In 1922 Stern and Gerlach sent a beam of silver atoms through a non-uniform magnetic field. Classical physics predicted one continuous smear; the plate showed exactly two spots. The awkward part comes with three analyzers in a row: measure along z, then along x, then along z again, and the last one still splits 50/50, because the x measurement erased everything known about z. In a uniform field the spin no longer splits but precesses about the axis at the Larmor frequency, proportional to the field strength.
In 1922 Stern and Gerlach sent a beam of silver atoms through a non-uniform magnetic field. Classical physics predicted one continuous smear; the plate showed exactly two spots. The awkward part comes with three analyzers in a row: measure along z, then along x, then along z again, and the last one still splits 50/50, because the x measurement erased everything known about z. In a uniform field the spin no longer splits but precesses about the axis at the Larmor frequency, proportional to the field strength.
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