What IS spin?

Discussion in 'Physics' started by magnet18, Feb 13, 2011.

  1. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Ive seen a lot of sites reference spin and i know its a property of subatomic particles, and certain molecules like fullerenes, but what physically IS it, are the particles actually spinning?

    Ive yet to find an answer to this anywhere...
     
  2. beenthere

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  3. Papabravo

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    In my view it does not hurt to consider that particles actually do spin. They have a component of angular momentum that is not due to their orbital motion. In quantum mechanics particles also have properties of waves. Quantum mechanics is all about using the statistical properties of a collection of particles to infer properties of the individual particles.

    The basic article is here:
    http://en.wikipedia.org/wiki/Spin_%28physics%29
    and the spin statistics article is here:
    http://en.wikipedia.org/wiki/Spin-statistics_theorem

    As your understanding of quantum physics becomes more sophisticated, it is convenient to let go of classical concepts. It is then easier to accept the intrinsic nature of their properties without reference to familiar concepts.
     
    Last edited: Feb 13, 2011
  4. Papabravo

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  5. someonesdad

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    It's unfortunately named, as it almost always makes a person think of a spinning ball in a classical context. It's a theoretical construct added to the formulation of quantum mechanics to explain experimentally-observed results. It has no classical analog (even though it behaves like angular momentum) and is one of those imponderables of the quantum world, just like the idea that some things exhibit both particle and wave characteristics. And these things are intimately and deeply tied up with the concept of observation and physical measurement. Their justification is that they lead to correct predictions.

    beenthere's link to that web page is nice -- it's an excellent web page.

    When I was a student, we'd come across the SO(3) group because it and the orthogonal group (among others) would pop out as symmetry groups of the differential equations we were studying. I had read about the double cover of SU(2) over SO(3), but never understood it physically. Ultimately, it's related to SO(3) not being a simply-connected group. You can read more e.g. here and here. And this stuff is tied up in relationships to spinors and the mathematical description of spin, and, ultimately to why there are bosons and fermions (I already knew why there are bozos though). Or so the theoreticians tell us. :p
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Just like the "colors" of quarks, the spin is a label. The colors were arbitrarily assigned to remind people that it was an abstraction. The same goes for "spin", the spin of particles needing to rotate twice to look the same, etc, is the best way a human brain can visualize "how it works".
     
  7. bertus

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    Hello,

    Do you mean the nuclear spin?
    That is also used in NMR (Nuclear Magnetic Resonance):
    http://en.wikipedia.org/wiki/Nuclear_magnetic_resonance
    Not all elements can be used for NMR.
    Ofthen certain isotopes of elements can be used due to the different spin properties.
    For instance C12 will not react with NMR, while C13 will work with NMR.

    Bertus
     
  8. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Bertus, not exactly what i was talking about, but looks interesting, thanks :)
    Thanks everyone, ill look into it...
    so much reading...
    Essentially i was wondering if it was related to physical spin, papabravo said yes, someonesdad said no...
     
  9. thatoneguy

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    Feb 19, 2009
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    Can a particle be a wave?

    Similar argument. If you set up the math abstractions and call a peculiar aspect "spin", and the math for it works out, then yes, "spin happens". Verified? No. What we may think of as spin can be some other relation in quantum mechanics, but we are using rudimentary labels so that the math works and an idea of what is happening can be described to match what is seen in colliders/"atom smashers".

    We are not much further into quantum mechanics now than Tesla and Edison were with electricity in their time. Some guesses and assumptions to make the math work are functional and fine. Others needed a bit of refinement. I believe the latter part is why some of Tesla's more "out there" ideas never came to fruition, as existing math explained it, but would find cases where the math 'broke' . The reason "wireless power from the ionosphere" isn't realized is that the frequencies involved are being used for millions of communications links, rather than relegated to white noise. Bouncing a signal off a satellite and receiving it on the other side of the world IS global wireless energy transmission, but with the added benefit that information is embedded into that energy.

    Until we get more info from the LHC and the tools to actually work at the subatomic level, spin and colors are mathematical constructs that do explain the results. We know the colors are arbitrary, but the spin? possibly.
     
  10. someonesdad

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    The notion is that an object like the electron with spin behaves like things with angular momentum. So you can see why papabravo said what he did -- it's a classical notion and perhaps pedagogically useful. But you have to also reconcile it with the fundamental fact that there is nothing there that is "spinning" in the classical sense. The electron is essentially a point particle. The other key notion, as pointed out on that lovely web page, is that the spin is quantized -- for electrons, it only comes in two flavors. How would you reconcile that with a classical viewpoint? A spinning golf ball, for example, can have a continuous spectrum of angular momentum values, both in magnitude and direction. Not so in the quantum world. That's why these quantum facts are so hard for us to grok -- they're not part of everyday experience. But they're extremely well-established experimental facts.
     
  11. magnet18

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    Dec 22, 2010
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    Soooo, long story short, we don't know.
    Correct?
     
  12. Papabravo

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    @someonesdad --That was very well put. As we started to learn physics the classical notions were helpful. At some point we had enough confidence in the experimental data and in the mathematics to let go of classical notions. It is worth noting that a common test for new theories is that they remain consistent with classical limits.

    @magnet18 -- What we know is that we have theories that are consistent with and provide explanations for experimental data. As we gather more data we can refine the theories and make new ones. What we have not done is found anything that would undermine the work of the previous century. There are still questions and mysteries and it is hoped by some that there is a grand unified field theory that will explain the four forces: gravity, electromagnetic, strong and weak. It is my understanding that we are not there yet.
     
