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Name:  Jack H.
Status: educator
Age: 60s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
Elementary particles have either a half or whole number spin. It is good to catagorize that way, but what is the spin? How is spin actually measured?


Replies:
Jack, Spin is a small amount of angular momentum "inside" a particle. It does not mean that the particle is spinning. It is an attribute that must exist in order to maintain "Conservation of Angular Momentum" at the level of individual particles. Experiments and calculations at the level of quantum mechanics indicate that angular momentum is transferred as integral multiples of a constant: h/(2*pi). We don't know why. It just happens to be so. A spin of one is an angular momentum of one times this constant.

Analysis indicates that particles have internal angular momentums equal to either an integer or a half-integer times this constant. Further analysis indicates that these two categories have very different properties:

Statistical analysis of integral spin particles by Bose and Einstein showed that there is no limit on how many can exist in one place at one time. They can overlap. These particles tend to be the ones that transfer energy: photons, gravitons, pions, etc. Such particles are called bosons.

Analysis of half-integral spin particles by Fermi and Dirac revealed that only one of each kind of fermion can exist in a specific state at a specific time. These particles cannot overlap. This is why atoms have orbits. At the lowest energy orbit of an atom, orbital angular momentum is zero. There are two ways for an electron to have a spin of 1/2: with a directional component of +1/2 or -1/2. As a result, there can only be two electrons in an atoms lowest orbit. Similar restrictions set the number of electrons in each orbit. Such particles tend to be those that form matter: protons, neutrons, electrons, quarks, etc. These are called fermions.

There really isn't room to go any deeper. You may find an introductory quantum mechanics book to be a source of interesting information.

Dr. Ken Mellendorf
Illinois Central College


Spin is a property of electrons and other atomic/molecular species that MUST be introduced into the quantum mechanical description of those species in order to bring the splitting of electronic spectral emissions and/or absorptions in the presence of electric and/or magnetic fields into agreement with the experimental observations. The term "spin" is borrowed from classical mechanical analogy, because the theoretical treatment follows a line of argument that would occur "if the electron, or whatever were spinning" but the analogy is strictly heuristic. It is a required property of the particles and one could attach any name to the effect. This was done with other properties of sub-atomic particles, e.g. the term "color" has no relation to the quantum mechanical property, it is just a name attached to the property.

I would recommend that you refer to an older text "Introduction to Quantum Mechanics: with chemical application" by Linus Pauling and E. Bright Wilson which gives a lucid description of spin from a perspective before it became overshadowed by the more elegant mathematical analysis of the property.

Vince Calder



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