The Double-Slit Experiment

The double-slit experiment challenges the very fabric of reality—fire individual electrons at a barrier with two slits, and 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, with 100% accuracy in detection. 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 in 2013, making it 800 times larger than the 1-atom tests of 1974. It's slower than the average quantum effect but fundamentally proves that reality changes based on observation.