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the double slit

wave · particle · detector
undetected — a wave pattern is building up
Fire a single particle at two slits, and it lands like a wave that went through both at once — building up a striped pattern over time. But detect which slit it actually uses — with any physical measurement, no one needs to be watching — and that pattern disappears, leaving two plain bands instead. It's the physical interaction that changes the result, not anyone's attention.