To understand entanglement, you must first accept that reality is unsure of itself at small scales. In 1927, Werner Heisenberg argued that it’s impossible to know both a particle’s position and momentum exactly. Measuring one simply makes the other fuzzier. Particles are fundamentally indecisive — neither precisely here nor there, and both at once. Making measurements effectively forces particles to choose how to behave. Einstein disliked this idea of a random universe; the atheist famously proclaimed his disbelief that God played at dice. (Quantum pioneer Niels Bohr supposedly replied, “Einstein, stop telling God what to do.”) So peeved was Einstein that he and his friends came up with a thought experiment to show how it was possible to learn both the positions and momentums of pairs of particles. Preserving their uncertainty would require one particle in the pair to instantly know and react when the other is measured — even at the other end of the universe. Every story about entanglement is obligated to include Einstein’s reaction to this apparent faster-than-light communication: “spooky action at a distance.” Unwilling to accept it, he declared quantum mechanics incomplete. In the end, though, Einstein would be undone by his own objection. “I like to think that he would have come around and accepted entanglement given the evidence today,” says Don Howard, a professor of philosophy at the University of Notre Dame.