The (Often) Overlooked Experiment That Revealed the Quantum World

Once the experiment was running, producing any legible result was still a challenge. The collector plate was only a fraction of the size of a nail head, so reading the patterns in the silver deposit required a microscope. Perhaps apocryphally, the scientists inadvertently helped themselves out with questionable laboratory etiquette: The silver deposit would have been invisible if it weren’t for the smoke trickling in from their cigars, which — because of their low salaries — were inexpensive and rich in sulfur that helped the silver develop into visible jet-black silver sulfide. (In 2003, Friedrich and a colleague reenacted this episode and confirmed that the silver signal appeared only in the presence of cheap cigar smoke.)

The Spin of Silver

After many months of troubleshooting, Gerlach spent the entire night of February 7, 1922, shooting silver at the detector. The next morning, he and colleagues developed the plate and struck gold: a silver deposit neatly split in two, like a kiss from the quantum realm. Gerlach documented the result in a microphotograph and shipped it as a postcard to Bohr, along with the message: “We congratulate you on the confirmation of your theory.”

The finding shook the physics community. Albert Einstein called it “the most interesting achievement at this point” and nominated the team for a Nobel Prize. Isidor Rabi said the experiment “convinced me once and for all that … quantum phenomena required a completely new orientation.” Stern’s dreams of impugning quantum theory had obviously backfired, though he did not hold to his promise of quitting physics; instead, he won a Nobel Prize in 1943 for a subsequent discovery. “I still have objections to the … beauty of quantum mechanics,” Stern said, “but she is correct.”

Today, physicists recognize that Stern and Gerlach were right in interpreting their experiment as a corroboration of the still-nascent quantum theory. But they were right for the wrong reason. The scientists assumed that a silver atom’s split trajectory is defined by the orbit of its outermost electron, which is fixed at certain angles. In reality, the splitting is due to the quantization of the electron’s internal angular momentum — a quantity known as spin, which wouldn’t be discovered for a few more years. Serendipitously, the interpretation worked out because the researchers were saved by what Friedrich calls a “strange coincidence, this conspiracy of nature”: Two yet-unknown properties of the electron — its spin and its anomalous magnetic moment — happened to cancel out.

Cracking Eggs

The textbook explanation of the Stern-Gerlach experiment holds that as the silver atom travels, the electron isn’t spin-up or spin-down. It’s in a quantum mixture or “superposition” of those states. The atom takes both paths simultaneously. Only upon smashing into the detector is its state measured, its path fixed.

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.

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 — theoretically — to recombine those ghostly identities. Doing so would generate a particular interference pattern on a detector when they realigned — an indication that the atom indeed navigated both routes.

The grand challenge is that, to preserve superposition and generate that final interference signal, the personas must be split so smoothly and quickly that the two separated entities have wholly indistinguishable histories, no knowledge of the other, and no way of telling which path they took. In the 1980s, multiple theorists determined that splitting and recombining the electron’s identities with such perfection would be as unfeasible as reconstructing Humpty Dumpty after his great fall from the wall.