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Thread: What does intrinsic spin mean?

  1. #1 What does intrinsic spin mean? 
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    I know spin is a trait that some particles have. They may have half spin or integer spin.

    But what does that concept mean when it is applied to a particle? Is doesn't mean the particle is actually spinning does it?

    What affects does it have on the particle's behavior? How do we even perceive or observe it?

    Does it have any gyroscopic properties, like causing the particle to stay oriented in a particular direction (like how the spin on a bullet fired by a rifle helps it fly straight? Or how the spin on a child's top will prevent it from tipping over while it is spinning?)
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    Quote Originally Posted by Kojax View Post
    I know spin is a trait that some particles have. They may have half spin or integer spin.

    But what does that concept mean when it is applied to a particle? Is doesn't mean the particle is actually spinning does it?

    What affects does it have on the particle's behavior? How do we even perceive or observe it?

    Does it have any gyroscopic properties, like causing the particle to stay oriented in a particular direction (like how the spin on a bullet fired by a rifle helps it fly straight? Or how the spin on a child's top will prevent it from tipping over while it is spinning?)
    "Spin" is a form of quantised intrinsic angular momentum, and it does not have a counterpart in classical mechanics. So no, you can't really equate it with the particle actually "rotating".
    The most noticeable effect of spin would be that it gives the particle a magnetic dipole moment, which is measureable with proper experimental setups; that is also how it is detected.

    I am not sure what you mean by "gyroscopic properties".
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    Quote Originally Posted by Kojax View Post
    I know spin is a trait that some particles have. They may have half spin or integer spin.

    Does it [the spin] have any gyroscopic properties, like causing the particle to stay oriented in a particular direction (like how the spin on a bullet fired by a rifle helps it fly straight? Or how the spin on a child's top will prevent it from tipping over while it is spinning?)
    A particle is essentially point-like, so there is no need or even meaning to stabilizing it gyroscopically.
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    Quote Originally Posted by Markus Hanke View Post
    I am not sure what you mean by "gyroscopic properties".
    I'm just trying to figure out what it means by comparing it with a similar word I do know the meaning of.

    Does the spin involve any kind of angular acceleration? Does General Relativity enter the mix?
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  5. #5  
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    Quote Originally Posted by Kojax View Post
    I'm just trying to figure out what it means by comparing it with a similar word I do know the meaning of.

    Does the spin involve any kind of angular acceleration? Does General Relativity enter the mix?
    In Particle Physics, spin is a bit complicated. I'll see if I can say anything understandable.

    Classically, angular momentum is given by L = r x p, which is easily converted to quantum mechanics by using the quantum version of p,
    p = - i hbar grad . [ The components of grad are d/dxi and are usually shown as an inverted triangle]. By ordinary algebra you can find that the components of L do not commute with each other, and that [Li , Lj] = i hbar ijk Lk, summed over k. These commutation relations can be used to get all the quantum properties of L .

    Then the train of interesting stuff begins. You find that all the eigenvalues of components of L must be an integer or a half-integer times hbar. But ordinary orbital angular momentum has only integer-valued eigenvalues. By using commutators of r and L you can verify that r x p itself can have only integer-valued eigenvalues. So quantum mechanics has the possibility of describing a kind of angular momentum which cannot be written as r x p even if r is an internal variable, not the position of the center-of-mass of the object.

    Well, objects in mathematics don't have to show up in physics. However, this one did. The electron has an internal quantity labelled spin that acts like angular momentum but has half-integer eigenvalues. Moreover, this spin and ordinary angular momentum are not separately conserved; only their combination is conserved. So you are pretty much stuck with labeling the sum as the physical angular momentum, even though the spin part has no direct relation to r .

    Finally, just to make things complicated, other particles have spins that have integer eigenvalues but are nonetheless not (as far as we can tell) represented by any form of r x p .

    Bottom line: spin is an angular momentum that has no representation as an angular acceleration.

    Spin is one of the things that make me feel that all correct physical theories are beautiful, but you can't appreciate the beauty until after you know you are stuck with them.

    In the above, L refers to orbital angular momentum, r to position, and p to momentum.
    Last edited by mvb; 05-10-2013 at 11:02 PM. Reason: Adding list of notation
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    Quote Originally Posted by Kojax View Post
    I'm just trying to figure out what it means by comparing it with a similar word I do know the meaning of.

    Does the spin involve any kind of angular acceleration? Does General Relativity enter the mix?
    General relativity? nothing to do with it. Why do you even think that?

    Angular acceleration is not present, since spin is discrete, and absolute. Only the direction of the angular momentum can change. And this does change, in a magnetic field a spin will act like a resistant-less gyro.
    In the age of information, ignorance is a choice.
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    I'm not quite as lost as I was before on this issue. But I'm still a long way from understanding it.

    What does a particle's angular momentum cause it to do (that's the p component, right?) Does it change the way it collides when it gets smashed into other particles in a particle accelerator?

    I like to take a fairly practical approach. If I know what something does then I can at least envision it. Then I have a map to guide me through the mathematical interpretation.


    Quote Originally Posted by Kerling View Post
    General relativity? nothing to do with it. Why do you even think that?

