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Spin up or spin down

  1. Aug 4, 2015 #1
    Hello,
    I have read about Schrodinger's cat saying we do not know whether or not a particle has a spin up or spin down. It is only when we check it that we know which one it is.
    I understand the concept however I do not understand what do scientists mean when they say "spin up/down".
    What are 'spins'?
     
  2. jcsd
  3. Aug 4, 2015 #2

    jtbell

    User Avatar

    Staff: Mentor

    Spin is intrinsic angular momentum, the quantum-mechanical analog to the classical angular momentum of an object spinning around its own axis, like a spinning top or the Earth's daily rotation. Each elementary particle has a fixed amount (magnitude) of spin, which is a fundamental property of that particle, like its mass and electric charge.

    There are only certain allowable values for the magnitude of spin, namely ##S = \sqrt {s(s+1)} \hbar## where ##s## can have either positive integer or half-integer values, or zero. Electrons have ##s = 1/2## so ##S = \sqrt{3/4} \hbar##. (beware the distinction between lower-case ##s## and upper-case ##S##)

    Analogous to the way that we can orient a spinning top so its axis of rotation points in different directions, we can (loosely speaking) think of a particle's spin as being oriented in different directions. We describe this using the component of spin along a given reference direction which we customarily call the z-direction although it can actually be any direction we like. The z-component of spin is restricted to a set of values which depend on the magnitude of spin: ##S_z = m_s \hbar## where ##m_s## can have values from ##-s## to ##+s## in steps of 1. Electrons have ##s = 1/2## so they must have either ##m_s = -1/2## (##S_z = -\hbar/2##) or ##m_s = +1/2## (##S_z = +\hbar/2##). We call these two states "spin down" and "spin up".

    Unlike a particle's spin magnitude ##S## which is fixed, we can change ##S_z## ("flip the spin") by various methods.

    For further information see e.g. Wikipedia: https://en.wikipedia.org/wiki/Spin_(physics)
     
    Last edited: Aug 4, 2015
  4. Aug 4, 2015 #3
    Thank you so much for your help. I am now reading more about spins and quantum numbers. :)
     
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