Observer effect primer
What makes something an observer, and what is still unexplained once you know?
Map / survey
Send particles through two slits and they pile up in stripes. Let anything at all keep a record of which slit each one used, and the stripes fade. That record is the observer. No mind is needed, only a trace left somewhere in the world.
What nobody has explained yet is the next step: why you see one outcome, with odds set by the Born rule (the chance of a result equals the wave's height squared).
Turn up the record, watch the stripes go
D = 0.00
left · source, two slits, and a detector that records the path with distinguishability D (0 = no record, 1 = perfect record) · right · where the hits landed. The dashed curve is the prediction with visibility V = √(1 − D²), the best a clean record allows. V is also estimated live from the hits.
In plain words
Decoherence explains why records kill interference. Once the path is copied into a detector, an air molecule, or a stray photon, the two routes no longer overlap, and the stripes have nothing to build from. The inequality V² + D² ≤ 1 says it with numbers: the more readable the record, the dimmer the stripes.
Three popular stories are wrong. Consciousness is not required. The effect is not just the probe bumping the particle. It is not the uncertainty principle either.
What is left open is the single outcome. Maudlin's trilemma lists three claims: the wavefunction is the whole story, it always evolves smoothly, and measurements have one definite result. Every interpretation drops one. Only objective collapse, which drops smooth evolution, predicts different lab results, so it is the testable one.
Prior artA survey page. The citations are on the primer itself.
Next clickQuantum Darwinism, done later: see the Darwinism explainer.