The quest to test quantum entanglement

Quantum entanglement, doubted by Einstein, has passed increasingly stringent tests. Over 12 billion years ago, speeding particles of light left an extremely luminous celestial object called a quasar and began a long journey toward a planet that did not yet exist. More than 4 billion years later, more photons left another quasar for a similar trek. As Earth and its solar system formed, life evolved, and humans began to study physics, the particles continued on their way. Ultimately, they landed i...