Watch a Particle and It Changes

Two panels side by side. In the first, single electrons land on a screen one at a time and slowly draw a pattern of stripes. In the second, a gadget is added that watches which of two gaps each electron actually used, and the stripes turn into two plain clumps. A third panel names the popular explanation for why - and then, to its credit, calls that explanation a myth. This lesson is the sober version: what really kills the stripes, and why it has nothing to do with anyone looking.

Before you start. You need nothing more than the picture of ripples spreading out from two gaps in a harbour wall, the kind you get by throwing two stones into a pond at once. If you have ever watched two sets of ripples cross and seen patches where the water goes flat and patches where it jumps extra high, you already have the one physical idea this lesson runs on.

The idea in 30 seconds

Fire single electrons one at a time and, given enough of them, the dots still draw stripes - the signature of a wave, not a pebble.
Stripes come from two possibilities crossing and either reinforcing or cancelling: interference.
Between being fired and landing, an electron carries more than one route at once: superposition, tracked by a running tally called the wave function.
Add a gadget that can tell which route was used, and the stripes turn into two plain clumps: collapse.
What causes that is not being watched. It is decoherence - any physical contact that leaves a trace anywhere, read or not.
A hot glowing molecule with no detector nearby proved this: the stripes died from its own leaked heat, with nobody looking at all.

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