- #1

- 51

- 3

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- #1

- 51

- 3

- #2

fzero

Science Advisor

Homework Helper

Gold Member

- 3,119

- 289

We do when we choose the convention for + vs -. Nature doesn't care which is which, it only knows that they are different.

- #3

- 51

- 3

Then what is the difference? How could one tell that the electron has a +1/2 or a -1/2?

- #4

fzero

Science Advisor

Homework Helper

Gold Member

- 3,119

- 289

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.

- #5

- 51

- 3

- #6

fzero

Science Advisor

Homework Helper

Gold Member

- 3,119

- 289

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.

- #7

- 51

- 3

- #8

fzero

Science Advisor

Homework Helper

Gold Member

- 3,119

- 289

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.

- #9

- 51

- 3

Thanks! This is exactly the kind of answers I was searching for!

- #10

Vanadium 50

Staff Emeritus

Science Advisor

Education Advisor

- 26,936

- 10,740

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.

Share: