The double-slit experiment

In 1801 Thomas Young shone light through two narrow slits onto a screen. Instead of two bright lines, he saw many stripes, light and dark. That is the signature of waves: where the waves from the two slits meet crest to crest they add up, and where crest meets trough they cancel.

Nearly two hundred years later, in 1989, Akira Tonomura and his team at Hitachi in Japan did something even stranger. They fired electrons, tiny particles, through a double slit one at a time. Each electron landed as a single dot on the screen, exactly like a particle.

But as thousands of dots built up, the same stripes appeared. Each electron, travelling alone, seemed to have gone through both slits and interfered with itself.

So the scientists put a detector at the slits to catch each electron in the act and see which slit it really used. The moment they did, the stripes disappeared. Just two plain bands, as if the electrons knew they were being watched.