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Spin imparting angular momentum

  1. Feb 6, 2016 #1
    Hey, I am new to the forum, and would certainly appreciate help in understanding spin - although I realize that perhaps no one really understands spin.

    If a beam of spin polarized electrons are absorbed in a target, then the target will start to rotate. This kind of makes sense on a macroscopic scale, since the beam has a collective angular momentum that must be conserved, i.e., apparently some fraction of the absorbed electrons will change their spin direction and therefore the target should rotate in order to conserve angular momentum.

    What I find peculiar on the microscopic scale is the electron really does not impart it's angular momentum to the target when it is absorbed unless it changes it spin direction, i.e., apparently the target does not rotate upon absorbing the electron if the electron maintains the same spin direction after being captured (correct me if I am wrong about this).

    Maybe the simplest example is when a positron captures an electron and forms positronium for a brief period before annihilation. Is there any spin dependency in the life time of positronium? Maybe it is not appropriate to think of positronium as rotating, but if the spins of the electron and/or positron have changed direction, wouldn't a resulting rotation of the collective pair be logical?
  2. jcsd
  3. Feb 8, 2016 #2


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    Do you mean spin-polarized photons?
  4. Feb 8, 2016 #3
    No, here I am specifically referring to electrons, or fermions in general. I suspect more high profile experiments have been done with spin polarized photons causing a target to rotate, but the same holds true for a spin polarized electron beam.

    The phenomenon with a photon beam seems to make more sense to me, since the photons are "absorbed" and their angular momentum "has to go somewhere." However, it seems something trickier is occurring with an electron beam, being that the electrons really do not lose their spin, but only a change in spin direction might occur when they are captured.

    Also, when a photon is absorbed in a target, it seems to "disappear" in a sense, whereas electrons when captured do not "disappear" in the same sense, (at least - in the case of positronium- not immediately "disappear," that is, there is a delay before the electron-positron pair convert to photons)

    I suspect the absorption of electrons might not be necessary: perhaps they only have to bounce off the target in order to cause the target to rotate. To add more speculation, perhaps the electron beam really does not have to come in "direct contact" with the target, perhaps the target could be negatively charged to a high enough voltage to cause the electrons to essentially "bounce off" the target at some distance away; as long as the "reflected" beam partially depolarizes, the target will rotate.
  5. Feb 8, 2016 #4
    As an addendum, which I should have brought up earlier, is the conservation of energy in this process: the electron's spin imparts energy to the target by causing the target to rotate, yet the magnitude of the electron's spin does not decrease or lose energy, the electron's spin only changes direction. Hence there seems to something odd here when electrons are involved, (photons, on the other hand, when absorbed, lose both their "linear" and "angular" momentum - all their energy - when absorbed). So I must be missing something here.
  6. Feb 9, 2016 #5
    well i am not definite but the 'spin' of electrons are stares of energy and it can not be visualized as a top rotating in a classical mechanical sense- they couple and provide spin-spin interaction or exchange interactions- am i right?
  7. Feb 9, 2016 #6


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    I've never heard of such experiments.

    Calling our resident expert in solid state physics, @ZapperZ.
  8. Feb 9, 2016 #7
    It seems strange to abandon the idea of rotation when talking about angular momentum, but there it is. Somehow particles have angular momentum, in almost every important sense, even acting like a gyroscope, but without doing all of the usual rotating. Instead, a particle’s angular momentum is just another property that it has, like charge or mass. Physicists use the word “spin” or “intrinsic spin” to distinguish the angular momentum that particles “just kinda have” from the regular angular momentum of physically rotating things.
  9. Feb 9, 2016 #8
    Electrons do each have a magnetic field (called the “magnetic moment” for some damn-fool reason), as do protons and neutrons. If enough of them “agree” and line up with each other you get a ferromagnetic material, or as most people call them: “regular magnets”.

    Herein lies the problem. For the charge and size of electrons in particular, their magnetic field is way too high. They’d need to be spinning faster than the speed of light in order to produce the fields we see. As fans of the physics are no doubt already aware: faster-than-light = no. And yet, they definitely have the angular momentum necessary to create their fields.

    It seems strange to abandon the idea of rotation when talking about angular momentum, but there it is. Somehow particles have angular momentum, in almost every important sense, even acting like a gyroscope, but without doing all of the usual rotating. Instead, a particle’s angular momentum is just another property that it has, like charge or mass. Physicists use the word “spin” or “intrinsic spin” to distinguish the angular momentum that particles “just kinda have” from the regular angular momentum of physically rotating things. See reference for above posts:
    <http://www.askamathematician.com/2011/10/q-what-is-spin-in-particle-physics-why-is-it-different-from-just-ordinary-rotation/> [Broken]
    Last edited by a moderator: May 7, 2017
  10. Feb 9, 2016 #9


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    You stated this without evidence. There is no such thing, and there is no such experimental observation so far. If this is your starting premise, then it is imperative that you provide sufficient evidence that this is the case, Otherwise we will be arguing about the color of the spots on a unicorn.

