Quantum mechanics is weird. The theory, which describes the workings of tiny particles and forces, notoriously made Albert Einstein so uneasy that in 1935 he and his colleagues claimed that it must be incomplete—it was too “spooky” to be real.
The trouble is that quantum physics seems to defy the common-sense notions of causality, locality and realism. For example, you know that the moon exists even when you’re not looking at it—that's realism. Causality tells us that if you flick a light switch, the bulb will illuminate. And thanks to a hard limit on the speed of light, if you flick a switch now, the related effect could not occur instantly a million light-years away according to locality. However, these principles break down in the quantum realm. Perhaps the most famous example is quantum entanglement, which says that particles on opposite sides of the universe can be intrinsically linked so that they share information instantly—an idea that made Einstein scoff.
But in 1964, physicist John Stewart Bell proved that quantum physics was in fact a complete and workable theory. His results, now called Bell’s Theorem, effectively proved that quantum properties like entanglement are as real as the moon, and today the bizarre behaviors of quantum systems are being harnessed for use in a variety of real-world applications. Here are five of the most intriguing: