Re: Do free particles have spin?

From: EjP (nospam_at_hackers.are.bad)
Date: 09/27/04 

Date: Mon, 27 Sep 2004 14:03:30 -0500

David Rutherford wrote:

>
>
> EjP wrote:
>
>> David Rutherford wrote:
>>
>>> Is there any evidence for the existence of particle spin for free
>>> particles, that is, for particles not bound to atoms? If so, please
>>> give references or links.
>>>
>>
>>
>> The SLC and SLAC made and studied polarized electron beams for
>> several years:
>> http://mitbates.mit.edu/pesp2002/talks/Clendenin_SLACPES.ppt
>>
>> Muon spin precessions are studied in detail in g-2 experiments:
>> http://www.g-2.bnl.gov/index.shtml
>>
>> Muon and tau spin is verified by their decay distributions (start
>> by reading a decent textbook, like Halzen and Martin, then reading
>> the about the corresponding measurments).
>>
>> Compton scattering is highly dependent on the polarization states
>> of both the target particle and the photon beam.
>> (Google "Compton Polarimetry" = 296 hits).
>>
>> The list of confirmations of particle spin is almost literally
>> endless. Perhaps if you ask a more specific question, someone
>> can give you a better answer.
>
>
> I'm still not convinced that polarization and spin are the same thing.
> What are the directions of the spin and polarization of an electron in
> quantum mechanics?
>

For Fermions, like elections, polarization is basically the net
average spin direction of a group of fermions. The relationship is a
little more complex for photons because polarization is defined in terms
of the electric field vector, which, while related to the spin
state, is not synonymous. There's a bit of a discussion here:
http://www.mathpages.com/rr/s9-04/9-04.htm

-E

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