Spin and Measurement
Name: Jack H.
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
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
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.
Click here to return to the Physics Archives
Update: June 2012