one dimension of time.


    A treatment which is necessarily impoverished of being able to relate angular momentum and the magnetic field of the electron, which requires in detail, Quantum Electrodynamics.

    You have again confused the map with the territory. The electron isn't a Dirac spinor, but its behavior can be modeled as a Dirac spinors.

    That is a terrible analogy. Rigid objects don't spin on "two axes" and college freshman courses spend at least a week on this topic alone.

    When we say something has spin-1/2, we mean it has intrinsic angular momentum of (½ √3 ℏ) but because of the uncertainty principle if we want to know what the value of the angular momentum in any particular direction, we measure only + ½ ℏ or – ½ ℏ with the information in the other directions lost to us.

    When we say a massive particle has spin-1, we mean it has intrinsic angular momentum of (√2 ℏ) but because of the uncertainty principle if we want to know what the value of the angular momentum in any particular direction, we measure only + ℏ, 0 or – ℏ with the information in the other directions lost to us. (Massless particles are a different case according to Wigner (1939) and reality.)
    https://en.wikipedia.org/wiki/Spin-½
     
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  8. The God Valued Senior Member

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    Did farsight say anything in his post? He is giving links, so its a matter of interpretation. For example 'same nature' is being interpreted differently by Rpenner.
     
  9. exchemist Valued Senior Member

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    Yes it was "same nature" I was worried about: all it means is it is indeed angular momentum. Rpenner however has covered this very well in the next post, so no harm done.
     
  10. The God Valued Senior Member

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    I meant in contrast with rpenner. It gives me a feeling that rpenner (whoever came up first with this interpretation) is taking a line of convenient fix. I am always of the opinion that if meaning is clear and relevant, then no complex or alternative meaning is to be derived. It would have been such a huge factor to deviate from classical meaning at that time, that authors would have been clear and precise if the intent was what Rpenner wants others to believe.

    PS: this is just a disagreement over Rpenner interpretation, not on the issue at hand.
     
  11. exchemist Valued Senior Member

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    It is a convenient fix in a sense, in that it is what we observe particles to do and it came as a rather puzzling surprise, back in the early 1900s. If it really were classical "spin", we would be able to stop them spinning, as you originally suggested. But experiment indicates otherwise (another example being that the magnetism of the electron is a bit more than twice what we would expect if it were a little ball physically spinning: https://en.wikipedia.org/wiki/Gyromagnetic_ratio) and this has led to the model in which there is this property - whatever it is - called "intrinsic angular momentum". Regarding what various authors meant by it, I think you can read quite a bit about the history, and you will see the model we now have emerging, by degrees.
     

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