The double-slit experiment

In 1801, physicist Thomas Young conducted the first doubleslit experiment, shooting a beam of light towards a barrier with two slits in it. Instead of forming two lines on a screen behind the barrier – in the same way that particles might – the beam formed a pattern of interference as if a wave had been pushed through the two slits. In 1908, Geoffrey Ingram Taylor repeated the experiment using a single photon. Even though the photon was a single particle, the wave interference pattern still appeared. That was strange enough, but then it got really weird: When scientists tracked the individual particles as they moved through the slits, the monitored particles abandoned their wave-like state and showed up as two separate lines on the screen. It’s as if they knew they were being watched.