Fire single electrons at two slits and they still make a wave pattern.
Send particles one at a time through two narrow slits and each lands as a single dot. Yet as the dots build up, they form the striped pattern of waves interfering. Try to catch which slit each one used and the stripes vanish. Richard Feynman called it the heart of quantum mechanics.
In 1801 Thomas Young presented work to the Royal Society arguing that light is a wave, and his demonstrations of interference, including a description of light passing through two slits, challenged Isaac Newton's view that light was made of particles. When two sets of waves overlap they reinforce in some places and cancel in others, producing alternating bright and dark bands on a screen. It is still an open question whether Young ever actually performed the double-slit version himself.
The twist came with quantum physics. Light also arrives in discrete packets, photons, and in 1927 experiments showed that electrons, clearly particles, diffract like waves too. The effect has since been seen with atoms and with molecules of up to about 2,000 atoms. Turn the source down so only one particle is in flight at a time and each is detected as a single point on the screen. But the points are not random: over thousands of arrivals, they pile up into interference stripes. Each particle's probability of landing somewhere behaves like a wave that passed through both slits.
Watching changes the result. If detectors reveal which slit each particle went through, the interference pattern disappears and you get two plain bands, as if the particles were ordinary bullets. Later versions, including 'quantum eraser' experiments, showed that scrambling that which-way information can bring the stripes back. Obtaining partial information, a 1987 experiment found, only partly washes the pattern out.
The single-electron version was voted 'the most beautiful experiment' by readers of Physics World in 2002, and electron interference now underpins electron microscopy. Feynman said it contained 'the only mystery' of quantum mechanics, and liked to say that all of quantum theory could be drawn out by thinking carefully about this one experiment.
Source: Wikipedia — Double-slit experiment · Text summarised from Wikipedia (CC BY-SA 4.0)
