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Particle Spin

  1. Apr 5, 2007 #1
    I know very little about quantum mechanics so I apologize for asking such a crazy question. I read all the time the talk of a particle's spin. I am curious at how this spin is detected in something that we can't see to begin with.

    For example: I read something linked to this forum that suggested that a physicist's thought of an up or down spin would actually cause a particles spin to match his/her thought of it. I'm curious at how we can be so completely positive of a particles spin as this.

    Thanks for you time,

  2. jcsd
  3. Apr 5, 2007 #2
    Something like this.

    (And that bit on thought isn't true.)
  4. Apr 5, 2007 #3

    Doc Al

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    Staff: Mentor

    When you say "can't see", I assume you mean with the naked eye. But we have instruments that detect things beyond what our unaided senses can detect.

    Associated with spin is a magnetic moment, which leads to detectable interactions. For example, the spin of the electron explains the fine structure splitting in the spectrum of the hydrogen atom. (The observed fine structure is evidence for electron spin.) The magnetic moment also allows the electron spin to interact with external magnetic fields--look up the Stern-Gerlach experiment and the Zeeman effect.

    Don't believe everything you read. That's just not true.
  5. Apr 6, 2007 #4
    Thank you very much for the information. I'll do a little more research on the subject as I'm very interested in how that all works. As for the story linked to this site about influencing the spin, I didn't believe it as there would be way too many things to track during that experiment that would lead to that conclusion... :-) But then he did write a book... just kidding...

  6. Apr 6, 2007 #5
    I think what was meant was that when you choose direction along which to measure the spin angular momentum, you will find that the spin angular momentum is always directed along (whether parallel or anti-parallel) to that axis. While for systems where the total spin angular momentum is greater than 1/2, we have varying values, for spin-1/2 objects, we find that it's either 1/2 or -1/2.
  7. Apr 6, 2007 #6
    I'm still reading up on the information you guys sent me but I'm still confused. Maybe you could put the answer in laymens terms so I could understand it better. Here is what I'm confused about:

    I've ready quite a few books on the subject but they are all off the shelf watered down versions that don't go into any depth. But as an example: One was talking about an experiment in which an electron was seperated and then measured and it was observed that measuring one would cause an instantaneous change in the other. It made reference to the fact that its spin changed.

    The thing that struck me as strange is how us humans can determine anything at all is instantaneous, but also how two things this small were contained in some way as to determine that measuring one caused changes in the others spin. I assume these would have to be free radicals or something but the book never went into it. Too many books try to add mystery to things where there is none and to keep you in the dark they just don't provide real data.

    Again, I'm new to the subject so all the technical jargen is only confusing me. I was just curious if the spin has to be measured inside a device, if it can measure the thing when it flies by or if its inside a reactor of some sort... There is a lot of specific data concerned about this and I'm interested and how its done...


  8. Apr 11, 2007 #7
    I do not understand you. You want go to depth, but you apparently do not want to study swimming. The electron spin, how and under what conditions it may be measured and why the things are as they are is, perhaps, the key question of the relativistic QT.

    For me, the spin is the most fascinating phenomenon of the Quantum world. It is new degree of freedom, semi internal, semi external. And apparently disappear in the Classical world (N.Bohr)! It perhaps provides the bridge for learning about space and time in QT.

    Why you assume that the physics may be expressed using words? It is obviously not a suitable tool. The adequate language of the physics is mathematics. Only speaking it you will understand others and will understand physics. There is no other way to depth. Then you will start to understand “jargon” also.

    Regards, Dany.
  9. Apr 11, 2007 #8
    The thing you mentioned about measuring one electron and the other changing is due to entangling the pair, not knowing the spin, but knowing the probabilities of spin (say 50/50). The electrons have to be the same, so if you put them on opposite sides of the galaxy, and measure one, then the other one has to be that measurement as well. It collapses the probabilities of both. I'm sure someone else can explain it better.

