principle, and Max Born’s overlapping probability waves, led Niels Bohr to develop the Copenhagen interpretation wherein spin directions and space itself are quantized. Supposedly,

particle spin dwells in a space of infinite parallel dimensions until observation causes the parallel existences to instantaneously collapse into a single observed state. However, a previously ignored three-dimensional classical mode of fast precession drives an initially quantized spin particle into quantized precession to produce the split-beam result of the Stern-Gerlach experiment. This obviates the need for any exotic explanation for “quantized spin space”.