The Double-Slit Experiment

Fire individual electrons at a barrier with two slits. They create an interference pattern — as if each electron went through both slits simultaneously. But observe which slit each electron passes through, and the pattern disappears. Particles behave like waves until you look at them, then they behave like particles. This experiment, first performed by Thomas Young in 1801 with light, has been replicated with electrons, atoms, and even molecules of 800 atoms. It fundamentally proves that reality changes based on observation. Let that sink in.