    No, it doesn't.

    When an atom emits a photon, it changes its "orbital" angular momentum quantum number by 1. This does not cause the atom to "spin". The same when it absorbs a photon. There is nothing in the mechanics that requires that the atom spins to conserve anything.

    So there is already evidence that the physics involved here is DIFFERENT than what you envisioned. So unless you have a proper, VALID description of what you are arguing, the rest that follows is moot. You haven't established the validity of your starting point, that this "spin" is the same physical spin as in classical mechanics (we have a FAQ on this), and that to conserve such spin, the "target" must physically spin when absorbing these electrons.

  11. Feb 13, 2016 #10
    Thanks for being skeptical - I am wondering how true this statement is:
    "When a beam of spin polarized electrons is absorbed by a target, then the target will start to rotate," which is stated in such an off hand manner in Robert Oerter's "The Theory of Almost Everything," I thought perhaps it was common knowledge, (see the chapter on "The Mystery of the Electron" for this statement). I had heard of this scenario for a circularly polarized photon beam, but was not familiar with it occurring with a spin polarized electron beam. Oerter's book, by the way, is extremely worthwhile, although it is mostly a popularization.

    I do not have access to most physics journals, so cannot readily find the experimental support for this statement. However, I am wondering if it is related to spin-transfer-torque used in solid state scenarios:

    " a spin polarized current is one with more electrons of either spin. By passing a current through a thick magnetic layer (usually called the “fixed layer”), one can produce a spin-polarized current. If this spin-polarized current is directed into a second, thinner magnetic layer (the “free layer”), angular momentum can be transferred to this layer, changing its orientation. This can be used to excite oscillations or even flip the orientation of the magnet. " (from https://en.wikipedia.org/wiki/Spin-transfer_torque)
  12. Feb 13, 2016 #11
    i think you should go over to uses of spin states of electrons in various devices or tunneling mechanisms where intrinsic spins of particles are being used- but its not a mechanical transfer of rotational angular momentum as in 'macroscopic' world- in microworld spin-spin interactions are well known -for example two types of hydrogen molecules are observed due to spin orientation of protons called ortho and para hydrogens and their scattering with other nucleons will have different features. see below;

    The spin of the electron is an intrinsic angular momentum that is separate from the angular momentum due to its orbital motion. The magnitude of the projection of the electron's spin along an arbitrary axis is [PLAIN]https://upload.wikimedia.org/math/1/b/c/1bc3821bd56da4cee39ce9fa2c70da4c.png, [Broken] implying that the electron acts as a Fermion by the spin-statistics theorem. Like orbital angular momentum, the spin has an associated magnetic moment, the magnitude of which is expressed as

    [PLAIN]https://upload.wikimedia.org/math/1/3/1/131d6b6e18bafa3ba9585923555611ec.png. [Broken]
    In a solid the spins of many electrons can act together to affect the magnetic and electronic properties of a material, for example endowing it with a permanent magnetic moment as in a ferromagnet.

    Spintronics (a portmanteau meaning spin transport electronics[1][2][3]), also known as spinelectronics or fluxtronics, is the study of the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices.

    Last edited by a moderator: May 7, 2017
  13. Feb 14, 2016 #12


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    Theoretically, absorbing polarized electrons by a medium at rest should lead to a rotation of the entire system, because total angular momentum is conserved. Whether or not there are concrete experiments verifying this, I can't say; obviously not, as ZapperZ says.

    Further, it's wrong to claim that nobody understands spin. I hope that all our students of the QM1 lecture understand spin. Theoretically you can understand spin via group theory. It's defined by the representation of the rotation group for particles of vanishing momentum.

    Also, you should not mix up magnetic fields and magnetic moments. Magnetic moments are sources of the electromagnetic fields as are electric charges. You also don't claim that electric charge is the same as an electric field.
  14. Feb 17, 2016 #13
    Apparently we have a difference of opinion here - not sure if the differences can be readily resolved.

    If the target does rotate, then the energy for the rotation has to come from somewhere - obviously the electrons do not lose some of their "spin energy" to the target, i.e., the absorbed electrons do not slow their spin in order to give rotational energy to the target, (electrons do not behave classically).

    Let's assume the rotational energy imparted to the target is coming from the linear momentum of the incident electrons. What if the incoming electron beam has almost zero kinetic energy, would this affect the amount of rotation imparted to the target? It shouldn't, right?

    The amount of energy imparted to the target for rotation should only depend on the number of electrons that change their spin direction, and this is independent of the beam's kinetic energy.

    If a single electron is incident on the target, and the electron changes its spin direction, then the target will rotate the same amount regardless whether the electron has high or low kinetic energy.

    Perhaps it is more appropriate to think of the target as a "negative well." In this scenario, the nearly "thermal" electron is falling into the "well" and imparting its "newly acquired" kinetic energy towards rotation.

    Thanks in advance to those that elaborate on this rather simple subject.
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