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Stern - Gerlach experiment with electrons

  1. Feb 15, 2015 #1
    Is it possible to perform am experiment like that of Stern-Gerlach but with electrons instead of silver atoms?
    Thank you.

  2. jcsd
  3. Feb 15, 2015 #2
  4. Feb 16, 2015 #3
    I don't understand. From the abstract it would seem it can be done:

    "The conflict between Bohr's assertion that the magnetic moment of the electron cannot be measured with experiments based on the concept of classical trajectories, and the measurement of the magnetic moment of electrons in a modified Penning trap by Dehmelt et al. has led us to reevaluate other implications of Bohr's assertion. We show that, contrary to the analysis of Bohr and Pauli, the assumption of classical trajectories in a Stern-Gerlach–like device can result in a high degree of spin separation for an electron beam. This effect may persist within a fully quantum-mechanical analysis. The magnetic fields considered are such that a tabletop Stern-Gerlach electron spin filter is feasible."

    Or I haven't understood well.

    In any case, thank you for your link!

  5. Feb 16, 2015 #4


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    Although the original Stern Gerlach experiments were done with neutral objects such as the silver atom, it was soon discovered that, yes, it definitely works with electrons. In fact the first time I encountered it, in a book called "Where Does the Weirdness Go?" it was specifically described with electrons.
  6. Feb 16, 2015 #5
    The link on the wik page is to a comment to the paper of which you quote the abstract.
    I have no access to the comment nor the paper.
    Just another example of why scientific papers should be accessible to the public.
  7. Feb 16, 2015 #6


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    It doesn't matter. Any paper that says it can't be done with electrons is so far wrong that it should be shunned.
  8. Feb 16, 2015 #7
    Extract of parts of that comment in PRL:

    "(1)[..] contrary to the authors’ conclusion a practical spinfilter device is not feasible; (2) extension of their investigation to more practical conditions shows no splitting; and (3) also in contrast to their assertion, transverse splitting is indeed possible for a suitable field configuration.
    In essence, it appears that the original Bohr/Pauli edict, while incorrect in full generality as the authors pointed out by their counterexample, yields the correct conclusion in practice, at least for beams: splitting is realized only with infeasible initial conditions, while achievable initial conditions yield no splitting"

    Interesting indeed!

    PS with Google Scholar I found that immediately following is a reply by Batelaan and Gay:

    "we were clearly not proposing it as an alternative to standard polarized electron sources [..] In retrospect, a better statement would have been that “experimental demonstration of such a spin-splitting effect may be possible.
    The measure of spin separation we provide is not quantitative but graphical".

    I have the impression that the disagreement was more about phrasing than about something fundamental.
    Last edited: Feb 16, 2015
  9. Feb 16, 2015 #8
    If you have a reference to a peer reviewed scientific paper demonstrating the Stern-Gerlach experiment on an electron beam, by all means, share it and update the wikipedia page.
  10. Feb 16, 2015 #9


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    Hm ... not so clear-cut as I thought. I did a bit of quick research and have found that while it apparently IS possible to do a Stern Gerlach experiment on electrons, it is not quite the same experiment because the normal Stern-Gerlach magnets don't work well with electrons. Instead something called a longitudinal method (as opposed to the transverse method of the standard SG experiment) is used.

    I went back to where I first read about the SG experiment and found this clear statement:

    Great. Clear. Unequivocal. This is what I remembered, but it's clearly not a peer-reviewed journal article.

