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What is Spin, Exactly?

  1. Mar 21, 2010 #1


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    I am having trouble understanding what this phenomenon is. The spin of an electron is 1/2 - but I don't understand what this is trying to say. What is 1/2? I know the electron isn't actually 'spinning'... I've heard the term "it's a kind of intrinsic angular momentum" a lot, but that doesn't really help. What is the difference between a spin of 1 and a spin of 1/2? What does it do? Why is it important?

  2. jcsd
  3. Mar 22, 2010 #2
    What is charge? It is an intrinsic property of fundamental paticles that can be measured by the particle's response to an electromagnetic field. What is inertial mass? It is an intrinsic property of a macroscopic body that can be measured by determining it's acceleration in response to a known force. What is spin? It's a property of a particle that can be measured by performing Stern-Gelasch experiments or Spin Resolved Photoemission experiments on the surface of a metal.

    The thing is all of these physical quantities are at there basis, something that can be measured. We can also talk about our mathematical mapping of these physical quantities to a theoretical representation. For example, mass and charge are scalar quantities represented by real numbers (charge has the additional constraint that it has to be an integer multiple of the electron charge - this constraint is experimentally derived).

    Spin must be represented a bit more abstractly. The spin degree of freedom of a particle (let's take a spin-1/2 particle for simplicity) is represented in an abstract vector space. The state of the electron (or electron ensemble depending on your interpretational preference) can be represented in a two-dimensional space. Why you ask? Well when we put a spin-1/2 through a Stern-Gelasch apparatus (wiki it) you get out two beams and only two beams. So the particles have some degree of freedom that only takes two values. That's spin.
  4. Mar 22, 2010 #3

    Char. Limit

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