From the record
Real cases, written from the published record. Predict first; the record and its sources come after.
1986 · Orsay
One photon, two paths
A half-silvered mirror lets half the light through and reflects half. Send light through two of them, with two paths between, and set it up so the paths recombine before two detectors, with the two paths made exactly equal in length.
Philippe Grangier, Gérard Roger and Alain Aspect sent single photons through such a setup, one at a time.
With both paths open, where did the single photons land?
What happened. Nearly every photon arrived at the same detector, even though each came through alone. Block one path and the clicks split half and half. Each photon behaves as if it took both paths and the two contributions add at one detector and cancel at the other.
Pause to think. If a photon can't split in half, what does "both paths" mean?
1922 · Frankfurt
Stern's two spots
Otto Stern and Walther Gerlach heated silver in an oven and let a thin beam of atoms fly between the poles of a magnet shaped to push them up or down, then onto a glass plate.
If each atom's tiny magnet could point any way, the beam should spread out evenly, from the top of its range to the bottom.
What built up on the plate?
What happened. Two spots. Each atom went up or down, never in between: the measurement gave only two answers. The deposit was so faint that it only showed after sulphur smoke from Stern's cheap cigar darkened the silver.
The first direct sight of space quantization: the atom's magnet could only point certain ways.