As described in the Wikipedia article about the Stern-Gerlach experiment, if an electron passes through the (Z-axis oriented) device it will hit a screen on the other side in one of 2 possible locations, call them the upper and lower locations. Sometimes the electron is "spin up" about the z-axis and it hits the screen in the upper location, other times it is "spin down" and hits the screen in the lower location.(adsbygoogle = window.adsbygoogle || []).push({});

This seems to imply that the electron is in the spin up or spin down state about the z-axis, but why can't it be in the spin up state about the x-axis or in any of the other infinite number of states? I know that a measurement of the spin about the z-axis will cause the spin state to be up or down about the z-axis, but I don't see that measurement taking place. I agree that the measurement of the location of the electron when it hits the screen gives away information about the spin (and therefore constitutes a spin measurement), but I don't see why that measurement is about the Z-axis. Why can't the electron hit the screen in other locations that are consistent with other spin orientations?

Looks like the total magnitude of the spin of an electron is sqrt(3)*hbar/2 and, when measured, its spin about a particular axis is either +hbar/2 or -hbar/2 (so there is another component of magnitude hbar/sqrt(2) in the plane normal to the axis of measurement). So, for example, why can't the electron hit the screen with no z-axis deflection corresponding to maybe a state with hbar/2 about the x-axis and hbar/sqrt(2) about the y-axis?

Thanks

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