N o other scientific theory can match the depth, range, and accuracy of quantum mechanics. It sheds light on deep theoretical questions — such as why matter doesn’t collapse — and abounds with practical applications — transistors, lasers, MRI scans. It has been validated by empirical tests with astonishing precision, comparable to predicting the distance between Los Angeles and New York to within the width of a human hair.

And no other theory is so weird: Light, electrons, and other fundamental constituents of the world sometimes behave as waves, spread out over space, and other times as particles, each localized to a certain place. These models are incompatible, and which one the world seems to reveal will be determined by what question is asked of it. The uncertainty principle says that trying to measure one property of an object more precisely will make measurements of other properties less precise. And the dominant interpretation of quantum mechanics says that those properties don’t even exist until they’re observed — the observation is what brings them about.