    As for measurement of spin, there are many ways. One is even a new chip being designed by UCLA:
  10. Apr 11, 2007 #9


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    Either that book is wrong or very sloppy in its language, or you misread it. It would make sense to say that the other electron's spin changed, only if we knew what its spin was, before we measured the first electron. But in these experiments, we don't know the spin of either electron before we make the measurement. All we know is that (from conservation of angular momentum and the way the electrons are produced) they must have opposite spin. Therefore if we measure the spin of one, we immediately know the spin of the other one without having to measure it directly.
  11. Apr 11, 2007 #10
    Thanks, I'll keep studying and try to figure it out on my own I guess. According to you guys it can't be communicated anyway so I guess asking questions and expecting answers in English is expecting too much. I'm currently learning the math so will get back in touch later.

    Thanks again,

  12. Apr 13, 2007 #11
    You sound disappointed. But according to the C.E. Shannon theory of communication (I consider it adequate) only the modulation (“interpretation” if you wish) may be communicated. The information is hidden- irrelevant in his words. Consider for example:

    This is mathematically exact statement, consistent with all existent experimental information and the mathematical formalism of QM and SR. Compare with:

    I am not curious with that statement. My personal judgment is rude and instant: the person who wrote that-idiot. Why? The person who made this statement either suggest his “ideas” in area where he has no knowledge or the way how he contrive to understand what he was taught. The conclusion in both cases is the same. One certainly may ask questions and expecting answers in English provided that it is build upon the mutually matched information. If you are interesting, you may read more about that in paper by J.B. Hartle, quant-ph/0610131.

    Regards, Dany.
  13. Apr 17, 2007 #12
    No, English is fine. It goes like this:

    An early concept was proposed based on wave mechanics (by Schrodinger). However, experiments showed that spectral lines within a magnetic field varied in a way that could not be explained by the proposed theory.

    Along comes Pauli who recognizes there is a spectral pattern (doublets), then Uhlenbeck and Gouldmit add an additional property to solve the problem: give each electron a magnetic moment, a "spin". Problem solved.

    Long ago a professor told me to ignore the word spin because it's a classical notion. Just use it as a label for the property of having the magnetic moment. It has two states... call them up or down, north or south, left or right, but write them as 1/2 or -1/2 and you are good to go.

    Just one more thing. Grab some of the older papers and textbooks on QM. I've found that the closer you are to the original source the more clear the expression of the concepts.
    Last edited: Apr 17, 2007
  14. Apr 18, 2007 #13


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    However, the "spin" of an electron is an actual angular momentum that (at least with a large number of electrons) can combine with the classical angular momentum of a macroscopic object, and must be taken into account in conservation of macroscopic angular momentum. See the Einstein-de Haas effect. I first read about it in the Feynman lectures, but I've forgotten which chapter.
  15. Apr 18, 2007 #14
    Ah yes, heard about it years ago... and trying to remember why. De Haas was into that sort of thing, but I've never actually read the original account of the E-dH experiment, and would be very interested in seeing that paper, especially how they account for the interaction of the two magnetic fields (the coil excitation WRT the induced) of which the latter lags in time. It seems to me that there is a magnetic shear force to account for. Also, would like to see how they handled for the eddy currents (if they used conducting materials).
    Last edited: Apr 18, 2007
  16. Apr 18, 2007 #15
    the spin is a quantum phenomenon arises relative to ROTATION in classical physics so to measure something related to quantum world u want to understand carefully the Heisenberg uncertainty principle
    for more information read about pauli matrices (spinors)
    to enter quantum world u want mathematical tools as much as u can
    3-equations of mathematical physics
    5-large backgroud about analytical mechanics
    lagrangian mechanics
    hamiltonain mechanics
    poisson brackets(classical)
    continuum limit and foundation of quantized phenomenon
    6-complex analysis
    7-classical quantum theory and planck's theory
  17. Apr 18, 2007 #16
    If I want to understand carefully the Heisenberg uncertainty principle, I find that it is the Heisenberg dispersion relation that 1) may be easily derived (as explained by W. Heisenberg, the most detailed discussion known to me in R.Jackiw, JMP (1968)); 2) the dispersion is eigenvalue of the well defined self-adjoint operator (depend on state indeed) and therefore is not connected with any uncertainty (historical development lead sometimes to confusing wording).

