The textbook explanation of the Stern-Gerlach experiment holds that as the silver atom travels, the electron isn\u2019t spin-up or spin-down. It\u2019s in a quantum mixture or \u201csuperposition\u201d of those states. The atom takes both paths simultaneously. Only upon smashing into the detector is its state measured, its path fixed.<\/p>\n

But starting in the 1930s, many prominent theorists opted for an interpretation that required less quantum magic. The argument held that the magnetic field effectively measures each electron and defines its spin. The idea that each atom takes both paths at once is absurd and unnecessary, these critics argued.<\/p>\n

In theory, these two hypotheses could be tested. If each atom really did traverse the magnetic field with two personas, then it should be possible \u2014 theoretically \u2014 to recombine those ghostly identities. Doing so would generate a particular interference pattern on a detector when they realigned \u2014\u00a0an indication that the atom indeed navigated both routes.<\/p>\n