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From: eric gisse on 18 Nov 2009 13:39
Jarek Duda wrote:

> Uncle Al, aluminum-27 and scandium-45 would be split in Stern-Gerlach
> - I have no problem with their nonzero spin.
> As You have said on the linked thread "Photons are inert to electric
> and magnetic fields in vacuum to at lest 10^9 gauss, lab on a small
> scale (particle accelerators' magnetic and electric detectors) and by
> observation of pulsars. "
> How to cope it with nonzero spin?
>
> eric gisse - if it's so obvious, please give me one good argument.

Polarization, and degrees of freedom in general. The number of modes
corresponds to the spin of the particle.


From: Jarek Duda on 18 Nov 2009 13:46
As I've written - changing electron's spin up to spin down can be also
made by just rotating it 180 deg - what photon has to do is just to
transmit angular momentum ... imagine lefthanded or righthanded swirl
behind marine propeller ...
From: Robert Higgins on 18 Nov 2009 15:05
On Nov 18, 1:46 pm, Jarek Duda <duda...(a)gmail.com> wrote:
> As I've written - changing electron's spin up to spin down can be also
> made by just rotating it 180 deg - what photon has to do is just to
> transmit angular momentum ... imagine lefthanded or righthanded swirl
> behind marine propeller ...

Spin "up" to spin "down" is usually strongly forbidden, so it has a
low probabliilty of occurance. Typically molar absorptivities for such
a process are 0.01 to 1.

BTW, photons DO transmit angular momentum. There are two types, spin
and orbital, each with different quantum numbers, depending on whether
they are fermion like electrons (halfpin integral spins) or bosons
like photons (integral spins). A simple and important process is the
absorption of a photon (spin=1) to an atom, causing an increase in
orbital angular momentum of 1. The photon is destroyed in the process,
and angular momentum (so long as you realize that spin and orbital
angular momenta can be interconverted) is conserved. These phenomena
are VERY well understood. More complicated processes involve the
coupling of spin and angular momenta.


From: Robert Higgins on 18 Nov 2009 15:08
On Nov 18, 1:46 pm, Jarek Duda <duda...(a)gmail.com> wrote:
> As I've written - changing electron's spin up to spin down can be also
> made by just rotating it 180 deg - what photon has to do is just to
> transmit angular momentum ... imagine lefthanded or righthanded swirl
> behind marine propeller ...

I should have added in my earlier post, that the idea of a spin being
"up" or "down" is a gross simplification. For simplicity, we often
consider just the z-component of the spin angular momentum (Sx), and
the total spin (S^2) agnular momentum, since they commute. By the HUP,
Sx, Sy, and Sz don't commute unless they are all identically 0.
From: Michael Moroney on 18 Nov 2009 15:18
Jarek Duda <dudajar(a)gmail.com> writes:

>As I've written - changing electron's spin up to spin down can be also
>made by just rotating it 180 deg - what photon has to do is just to
>transmit angular momentum ... imagine lefthanded or righthanded swirl
>behind marine propeller ...

Violates conservation of angular momentum.
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