    Angular acceleration is not present, since spin is discrete, and absolute. Only the direction of the angular momentum can change. And this does change, in a magnetic field a spin will act like a resistant-less gyro.
    GR applies to all forms of acceleration, not just gravity. If a not-quantum object is spinning, then it experiences acceleration. That's why I asked.
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    Quote Originally Posted by Kojax View Post
    I'm not quite as lost as I was before on this issue. But I'm still a long way from understanding it.

    What does a particle's angular momentum cause it to do (that's the p component, right?) Does it change the way it collides when it gets smashed into other particles in a particle accelerator?

    I like to take a fairly practical approach. If I know what something does then I can at least envision it. Then I have a map to guide me through the mathematical interpretation.
    In my post below, L is the angular momentum, r is the distance to the axis used to define the angular momentum, and p is the momentum of the object. In analyzing particles which have orbital angular momentum and intrinsic angular momentum, the effect of having the spin around is that it is the sum of orbital and intrinsic angular momentum which is the same before and after a collision, rather than having the orbital angular momentum alone to be unchanged [conserved]. Using quantum mechanics, it is seldom if ever useful to think in terms of angular acceleration. You work directly with angular momentum itself.
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    Quote Originally Posted by Kojax View Post
    But what does that concept mean when it is applied to a particle?
    Spin is a form of angular momentum. There are two types of physical “revolving” that occurs in physics. One is when one body is orbiting around a point and the other is when a body is rotating about its axis. The angular momentum associated with the former is called “orbital angular momentum" the other is called “spin angular momentun.”

    When Goudsmit and Uhlenbeck proposed the spin of the electron they visualized the electron as a small body charged body rotating about its axis. The term "spin" was thus applied to it. The spining charge thus also has a magnetic moment. This view was abandoned when a calculation showed that it would require speeds greater than the speed of light.

    However there is the fact that an electron has both a charge and a magnetic moment. Therefore the Poynting vetor S, which represents the momentum density in an electromagnetic field, is non-zero. There is no radial component, i.e. S*r = 0 showing that there is no flow of energy in the radial direction. The flow of energy is around the electron. I.e. there is flow of electromagnetic energy around the electron. This energy carries angular momentum. Ohanian showed that the spin angular momentum carried by the electron ay be regarded as an angular momentum generated by this circulating flow of energy. See What is Spin? by Hans C. Ohanian, Am. J. Phys. 54(6), 1986 pgs 500-504.

    I'd like to point out that the above treatment does not apply to non-charged particles. Only particles which are charged. Thus the magnetic moment for, say, a neutron cannot be attributed to its charge and magnetic moment.

    My question is whether particles exist which have spin but no magnetic moment?
    Last edited by PhyMan; 05-24-2013 at 02:25 AM.
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    The question about what this means for spin for particles with a magnetic moment but no charge is the obvious question. I was able to find the authors e-mail address. So I contacted him and asked him the following question
    I read your artilce "What is spin?" Very interesting. That Poynting vector always brings me suprises. What about particles that have spin but no electric charge? Your article does't seem to be able to cover that instance.

    There's something I don't understand. It's never made clear in QM textbooks. Is spin always connected with the magnetic moment of a particle or can it simply represent part of the angular momentum that is concerned?
    I don't think he'd mind if I posted his response which reads
    My article was focused on electrons, although it can also be applied to other spin 1/2 particles, provided they are truly elementary, that is, without substructures. This excludes the neutron and all hadrons, because these particles are composites, consisting of quarks and gluons.

    The magnetic moment of the neutron arises from the internal charge distribution and currents inside the neutron. Recent scattering data actually indicate that most of the spin of neutrons and protons (about 60%) is contributed by the spin and orbital angular momentum of the gluons. But since the gluons are neutral, the magnetic moment must arise from the quarks, both from their spin and their orbital angular momentum. So it's not surprising that the neutron and the proton do not have the same magnetic moment as electrons--their "anomalous" values of magnetic moments are related to their composite internal structure.
    Since the photon has an EM field and a non-vanishing Poynting vector then this interpretation makes a lot of sense when addressing a circular polarized photon. Details of this can be found at http://en.wikipedia.org/wiki/Spin_an...entum_of_light
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  11. #11  
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    I just realize that there are elementary particles that have spin but which have no magnetic moment. A good example are neutrinos. Therefore the interpretation of spin as the angular momentum of energy in a particles field has its limitations.
    The most important thing to keep in mind is that you don't know everything and nobody else does either.
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    So let's break it down. In QFT spin is found by a symmetry of a field. In such that it's rotational symmetry has a certain degree.
    Basically it means that if I rotate my spin object twice about its rotating axis, and only then the rotated object equals the original object. Then my spin is a half.
    In other words, you'd have to rotate a spin 1/2 particle twice before it returns to its original position.

    If you think this weird, lay down two coins, and hold one fixed. Now rotate one around the other. You'll see that the coin has to rotate its own axis twice before it returns to its original position.
    That is why viewing spin as a 'rotation' is tricky. And Pauli knew this, so he didn't use it. Uhlenbeck and Goudsmit did, so they god the nobel prize because the idea of it rotating was more catchy. :P
    In the age of information, ignorance is a choice.
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