    Last edited: Feb 14, 2011
  13. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Ive heard of it, the BIG TOE, theory of everything. :)
    so then this is one of the few fields where we still don't know whats going on, and there is still work to be done in understanding everything?
    *interest level rising...
    [edit]

    would does a bear $#!7 it the woods be a good reply?? ;)
    if i understand this one correctly, everything is vibrating, in large masses it is simply irrelevant, it becomes prominent on the quantum level.
     
  14. someonesdad

    Senior Member

    Jul 7, 2009
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    I guess it depends on your definition of "know". Science wants to "understand", but you must admit that when you scan over the efforts of the last few millenia is that it is inherently a descriptive discipline. Observed behaviors get codified in mathematical descriptions which get more sophisticated over time via generalizations, new notions, and unifications. They feed off each other too. For example, Lagrange and Hamilton formulated mechanics in new ways that let people attack problems they couldn't deal with before -- but they didn't really add any new physics (I'll waffle on the variational principles and Noether's theorems a bit). These formulations led to new things like field theories and the Schrodinger equation. But, at the heart of things, they're still descriptive -- i.e., they're used to make predictions about how things will behave.

    I still have trouble with "simple" things like mass and velocity. You multiply them together and get something called momentum, a very fundamentally important thing in both classical and quantum physics. But what is it "physically" (to echo the OP's original question)? What is mass? Pretty soon I begin jiggling like an epileptic jellyfish and retreat into my epistemological solipsism house of cards... :p

    My practical answer to your original question is "we don't know -- but we do know how to make some pretty good (but not perfect) predictions with what we don't know".
     
  15. debjit625

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    Apr 17, 2010
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    The wave associated with a particle is called a "Matter wave" or De Broglie wave" and its all around with any material objects but it only have significance in case of microscopic particals (at quantum level ).Notic Matter waves are different than Electromagnetic waves.Many people have confusion over this at first but they are not the same.

    By the way Louis De Broglie was the physicist who gave the equation of matter wave stuff,its interesting to see that the equation of matter wave was nothing but combination of two well known equations i.e.. Planck's quantum theory and Einstein's equation.

    We dont see the effect of matter wave over large(macroscopic) objects because,the wave's wavelength is too small to be noticed.On the other hand this matter wave have enough wave length to be observed by us over small (microscopic) particals at quantum level.

    Good luck
     
  16. Black-Bird

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    Jan 26, 2011
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    when the wave function of the electron = 0 , can we say the particle has a spin at this moment ? maybe the spin itself doesn't exist and the notation of spin is just a mathematical consequence of Quantum Mechanics?

    has anyone thought about that ?
     
  17. Papabravo

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    Feb 24, 2006
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    Wave functions are complex functions of time and space. The implication of a wave function with a value 0 would describe a place and time where the electron could not be. If it is not there and can't ever be there then why would we ever be interested in "there and then"

    In short I don't think a wave function with a 0 value is possible. I could be wrong, but I don't think so.

    In short, spin is an intrinsic feature of the solutions to the equations, and thus to the particles/waves they represent. We call it "spin" for convenience, not because anything is literally spinning. As I said in an earlier post it doesn't hurt to start with that mental picture, but be prepared to discard that notion as your mathematical and physical sophistication increases.

    In Quantum Chromodynamics we say that quarks have an intrinsic property called "color" and we assign the normal primary colors Red, Green and Blue. We do this not because they actually have those colors in any real sense, but for convenience to account for certain properties that are observed in experiments.

    http://en.wikipedia.org/wiki/Quantum_chromodynamics
     
    Last edited: Feb 15, 2011
  18. someonesdad

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    Jul 7, 2009
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    Actually, it does happen, for precisely the reason Papabravo states. The first Schrodinger equation problem the atomic physics student usually sees attacked after the free particle is the square potential well problem (and the related problem, the infinite potential well, also known as the particle in a box problem). The particle in a box problem is one of the few quantum problems that can be solved analytically. For the particle in a box, the spatial part of the wave functions are sinusoids of position, the quantum number, and the width of the box. So there are points inside the box where the wave function is zero and, thus, where you could not find the particle. This is distinctly different than the classical case, where the distribution function inside the potential well is just a constant.

    Papabravo, your last paragraph is perfect -- that is exactly what I was trying to say earlier, but didn't say it as well as you did. The justification for our models is that they can be used to predict what we observe experimentally. Experimental observation is the absolute essence of science.
     
  19. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    In relation to the particle in a box;
    Sorry I dont speak calculus, but what exactly prevents the particle from having infinite possibilities?
    I understand that this is the quantum level, where everything is quantized, but im gonna jump right to the base here and ask; why is everything quantized?

    And are you guys physicists?
     
  20. someonesdad

    Senior Member

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    I'm going to have to say that you'd best take a freshman physics class, then follow it with the atomic physics class usually taught to sophomores or juniors. The reason is that you'll also get (hopefully) some useful exposure to the experimental side of things, which is important to start developing mental models for this stuff. The best advice I can give you is pay deep attention to the experiments -- they are why we have the current view of the world that we do.

    As to why things are quantized, nobody knows. If you want to read a good discussion of this stuff, take a look at Feynman's description of the double slit experiment with electrons in his basic physics book (I think it's in the third volume, but I'm too lazy to go find my copy). It captures the quintessential mystery of quantum behavior. And, as Feynman points out, nobody "understands" this stuff -- the best people have been able to do is to describe the observed behavior.
     
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