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Author Topic: Photons, Spin, Polarisation and stuff  (Read 3566 times)

Offline Solvay_1927

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Photons, Spin, Polarisation and stuff
« on: 01/08/2005 13:19:23 »
Iím sure Iíve read somewhere (but I canít remember where) that there is a connection between the polarisation of light (or EM waves more generally) and the spin of the photons.  Can anyone explain this to me (or point me in the direction of an appropriate website or book, say)?

Also, I understand that photons have a spin number of 1, so they can have spin states of +1, 0, or Ė1.  But how can you tell what spin state a photon is in?  What different properties do the different spin states have and what different phenomena do they give rise to?  What gives rise to the different spin states in the first place and are all three equally likely to occur in nature?  (Or am I just displaying a complete lack of understanding of what spin really is? Or of physics, for that matter.)

Hereís the speculative bit:  Could the spin states of photons be used to explain physical phenomena such as the difference between attractive and repulsive forces between charged particles?  (This thought is based on my confusion re: how can the same photon, when exchanged in an EM interaction, cause two different effects Ė repulsion between same-charge particles but attraction between opposite-charged ones.)

Any views, anyone?  Can anyone "shed any light" on this? (Sorry, excuse the pun.)

Thanks.
Solvay.


 

Offline gsmollin

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Re: Photons, Spin, Polarisation and stuff
« Reply #1 on: 02/08/2005 03:37:25 »
Real photons have spin of +1 or -1. The spin axis is always parallel to the velocity vector, so the photon spin will appear to be left handed or right handed. A photon of a given spin has angular momentum, and this angular momentum can be measured. If you had photons of -1 or +1 spin, you would have right or left circularly polarized light. With 50% +1, and 50% -1, you have a plane polarized wave. I suppose this is one aspect of wave-particle duality, where the polarization of the electromagnetic wave corresponds to the phase function of the Schrodinger equation describing the photon.

There is always more reading on the subject. Here is an interesting thread archived from another forum:
http://www.physicsforums.com/archive/t-41358_photon_spin.html

The repulsive or attractive forces between charged particles involve the exchange of virtual photons. I would have to study this question further. If I understand it, you are asking how the exchange of virtual photons is different between particles of like charge and particles of opposite charge, i.e., how the appropriate mechanical force or repulsion or attraction is manifested through the field.
« Last Edit: 02/08/2005 04:03:29 by gsmollin »
 

Offline David Sparkman

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Re: Photons, Spin, Polarisation and stuff
« Reply #2 on: 19/08/2005 05:09:41 »
The waves also have a polorization as in the wave has a width but no thickness. Hence polorized lenses work, and the sun reflected off of your car hood er Bonnet for you Brits, is partially polorized.

Light is a facinating study. They are doing experiments with individual photons on wave patterns that are coming up with answers right out of the Asylem (one photon knows what the other photon is doing no matter what the seperation), slowing photons down to a stand still, and other mysterious tricks that beg the question what is a photon?

David
 

Offline Solvay_1927

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Re: Photons, Spin, Polarisation and stuff
« Reply #3 on: 06/09/2005 14:32:19 »
Thanks for the replies.  (I would have thanked you sooner, but I've been away on hols / out of action for some weeks.)

I've now got off my ar*e and done some proper searching on this myself, and found a few websites that give lots more very useful detail.  So if there's anyone else out there who wants to know more detail about the physics of light, they might like to browse among the following:

http://hep.ph.liv.ac.uk/~hutchcroft/Phys258/index.html

http://www.abdn.ac.uk/physics/px2013/

Cheers.
Solvay.

P.S. gsmollin - yes, i am asking "how the exchange of virtual photons is different between particles of like charge and particles of opposite charge, i.e., how the appropriate mechanical force or repulsion or attraction is manifested through the field".  I've not yet found any useful websites to help with this point.  Have you any ideas?
« Last Edit: 06/09/2005 15:01:58 by Solvay_1927 »
 

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Re: Photons, Spin, Polarisation and stuff
« Reply #3 on: 06/09/2005 14:32:19 »

 

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