Back in 1935, Albert Einstein had thrown down a gauntlet to the champions of quantum theory. He had realized that if quantum theory were true it should be possible for two photons separated to opposite sides of the Universe to influence each other. This was such a blatant contradiction of special relativity that he was sure that his EPR paper on this ‘quantum entanglement’ effect would spell the end of quantum physics and a return to sanity. Einstein was rarely wrong when it came to science, but this time he was wonderfully far off the mark.
Entanglement has become a hot topic in nuclear and particle physics, although many physicists are not sure they know what it means. We maintain that an era of understanding and using quantum mechanics on a dramatically new basis has arrived. We review a viewpoint that treats the subject as being primarily descriptive and completely free of the intellectual straitjackets and mysticism argued over long ago. Quantum probability is an extension of classical probability, but with universal uses. Density matrices describe systems where entanglement or its absence is a classification tool. Most of these have been known for decades, but there is a new way of understanding them that is liberated from the narrow outlook of the early days.