    THEN, just a bit later in the same book I found the following weasle-words:

    Not at all as simple and straight forward as I had remember. Thanks for bringing that to my attention.
  11. Feb 16, 2015 #10
    I see, I haven't understood it was a comment to that paper. Anyway, looking in the web for "Stern-Gerlach Effect for Electron Beams" I have found this pdf document probably much similar to the "paper" we are discussing:


    For the sake of ease I quote some of their conclusions:

    "In summary we have presented a semiclassical analysis
    of an electron beam passing through an inhomogeneous
    magnetic field. The main results and conclusions of this
    work are the following: (a) The outgoing beam has complete
    spatial separation of the electron spin components,
    (b) the Bohr-Pauli analysis of Brillouin’s thought experiment
    is incorrect, (c) Bohr’s general assertion concerning
    observation of electron spin is not universally applicable,
    (d) a provisional estimate of the quantum-mechanical result
    shows that the spin splitting is blurred to the same
    order as the splitting itself, but that nonnegligible polarization
    effects are still extant, and (e) our geometries,
    chosen in accordance with Dehmelt’s three criteria, indicate
    that it is reasonable to attempt the design of a Stern-
    Gerlach device for an electron beam.
    This work was supported by NSF".

    Last edited: Feb 16, 2015
  12. Feb 16, 2015 #11


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    There is a more recent paper by Batelaan:
    S. McGregor et al., Transverse quantum Stern–Gerlach magnets for electrons, New J. Phys. 13 065018 (2011).

    In the Stern–Gerlach experiment, silver atoms were separated according to their spin state (Gerlach and Stern 1922 Z. Phys. 9353–355). This experiment demonstrates the quantization of spin and relies on the classical description of motion. However, so far, no design has led to a functional Stern–Gerlach magnet for free electrons. Bohr and Pauli showed in the 1930 Solvay conference that Stern–Gerlach magnets for electrons cannot work, at least if the design is based on classical trajectories (Pauli W 1932 Proc. of the 6th Solvay Conf. 2 (1930) (Brussels: Gauthier-Villars) pp 183–86, 217–20, 275–80; Pauli W 1964 Collected Scientific Papers ed R Kronig and V F Weiskopf, vol 2 (New York: Wiley)). Here, we present ideas for the realization of a Stern–Gerlach magnet for electrons in which spin and motion are treated fully quantum mechanically. We show that a magnetic phase grating composed of a regular array of microscopic current loops can separate electron diffraction peaks according to their spin states. The experimental feasibility of a diffractive approach is compared to that of an interferometric approach. We show that an interferometric arrangement with magnetic phase control is the functional equivalent of an electron Stern–Gerlach magnet.
  13. Feb 17, 2015 #12
    Thank you. At the end they write:

    "4. Conclusion
    The following question is addressed: ‘Is it possible to observe the spin of the electron, separated fully from its orbital momentum, by means of experiments based on the concept of quantum particle trajectories?’ As this applies to Stern–Gerlach ‘magnets’, the answer is affirmative. For the longitudinal case, this has been analyzed previously [10], while in this paper a transverse case is analyzed. The arrangement is not optimized for practical applications; magnetic Bragg crystals would be interesting to study in this context. Nevertheless, the logical argument is made for a scenario where the physical elements have been individually realized. The answer to the above question appears to be ‘Yes’. For example, spin can be observed, fully separated from its orbital momentum, by energy jumps associated with spin flips, in the lowest quantum motional states (cyclotron and magnetron) [23]. Dehmelt has observed such spin flips [23] for individual electrons, and attacked Bohr and Pauli’s dictum [24], suggesting the above formulated general rule."

  14. Feb 17, 2015 #13
    Aha so that is the root issue of the SG statement on the wikipedia page.
    Here's my opinion on the observability of electron spin without orbital angular momentum involvement.
    Electron spin can be observed, without any influence of orbital momentum, by ESR on for example a substitutional phosphorus atom in silicon.
    This is a very pure case of electron spin, with a spectroscopic splitting factor of 1.9985, close to the free electron value of 2.00232.
    This observation was done in the 1950's. So imho the discussion could in fact have been concluded already 60 years ago.
    Also the observation of the 21 cm resonance of neutral hydrogen in 1951 is an observation of pure electron spin.
    Still it is very nice that the SG can work for free electrons, although not in a straightforward way.

    Last edited: Feb 17, 2015
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