What is Spin 1/2: Definition and 107 Discussions

In quantum mechanics, spin is an intrinsic property of all elementary particles. All known fermions, the particles that constitute ordinary matter, have a spin of 1/2. The spin number describes how many symmetrical facets a particle has in one full rotation; a spin of 1/2 means that the particle must be rotated by two full turns (through 720°) before it has the same configuration as when it started.
Particles having net spin 1/2 include the proton, neutron, electron, neutrino, and quarks. The dynamics of spin-1/2 objects cannot be accurately described using classical physics; they are among the simplest systems which require quantum mechanics to describe them. As such, the study of the behavior of spin-1/2 systems forms a central part of quantum mechanics.

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  1. T

    Problem with a uuu particle with spin 1/2?

    A baryon comprising uuu (delta++, spin 3/2) exists albeit briefly giving rise to evidence for colour as an additional quantum number, but what's the problem with a uuu particle with spin 1/2? Why can't the individual quarks be RGB, giving a colourless result with the spins being +,+,- to give...
  2. quasar987

    Hamiltonian of a spin 1/2 particle in a constant mag. field

    Why is there no kinetic energy term in said hamiltonian? Suppose I take a magnetic dipole in my hand, and I throw it in the field. Then surely its classical energy is E = p²/2m - \vec{\mu} \cdot \vec{B}. Then why is the p²/2m term absent in the hamiltonian?
  3. K

    Two Spin 1/2 Particles: Stot=0 Outcome?

    I have a hypothetical situation that I am trying to work through... Say there are two spin 1/2 particles, and the system is known to be in a total spin state of Stot=0. An observer comes along and determines the first particle to have a spin component (S1z) of hbar/2. Now say another...
  4. C

    Is Spin 1/2 Really That Difficult to Understand?

    Why people say that to understand spin is difficult because there in no a classical analogous...bla bla bla? we got an OBSERVABLE, S associated with the physical quantity S, why all that confusion about the not classical analogous, isn't S the physical quantity associated with the observable...
  5. humanino

    Proving Spin 1/2 of Nucleons: Theory & Experiment

    How well are we certain that nucleon have spin one half ? I see several experimental answers. I would like to have theoretical arguments, considering the fact that nucleons are not fundamental particles. How well can one apply lorentz symmetry to the free neutron which is not stable ? :confused:
  6. A

    Spin 1/2 Particles: Understand Complex Physics

    Well, I just read about particles which you have to spin around twice to get back to their original form (If you spin it 360 degrees it won't be the same...) Now, this is sortof counter-intuitive so I need a bit of clarification. The closest I could get to explaining this in my mind was a...
  7. S

    Is a mobius strip spin 1/2 object?

    Remember Phiysicist always say electron has a spin of 1/2; I can't remember how it was derived? But I noticed a mobius strip exihibit interesting attribute. Can we consider a mobius strip a spin 1/2 object?
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