# Stern–Gerlach experiment with photons?

1. ### cragar

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 .

2. ### xlines

96
No, because photons are unaffected by gradient of magnetic field. I believe we discussed this three weeks ago. See:

There is a difference between spin and magnetic dipole moment.

3. ### cragar

ok i see , thanks for the answer. Then why are neutrons affected by the magnetic field

Last edited: Mar 29, 2010
4. ### xlines

96
They have magnetic dipole moment. Reason this is so is neutron is composite particle made of three charged quarks.

Last edited: Mar 29, 2010
5. ### xlines

96
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.

6. ### eaglelake

128
If you replace the Stern-Gerlach apparatus with a calcite crystal then you can do the analogous experiment with photons.

7. ### Frame Dragger

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'.

8. ### eaglelake

128
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

9. ### Frame Dragger

Hence the "fair enough" that preceeded everything I said. I take your point.

10. ### cragar

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

Last edited: Mar 29, 2010
11. ### Frame Dragger

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!

Last edited: Mar 30, 2010
12. ### cragar

so a neutrino with no charge has a dipole moment , what gives it a dipole moment

13. ### Frame Dragger

EDIT: Sorry, tired... I forget we're talking about a neutrino, not a neutron; In my post #11 I was thinking "neutron".

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.

Last edited: Mar 30, 2010
14. ### cragar

But the neutrino has no subparticles

15. ### Frame Dragger

Yeah, I'm in EST (Eastern Standard time aka GMT -5:00) so... yeah, I was just being stupid and sleepy. Sorry crager.