Stern-Gerlach experiment

In plain terms

An atom can behave like a tiny bar magnet, and an uneven magnetic field pushes a small magnet up or down by an amount that depends on how its axis is tilted relative to the field. Classical physics therefore predicted that a beam of atoms, their axes pointing every which way, would spread into a continuous smear, while the old quantum theory predicted discrete lumps instead, so Stern built the apparatus to referee.

The atoms split into two sharp beams, a point for quantisation, but the deeper shock is the second act. Take the upper beam from one magnet and pass it through a second magnet tilted at an angle. It splits again, and the original sorting is gone, so if you send the result through a third magnet aligned with the first, the atoms that were all found to be up no longer all come out up.

Measurements don't read off labels the atoms already carry, as asking a new question erases the answer to the old one. That's the finding underneath the finding, and it has never stopped being strange.

In CSD

The Stern-Gerlach experiment is the corpus's reference measurement, and the second act is the part that earns it the place. Discreteness is cheap, as any theory with the right list of allowed values delivers it. The unsorting is contextuality in its most concrete laboratory form, the dependence of an outcome on which question is asked and in what company, and a substrate account, one in which there's an underlying definite state, must produce that behaviour rather than excuse it.

CSD produces it structurally. The measurement context, here the orientation of the magnet, fixes which partition of the epistemic space the outcome is read against, that is, which way the space of quantum states is divided into outcome regions. The tilted magnet is a different context, hence a different partition, and a second sorting asks a different question of the same microstate, the underlying definite state, rather than reading out a stored answer to the old one. The forgetting isn't the decay of a label, as there was never a label, only a state and a context.

Mathematically

Spatial separation of beams according to the eigenvalues of the spin component along the field gradient, where sequential measurements along non-commuting axes fail to preserve earlier outcomes.

A historical footnote has content, as the experiment predates spin. It was designed for orbital angular momentum in the old quantum theory, and the right answer emerged from two errors cancelling, understood only years later.

The name

Otto Stern (1888-1969), from a Jewish grain-milling family at Sohrau in Upper Silesia, was Einstein's first pupil in Prague and followed him to Zurich before settling at Frankfurt, where the experiment ran in 1922. Walther Gerlach (1889-1979), from Biebrich, took his doctorate at Tubingen and was the experimental hand.

The beloved detail is documented, as the silver deposit was invisible until Stern's breath, heavy with sulphur from cheap cigars, developed it like a photograph. The aftermath is the era in miniature. Stern left in 1933 for Carnegie Tech in Pittsburgh and received the 1943 Nobel Prize, while Gerlach stayed, headed the Reich's uranium programme by the war's end, and spent 1945 interned at Farm Hall, where the captured transcripts record the German physicists learning of Hiroshima.

Related entries

• Malus's lawMalus's law is a cosine-squared rule discovered in 1808 with a crystal and a sunset, and it turns out to be the oldest data point for the Born rule.
• Bloch sphereThe states of the simplest quantum system form an ordinary ball, which is a lovely accident of low dimension and the single most misleading picture in quantum mechanics.
• Pointer basisA measuring apparatus doesn't record what you ask it to, as it records what it can keep, and what it can keep is decided by its interaction with the surroundings and not by the experimenter.
• Measurement flowA deterministic, volume-preserving flow carries out a measurement, and the volumes of its pointer blocks are the Born weights, for every preparation.
• Ramsey interferometryTwo brief nudges separated by a long stretch of doing nothing measure a frequency more sharply than any amount of continuous pushing, and every atomic clock runs on this trick.