    By spinors you mean here 2-component wavefunction (P.A.M.Dirac use 4-component spinors and gamma matrices accordingly). The Pauli matrices may be treated as operators in the space of 2-component spinors. The simplest way to demonstrate that the spin is related to rotations presented by P.A.M.Dirac (orbital angular momentum together with spin is conserved quantity), more detailed consideration by E.P. Wigner. All that within the QT.

    Your statement “the spin is a quantum phenomenon arises relative to ROTATION in classical physics” is very interesting provided you are able to defend it. Please explain what you have in mind.

    Regards, Dany.
  18. Apr 19, 2007 #17
    first i understand what you wrote about spinors and appreciate your wide knowledge about the Dirac's work...

    what i have in mind is a unification of micro-macrowrolds
    i have a proposal for a research about the unification in nature it is not as the grand-unified theory (GUT) but it is a unification from the mathematical point of view, does the mathematics used in quantum mechanics is much more complicated than classical mechanics?...the answer is no, so why?
    the unification come fromtopological aspects
    the spacetime topology is able to define what is SPIN!
    so the basic principle of unification is based on topologies
    other statements such as symmetries,supersymmeter(winberg) are used in defining the topological aspects
    for uncertainty relation it arises also in information theory i has a quanutum flavor but really it define a limiation for quantum world and how the transition to classical world is maintained if you simply reverse the inequality

    quantum mechanics in large scale topology is the title of the research
    sincerly Mina
    Last edited: Apr 19, 2007
  19. Apr 19, 2007 #18
    My guess was that you are talking about E.Cartan “Theory of spinors” (for example, Lecons sur La Theorie der Spineurs, 1938) and the Cartan’s torsion (for example, F.W. Hehl et al. Rev. Mod. Phys., 48 , 393 (1976)). I only reformulated slightly your statements to provide the common platform to understand your explanation. However, my guess was wrong.

    I think D.Hilbert have different POV:

    “This will not do. Physics is obviously far too difficult to be left to the physicists!”

    Good luck! But remember that the accuracy should be maintained in research.

    Regards, Dany.
  20. Apr 19, 2007 #19

    you know i was a communication engineer and i left it and i become a researcher in quantum mechanics
    i didn't find myself in engineering applications and here is another cause
    when i read the theory oflinear operators and especially the linear transformations i felt that this part was creates for physics when you define the next equation AX=B where A is the transformation matrix, you can say simply that X is transformed to B through A this is applied in nature in the form of energy transfomration(1st law of thermodynamics),such transformations are classified according to the space topology such that if you work in cartesian,banachian,hilbertian,...

    please tell me your opinion about my quotes fairly
  21. Apr 20, 2007 #20
    I think I did that already. In addition, I do not see the relation to the OP original question and intention. I am sure that you may find more appropriate session PF to what is really interested you.

    I consider the development of the theory of measurements the most important problem in the theoretical physics now. It is still terra incognita. This will not do. Your background as a communication engineer may be very important and useful. But you must start to think as a physicist.

    Consider for example SR. It tells us only how the single point identification in the space-time is communicated from one reference frame to another. This is enough for Newtonian mechanics and field theory of massless waves. QM is the field theory of massive waves (v<c). I must know how the information (identification) is communicated from one reference frame to another in case of the extended objects which are not rigid. Leave the unification and supersymmetries for a while. Believe me, Galileo, Lorentz, Poincare and Yang-Mills are enough. May be the large scale topology approach is fruitful. I do not know. Try to make your point as simple as possible. Then apply all you know.

    Regards, Dany.
    Last edited: Apr 20, 2007
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