# Spin 360 rotation - experiment

1. Aug 4, 2009

### neworder1

Rotating an electron by 360 degrees causes its spinor to flip sing ($$\vert \psi \rangle \rightarrow -\vert \psi \rangle$$). Has this effect been observed experimentally (e.g. in an appropriate modification of the Stern-Gerlach experiment)?

2. Aug 4, 2009

### tiny-tim

Hi neworder1!
Wouldn't it be easier to send it along two paths, along one of which it is rotated in a magnetic field, then join them and see how they interfere?

(I assume it's been done, but I don't know)

3. Aug 4, 2009

### ytuab

For electrons, I think it is difficult, and I don't know that experiment.

For neutrons, there are some experiments in which the spinning neutrons went back to the original forms when they are rotated by an angle of 4pai (not 2pai).
(H. Rauch et al. Phys.Lett. 54A (1975) 425)

But in this real world, is it really possible?

In these experiments, they rotated the neutrons around the spin axis by using " precession".
The angular frequency of the precession is

$$\omega = \frac{g \mu_N}{\hbar} H$$

So, if spin g-factor becomes half, the angular frequency becomes half and the neutrons will go back to their original when they are rotated by 2pai (not 4pai).

There are some problems about spin.
For example, the electron spin g-factor is 2, so this means that the charge and mass of one electron is unequally distributed.
And the electron is too small by scatteing experiments, so by equating the spin angular momentum to 1/2 hbar, spinning sphere speed leads to more than 100 times the speed of light.

We can experimentally measure the (spin) magnetic moment.
If spin is simple circular movement like Bohr model, angular momentum becomes hbar (not 1/2 hbar) and (spin) g-factor becomes half (2 >> 1) (due to 2 x 1/2 = 1 x 1, the magnetic moment will not change).

Which case is more natural?