Stern–Gerlach experiment with photons?

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Discussion Overview

The discussion revolves around the application of the Stern–Gerlach experiment to photons, exploring the implications of photon spin, the behavior of neutrons and neutrinos in magnetic fields, and the use of alternative methods for measuring polarization.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that photons can have spin values of 1, 0, and -1, suggesting that three paths would be expected in a Stern-Gerlach-like experiment.
  • Others argue that photons are not affected by magnetic fields due to their lack of a magnetic dipole moment, emphasizing the distinction between spin and magnetic properties.
  • There is a discussion about why neutrons are affected by magnetic fields, attributed to their magnetic dipole moment as composite particles made of quarks.
  • Some participants clarify that photons always have spin 1, with projections of -1 and 1, and that the absence of rest mass prevents the existence of a 0 projection state.
  • A suggestion is made that using a calcite crystal could serve as an analogous experiment for measuring polarization in photons, similar to the Stern-Gerlach experiment for spin measurements.
  • Questions arise about the behavior of neutrons in a similar experiment, specifically whether their deflection would be due to spin or magnetic moment, and what would happen with neutrinos, which are neutral but have spin 1/2.
  • One participant corrects themselves regarding the behavior of neutrinos, noting that they have no magnetic dipole moment and would not interact with the apparatus in the same way as charged particles.
  • There is a recognition of confusion between neutrons and neutrinos in the discussion, leading to clarifications about their respective properties.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the Stern-Gerlach experiment to photons, with some asserting it is not feasible while others propose alternative methods. The discussion remains unresolved regarding the specifics of neutrino behavior and the implications of spin versus magnetic moment for neutrons.

Contextual Notes

Limitations include the lack of consensus on the applicability of the Stern-Gerlach experiment to photons and the nuances surrounding the properties of neutrinos and neutrons, particularly in relation to their magnetic moments and spin characteristics.

cragar
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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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cragar said:
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:

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

There is a difference between spin and magnetic dipole moment.
 
ok i see , thanks for the answer. Then why are neutrons affected by the magnetic field
 
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cragar said:
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.
 
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cragar said:
... 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.
 
cragar said:
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.
 
eaglelake said:
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'.
 
Frame Dragger said:
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
 
eaglelake said:
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.
 
  • #10
xlines said:
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 experiment with neutrinos what would we excpect to see , Neutrino's are neutral in charge but have a spin of 1/2
 
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  • #11
cragar said:
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 experiment 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 apparently ran with it an ignored that you were talking about neutrinos. The spin really should have given it away... sorry!
 
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  • #12
so a neutrino with no charge has a dipole moment , what gives it a dipole moment
 
  • #13
cragar said:
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/questions/neutrinospin.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.
 
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  • #14
Frame Dragger said:
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
 
  • #15
cragar said:
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.
 
  • #16
i see , thanks for your answers .
 
  • #17
cragar said:
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. :wink:
 

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