Re: Do free particles have spin?

From: Gregory L. Hansen (glhansen_at_steel.ucs.indiana.edu)
Date: 09/25/04 

Date: Sat, 25 Sep 2004 20:51:58 +0000 (UTC)

In article <ts-dnWF56rwROcjcRVn-rg@softcom.net>,
David Rutherford <drutherford@softcom.net> wrote:
>
>Gregory L. Hansen wrote:
>
>> In article <VqmdnTu5IONwUcncRVn-oQ@softcom.net>,
>> David Rutherford <drutherford@softcom.net> wrote:
>>
>>>Thanks Gregory. Do you know of any evidence in support of free charged
>>>particle spin?
>>
>> Neutrons are my thing, not charged particles. But Google has quite a lot
>> to say about "polarized electron beam".
>
>I searched Google under "polarized electron beam" and none of the links
>explained the difference between polarization and spin orientation. I
>know that, for a photon, they're different things. Is a polarized
>electron beam the same as a spin oriented electron beam? If so, how do
>you know if you are measuring polarization or spin?

To polarize something is to orient its spin. Back to the neutron world, a
neutron polarizer is also called a spin filter, because the beam is
polarized by removing the 50% whose spins are oriented in the wrong
direction.

Photons, with spin 1, have two orthogonal polarization directions, e.g.
vertical and horizontal. (They're massless, so they can't be
longitudinally polarized.) Neutrons, with spin 1/2, have two orthogonal
polarization directions, e.g. up and down. Why aren't the neutron
polarizations vertical and horizontal? Because that's not an orthogonal
basis for spin 1/2. The number of polarization directions for a spin 1/2
particle corresponds to the number of values m_z can take: -1/2 or 1/2.
The number for a massive spin 1 particle would correspond to m_z = -1, 0,
1.

Classically speaking, it's related to the way a field transforms.
Electromagnetism is a vector field, neutrons or electrons would correspond
to a spinor field. Gravity in general relativity is a tensor field that
would correspond to a graviton with spin 2. 

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Relevant Pages

  • Re: Do free particles have spin?
    ... >> Neutrons are my thing, ... >explained the difference between polarization and spin orientation. ... Photons, with spin 1, have two orthogonal polarization directions, e.g. ...
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  • Re: Narrow-mindedness and insolence of "Optics Connumications" editors
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  • Re: What keeps an electron from being "absorbed" by a proton?
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