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Stern–Gerlach experiment with photons?

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cragar
#1
Mar29-10, 03:38 AM
P: 2,466
If we did this experiment with photons , the photon can have a spin of 1, 0 , -1 ,
right , so then we would expect to see 3 paths correct .
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xlines
#2
Mar29-10, 04:30 AM
P: 96
Quote Quote by cragar View Post
If we did this experiment with photons , the photon can have a spin of 1, 0 , -1 ,
right , so then we would expect to see 3 paths correct .
No, because photons are unaffected by gradient of magnetic field. I believe we discussed this three weeks ago. See:

http://www.physicsforums.com/showthread.php?t=385607

There is a difference between spin and magnetic dipole moment.
cragar
#3
Mar29-10, 04:35 AM
P: 2,466
ok i see , thanks for the answer. Then why are neutrons affected by the magnetic field

xlines
#4
Mar29-10, 04:49 AM
P: 96
Stern–Gerlach experiment with photons?

Quote Quote by cragar View Post
ok i see , thanks for the answer. Then why are neutrons affected by the magnetic field
They have magnetic dipole moment. Reason this is so is neutron is composite particle made of three charged quarks.
xlines
#5
Mar29-10, 06:10 AM
P: 96
Quote Quote by cragar View Post
... the photon can have a spin of 1, 0 , -1 , right , so then we would expect to see 3 paths correct .
And, just to be clear - photons always have spin 1. They can have different projections of spin 1 and -1. If it was ordinary particle, it would also have 0 projection, but it doesn't. Long story short: lack of rest mass removes that state. If photons had 0 projection state, that would be seen as longitudinal polarized light.
eaglelake
#6
Mar29-10, 11:56 AM
P: 128
Quote Quote by cragar View Post
If we did this experiment with photons , the photon can have a spin of 1, 0 , -1 ,
right , so then we would expect to see 3 paths correct .
If you replace the Stern-Gerlach apparatus with a calcite crystal then you can do the analogous experiment with photons.
Frame Dragger
#7
Mar29-10, 12:46 PM
P: 1,540
Quote Quote by eaglelake View Post
If you replace the Stern-Gerlach apparatus with a calcite crystal then you can do the analogous experiment with photons.
Fair enough, but what would be the point? With AgBr (or some other silver) -> Silver Halide you're exploring the photographic process. With photons you'd be doing some kind of bastardized version of better experimental apparatus'.
eaglelake
#8
Mar29-10, 02:58 PM
P: 128
Quote Quote by Frame Dragger View Post
Fair enough, but what would be the point? With AgBr (or some other silver) -> Silver Halide you're exploring the photographic process. With photons you'd be doing some kind of bastardized version of better experimental apparatus'.
I thought that the original post had to do with doing the
Stern-Gerlach experiment with photons. As pointed out, we cannot do the Stern-Gerlach experiment with photons. I am only pointing out that, just as Stern-Gerlach magnets are used to measure spin, Calcite crystals can be used to measure polarization. As you know, spin 1/2 particles have two eigenvalues, as does polarization. The Stern-Gerlach (Calcite crystal) experiment has two output channels, one for each possible value the spin (polarization). The physics of the two experiments is almost identical. If you understand one, then you understand the other. That's all I was trying to say!
Best wishes
Frame Dragger
#9
Mar29-10, 04:40 PM
P: 1,540
Quote Quote by eaglelake View Post
I thought that the original post had to do with doing the
Stern-Gerlach experiment with photons. As pointed out, we cannot do the Stern-Gerlach experiment with photons. I am only pointing out that, just as Stern-Gerlach magnets are used to measure spin, Calcite crystals can be used to measure polarization. As you know, spin 1/2 particles have two eigenvalues, as does polarization. The Stern-Gerlach (Calcite crystal) experiment has two output channels, one for each possible value the spin (polarization). The physics of the two experiments is almost identical. If you understand one, then you understand the other. That's all I was trying to say!
Best wishes
Hence the "fair enough" that preceeded everything I said. I take your point.
cragar
#10
Mar29-10, 07:47 PM
P: 2,466
Quote Quote by xlines View Post
They have magnetic dipole moment. Reason this is so is neutron is composite particle made of three charged quarks.
ok so if we did this experiment with neutrons , then would we see two different paths for the neutrons , and would the neutrons deflect because of their spin or because of their magnetic moment .
And if we did this expirment with neutrinos what would we excpect to see , Neutrino's are neutral in charge but have a spin of 1/2
Frame Dragger
#11
Mar30-10, 02:45 AM
P: 1,540
Quote Quote by cragar View Post
ok so if we did this experiment with neutrons , then would we see two different paths for the neutrons , and would the neutrons deflect because of their spin or because of their magnetic moment .
And if we did this expirment with neutrinos what would we excpect to see , Neutrino's are neutral in charge but have a spin of 1/2
WRONG>They would behave as a silver atom would, determined by their dipole moment, not their spin.<WRONG

EDIT: Ok... that's true for neutrons, which is what I read in the first sentence, then apparantly ran with it an ignored that you were talking about neutrinos. The spin really should have given it away... sorry!
cragar
#12
Mar30-10, 04:33 AM
P: 2,466
so a neutrino with no charge has a dipole moment , what gives it a dipole moment
Frame Dragger
#13
Mar30-10, 05:07 AM
P: 1,540
Quote Quote by cragar View Post
so a neutrino with no charge has a dipole moment , what gives it a dipole moment
EDIT: Sorry, tired... I forget we're talking about a neutrino, not a neutron; In my post #11 I was thinking "neutron".

Here is the answer to your question about neutrinos: http://www.fnal.gov/pub/inquiring/qu...trinospin.html

So, yes, their spin can be determined, but they have no magnetic dipole moment, but maybe a gravitoelectric one. and would fly past the apparatus as they would pass through the Earth.

Truly sorry, I shouldn't comment when tired... I tend to miss things like "-ino" at the end of "neutron" and I probably could have spared you confusion.
cragar
#14
Mar30-10, 05:09 AM
P: 2,466
Quote Quote by Frame Dragger View Post
See post #4. If you want to do a bit of research you could take a crack at QCD, a fasciniting subject, but maybe basic E&M first.
But the neutrino has no subparticles
Frame Dragger
#15
Mar30-10, 05:21 AM
P: 1,540
Quote Quote by cragar View Post
But the neutrino has no subparticles
Yeah, I'm in EST (Eastern Standard time aka GMT -5:00) so... yeah, I was just being stupid and sleepy. Sorry crager.
cragar
#16
Mar30-10, 05:26 AM
P: 2,466
i see , thanks for your answers .
Frame Dragger
#17
Mar30-10, 05:32 AM
P: 1,540
Quote Quote by cragar View Post
i see , thanks for your answers .
Yeah, pity I couldn't have just made it an "answer" (singular) and not contradictory ones relating to completely different particles!

Thanks for letting me off easy.


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