Over the past century, scientists have made huge strides in understanding the mind-bending rules that govern the microworld of atoms and subatomic particles. But these rules, called quantum mechanics, contain one particularly bizarre, unexplained phenomenon: quantum entanglement. Imagine two subatomic particles that mirror changes in each other instantaneously over any distance – apparently without communicating. Einstein called it “spooky action at a distance” and although no one can explain how it works, scientists have already begun to harness its power. So-called quantum computing could give rise to computers that can shatter existing data encryption in minutes – and use the laws of physics to create a totally secure, unhackable network. But even as researchers are poised to transform the digital world with entanglement, a few doubts about it remain, and to rule them out will take a ground-breaking experiment. NOVA takes you to a frigid mountaintop in the Canary Islands where physicists will use massive telescopes to catch light from quasars at opposite ends of the universe to control detectors in a record-breaking experiment that might settle the remaining questions once and for all.