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What is spin and why does is particle emission possible

  1. Nov 30, 2012 #1
    Don't know why i put "does is" in the title...oh well...

    Hey guys, i'm taking Physics in high school and have always been very interested in things such as quantum physics, black holes and the like, but there are a few things that i've been searching for but can't seem to find a non-technical explanation for.

    Can someone please explain to me what the 'spin' of a particle is. I know that particles can have integer or half-integer spins, but what is the difference. I assume that spin has to do with the actual spinning movement of a particle around it's axis, but can someone elaborate for me, please. For example, does a particle with a spin of 1 rotate twice as fast as a particle with a spin of 1/2?

    Also, what has always bothered me is how particle emission does not violate the Conservation of Energy law. Where does the energy of the particle come from? And I also can't seem to understand WHY particles are released. I assume this is different for different particles, however. Whenever they are described, the focus seems to be on what happens during the interaction rather than why the interaction happens in the first place.

    I really appreciate it :)
    Last edited: Nov 30, 2012
  2. jcsd
  3. Nov 30, 2012 #2


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    Spin, quite to the contrary, does not actually consist of the particle spinning around it's axis. The particle has properties that, classically, are the result of something spinning around an axis. At the quantum level particles are not solid little spheres spinning around, it is FAR more complicated than that. The spin of the particle is an intrinsic property of the particle itself, much like mass is. You cannot increase or decrease the spin at all. It will always stay the same. In fact, you cannot even associate a "speed" to this spin, as it is not a physical rotation.

    When we talk about spin 1/2, what we mean is that if we rotate the wavefunction describing the particle, we need to spin it around TWICE to get the particle back to it's "original" state. If we only do it once the wavefunction is actually opposite of what it was when we started the rotation. Spin 1 only requires one rotation to get back to the original state.

    The energy needed to create particles comes from either the mass of the decaying parent particle, or the energy of the system that creates it. For example, two protons that collide at high energies can release hundreds of other particles. The energy and mass required to create them comes from the enormous kinetic energy the two protons had at the time of the collision. As for WHY this happens, it's simply how the universe works. Particle physics can tell you HOW it happens and accurately predict what will happen, but we cannot explain WHY it happens other than to say it DOES happen.
  4. Nov 30, 2012 #3
    Ok, I think I understand the spin concept now, I just didn't consider the wave properties of matter. So it is like axial symmetry in math?

    However, where does the kinetic energy of the particles come from in the first place? What kind of machinery could accelerate a particle to near-light speed, the only thing i can think of is extreme magnetism. And I assume the reason that the protons emit those particles is because the energy can not be lost in any other way (such as deformation in a car accident)?
  5. Dec 1, 2012 #4
    These emitted particles have such low masses that it does not take much energy to accelerate them near the speed of light--see neutrinos for example. The kinetic energy of the particles comes from the rest mass of the parent particle. Mass is not conserved in particle decay--only energy is, which includes the rest mass energy of the particles. Since rest mass energies are huge, if you had a particle decay in which a little mass was lost, the resulting particles would have a lot of energy
  6. Dec 1, 2012 #5
    Electric field can accelerate particles. The field does not have to be extremely strong; it depends on how close to the speed of light you want to get. If 99% of light speed is enough you want, this is done every day in lots of hospitals all over the world on medical accelerators: they accelerate electrons to about 6 MeV (or more) energy on a device that fits into 1 room and is not using extreme fields.
  7. Dec 1, 2012 #6


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    The newly-produced particles in a decay process are not accelerated to their velocities. They are created with those velocities. At the moment of decay, the rest-energy (the energy corresponding to the mass) of the original decaying particle is distributed among the rest-energies and kinetic energies of the newly-produced particles. Total energy after the decay = total energy before the decay.

    Similarly for a collision process that produces new particles. The only difference is that the incoming particles have both rest-energy and kinetic energy. It all gets "thrown into a pot" so to speak, and distributed among the outgoing particles.
    Last edited: Dec 1, 2012
  8. Dec 1, 2012 #7


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    Don't lose sight of the basic fact that physics is invariant under a 360-degree rotation. Spinors don't violate this. Spinor wavefunctions are double-valued functions. That is, there's an implied ± sign in front. Both +ψ and -ψ represent the same state. It's true that if you rotate a spinor by 360 degrees, it changes sign, but this IS the original state.
  9. Dec 1, 2012 #8


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    I was unaware of that, as QM is not my strongest area. Thanks Bill!
  10. Dec 2, 2012 #9
    The way cartan has given the idea of spinors,there is an indeterminacy in the sign of spinors.So both signs are supposed to represent same state.
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