What Dictates the Spin Direction of an Electron?

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Discussion Overview

The discussion revolves around the nature of electron spin, specifically what determines the direction of an electron's spin (+1/2 or -1/2) and whether this spin can change. Participants explore the intrinsic properties of spin, measurement techniques, and the implications of different coordinate systems.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that spin is an intrinsic property of particles, questioning what dictates whether an electron has +1/2 or -1/2 spin.
  • Others propose that the choice of convention for + and - spin is arbitrary, suggesting that nature does not favor one over the other.
  • Participants inquire about the differences between +1/2 and -1/2 spin and how these can be distinguished experimentally, referencing the Stern-Gerlach experiment.
  • There are questions regarding the static nature of spin and whether it can be flipped through interactions, with some suggesting that interactions with external fields can change the spin state.
  • The relationship between intrinsic spin and magnetic dipoles is discussed, with references to how dipoles can be measured through experimental setups.
  • Some participants mention that the choice of coordinate system can affect the representation of spin, indicating that observable quantities remain unchanged despite sign flips in different frames.

Areas of Agreement / Disagreement

Participants express a range of views on the nature of spin and its measurement, with no clear consensus on the fundamental reasons behind the direction of spin or the implications of coordinate systems. The discussion remains unresolved regarding the static versus dynamic nature of spin.

Contextual Notes

Limitations include the dependence on specific experimental setups for measuring spin and the unresolved nature of how different coordinate systems influence the interpretation of spin direction.

Ryan Reed
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Spin is an intrinsic property of particles meaning that they have it naturally. Electrons can have either +1/2 spin or -1/2, what dictates that it is one instead of the other?
 
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Ryan Reed said:
Spin is an intrinsic property of particles meaning that they have it naturally. Electrons can have either +1/2 spin or -1/2, what dictates that it is one instead of the other?

We do when we choose the convention for + vs -. Nature doesn't care which is which, it only knows that they are different.
 
Then what is the difference? How could one tell that the electron has a +1/2 or a -1/2?
 
Ryan Reed said:
Then what is the difference? How could one tell that the electron has a +1/2 or a -1/2?

Google for Stern-Gerlach experiment. Briefly, in an external magnetic field a magnetic dipole has an interaction energy ## H_d = - \vec{\mu}\cdot \vec{B}##. Spin up and down have opposite signs in this formula (if all else is equal), so you could conspire to make a measurement that would distinguish them.
 
And is the spin of the particle static, as in it doesn't change? Or can particle spin be flipped and changed?
 
Ryan Reed said:
And is the spin of the particle static, as in it doesn't change? Or can particle spin be flipped and changed?

If we have a particle that we know to be in a definite spin state, then the spin can be flipped if we interact with it. From the same interaction, we can imagine that we can apply a nonconstant field that will rotate the dipole. If the dipole has rotated enough to now have the negative projection onto what we have decided is the axis we are using to measure spin, then we would say that the spin has flipped. This is really a classical point of view though. In the quantum picture, we are adding a small bit of the spin down state to the quantum state when we turn on the field. So there is always a nonzero probability to find that the spin has flipped due to the applied field. Tuning the field can have the effect of increasing the probability that the spin flips.
 
How would you find these dipoles? I'm guessing that there is some way to tell since the magnetic field isn't distributed in the same way on the equator as on the poles.
 
Ryan Reed said:
How would you find these dipoles? I'm guessing that there is some way to tell since the magnetic field isn't distributed in the same way on the equator as on the poles.

A particle with intrinsic spin is already a magnetic dipole, with dipole moment proportional to the spin vector. One learns this from the Stern-Gerlach type experiments that I've already mentioned.
 
Thanks! This is exactly the kind of answers I was searching for!
 
  • #10
Ryan Reed said:
Electrons can have either +1/2 spin or -1/2, what dictates that it is one instead of the other?

Choice of coordinate system. Stand on your head, and everything flips sign, but the observable quantities remain unchanged.
 
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