# Entanglement

## he Unlikely Story of How Scientists, Mathematicians, and Philosophers Proved Einstein's Spookiest Theory

From the bestselling author of **Fermat's Last Theorem**, the story of a group of scientists who set out to finish what Einstein started

From the bestselling author of Fermat's Last Theorem, the story of a group of scientists who set out to finish what Einstein started Can two particles become inextricably linked, so that a change in one is instantly reflected in its counterpart, even if a universe separates them? Albert Einstein's work suggested it was possible, but it was too bizarre, and too contrary to how we then understood space and time, for him to prove. No one could. Until now. Entanglement tells the astounding story of the scientists who set out to complete Einstein's work. With accesible language and a highly entertaining tone, Amir Aczel shows us a world where the improbable-from unbreakable codes to teleportation-becomes possible.
"Alas, to wear the mantle of Galileo it is not enough that you be persecuted by an unkind establishment, you must also be right." —Robert Park Is it possible that something that happens here will As these scientists studied such effects, and produced definitive evidence that entanglement is a reality, they have also discovered other, equally perplexing, aspects of the phenomenon. Imagine Alice and Bob, two happily married people. WhileAlice is away on a business trip, Bob meets Carol, who is married to Dave. Dave is also away at that time, on the other side of the world and nowhere near any of the other three. Bob and Carol become entangled with each other; they forget their respective spouses and now strongly feel that they are meant to stay a couple forever. Mysteriously, Alice and Dave-who have never met-are now also entangled with each other. They suddenly share things that married people do, without ever having met. If you substitute for the people in this story particles labeled A, B, C, and D, then the bizarre outcome above actually occurs. If particles A and B are entangled, and so are C with D, then we can entangle the separated particles A and D by passing B and C through an apparatus that entangles them together. Using entanglement, the state of a particle can also be In such futuristic applications of technology, the entanglement is often extended to more than two particles. It is possible to create triples of particles, for example, such that all three are 100% correlated with each other-whatever happens to one particle causes a similar instantaneous change in the other two. The three entities are thus inexorably interlinked, wherever they may be. One day in 1968, physicist Abner Shimony was sitting in his office at Boston University. His attention was pulled, as if by a mysterious force, to a paper that had appeared two years earlier in a little-known physics journal. Its author was John Bell, an Irish physicist working in Geneva. Shimony was one of very few people who had both the ability and the desire to truly understand Bell's ideas. He knew that Bell's theorem, as explained and proved in the paper, allowed for the possibility of testing whether two particles, located far apart from each other, could act in concert. Shimony had just been asked by a fellow professor at Boston University, Charles Willis, if he would be willing to direct a new doctoral student, Michael Horne, in a thesis on statistical mechanics. Shimony agreed to see the student, but was not eager to take on a Ph.D. student in his first year of teaching at Boston University. In any case, he said, he had no good problem to suggest in statistical mechanics. But, thinking that Horne might find a problem in the foundations of quantum mechanics interesting, he handed him Bell's paper. As Shimony put it, "Horne was bright enough to see quickly that Bell's problem was interesting." Michael Horne took Bell's paper home to study, and began work on the design of an experiment that would use Bell's theorem. Unbeknownst to the two physicists in Boston, at Columbia University in New York, John F. Clauser was reading the same paper by Bell. He, too, was mysteriously drawn to the problem suggested by Bell, and recognized the opportunity for an actual experiment. Clauser had read the paper by Einstein, Podolsky, and Rosen, and thought that their suggestion was very plausible. Bell's theorem showed a discrepancy between quantum mechanics and the "local hidden variables" interpretation of quantum mechanics offered by Einstein and his colleagues as an alternative to the "incomplete" quantum theory, and Clauser was excited about the possibility of an experiment exploiting this discrepancy. Clauser was skeptical, but he couldn't resist testing Bell's predictions. He was a graduate student, and everyone he talked to told him to leave it alone, to get his Ph.D., and not to dabble in science fiction. But Clauser knew better. The key to quantum mechanics was hidden within Bell's paper, and Clauser was determined to find it. Across the Atlantic, a few years later, Alain Aspect was feverishly working in his lab in the basement of the Center for Research on Optics of the University of Paris in Orsay. He was racing to construct an ingenious experiment: one that would prove that two photons, at two opposite sides of his lab, could instantaneously affect each other. Aspect was led to his ideas by the same abstruse paper by John Bell. In Geneva, Nicholas Gisin met John Bell, read his papers and was also thinking about Bell's ideas. He, too, was in the race to find an answer to the same crucial question: a question that had deep implications about the very nature of reality. But we are getting ahead of ourselves. The story of Bell's ideas, which goes back to a suggestion made thirty-five years earlier by Albert Einstein, has its origins in humanity's quest for knowledge of the physical world. And in order to truly understand these deep ideas, we must return to the past. 1. A Mysterious Force of Harmony 2. Before the Beginning 3. Thomas Young's Experiment 4. Planck's Constant 5. The Copenhagen School 6. De Broglie's Pilot Waves 7. Schrodinger and His Equation 8. Heisenberg's Microscope 9. Wheeler's Cat 10. The Hungarian Mathematician 11. Enter Einstein 12. Bohm and Aharanov 13. John Bell's Theorem 14. The Dream of Clauser, Horne, and Shimony 15. Alain Aspect 16. Laser Guns 17. Triple Entanglement 18. The Ten-Kilometer Experiment 19. Teleportation: "Beam Me Up, Scotty" 20. Quantum Magic: What Does It All Mean? Acknowledgments Notes References Index [ Entanglement is] perhaps the best lay description of the evolution and current state of quantum physics available today. (Focus) |

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