MadSci Network: Physics

Re: How do electrons have spin?

Date: Tue May 19 12:15:34 1998
Posted By: Everett Rubel, Degree in Physics
Area of science: Physics
ID: 895366490.Ph


You are right in saying that there is a spatial orientation to the 
electron's spin.  However, the statement about the momentum should be 
clarified.  When we talk about electon spin, we are talking about angular 
momentum,  the momentum about a point in space.  When we know the radius 
from this point to the object(s) moving about the point, and the mass and 
velocity of the object(s), then we can calculate the angular momentum of 
the system. 
For electrons, we do have a mass.  However, we consider them to be point 
particles, so there is no value for a radius, so there is no way we can 
calculate velocity,( or frequency ), given the measured value for the 
angular momentum.  
The electron's spin is an intrinsic property of the electron as far as we 
know.  The axis of the spin defines the direction, according to the right 
hand rule.  If you have a vertically oriented spin for an electron that is 
spin up, and you drop the electron onto some object that absorbs the 
electron, that object will experience a torque in the counter clockwise 
direction when viewed from above.
It turns out that the mathematical formalism developed for handling angular 
momentun (spin) is also quite useful in describing various other more 
abstract quantities in the quantum domain.  When the formalism is adapted 
for use with such a quantity, people start talking about another new type 
of spin which may not have anything to do with angular momentum at all.  
One example is isospin, which is really just a bookkeeping thing that 
started out keeping track of proton and neutron interactions, and extends 
to help keep track of various other related particles.  It views the proton 
and neutron as just "up" and "down" states of some more abstract nucleon 

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