We can test this by switching off the left detector. It’s still there, so we would expect its influence to be pretty much the same. But now, with the detector present but switched off, the interference pattern builds up on the screen once again! All the atoms going through the experiment have gone back to behaving as waves. How is it that atoms behaved as particles when the detector over the left slit was switched on, but as soon as it was switched off they behaved like waves? How does a particle going through the right slit know that the detector over the left slit is switched on or off?
