Stern–Gerlach experiment

Send spin-½ particles through sequential magnetic analysers and watch quantum measurement reshape the beam.

Unpolarised → SGzRunning

Spin +½Spin −½Blocked

Emitted0

Detected0

Blocked0

Positive0 · 0%

Negative0 · 0%

Born prediction50% / 50%

Outcome histogram

Detector impacts

Prepared spin on Bloch sphere

Two discrete results

A non-uniform magnetic field converts spin projection into two spatially separated beams. Spin-½ gives only +ħ/2 or −ħ/2 along the chosen axis—not a continuous range.

Measurement changes the state

After an outcome, the model collapses the spinor to that analyser’s eigenstate. Repeating the same axis is certain; measuring a perpendicular axis gives equal probabilities.

Incompatible questions

The sequence SGz+ → SGx+ → SGz restores a 50/50 z result. The intervening x measurement erases certainty about z, illustrating incompatible quantum observables.

Interpretation: Quantum spin is intrinsic angular momentum, not a little sphere physically rotating. Particle lines are schematic representative paths; detector statistics are sampled from the Born rule with a repeatable seeded generator.