Entanglement [Paperback]

This is one of the best books I've read this year. It's easy to read, highly informative, accurate and fun. Aczel has done a masterful job of combining a book on scientific history with an introductory tomb explaining one of the especially non-intuitive aspects of the quantum world - the entanglement of multiple quantum particles.

Entanglement as a consequence of quantum mechanics was actually predicted by Einstein and used in a thought experiment to try and discredit the new theory. Einstein believed in strict determinism and considered quantum mechanics to be incomplete. He had a lifelong friendship and debate with Bohr, who was one of the founders of quantum theory. Einstein, along with Podolsky and Rosen developed a "thought experiment" in which the outcome was so weird - if quantum physics was correct - that it simply couldn't be accepted (by Einstein, anyway). Einstein considered this weird outcome in quantum mechanics to essentially prove that it was incomplete.

In Einstein's thought experiment he imagined two entangled quantum particles whose quantum properties were dependent upon each other. For example, the particles could be photons produced by a reaction that starts out with zero spin. Since spin is conserved, and photons have integral spin, if one photon has spin +1 the other must have spin -1.

Quantum mechanics says that, until the particles are measured, their spins are in a superposition of states, and when one photon's spin is measured, the other photon instantly assumes the opposite spin - no matter how far apart the two are. Indeed, before they are measured, quantum mechanics doesn't treat the two photons as distinctly different particles at all. Before the measurement (before collapse of the wave function) they are a superposition - something for which there is no classical analog.

Quantum mechanics is thus non-local, and Einstein thought the result of this thought experiment, weird as it is, left no other option but to conclude that quantum mechanics is incomplete. The thought experiment proposed by Einstein, Podolsky, and Rosen was a serious challenge to Bohr and other scientists who subscribed to the Copenhagen view on quantum mechanics.

Years passed after Einstein proposed this experiment, and it remained technically out of reach. Later, the idea was refined by John Bell who showed that the central logical question in the EPR experiment can be reduced to an inequality that can be measured in actual experiments.

This is the story of how scientists built upon Einstein's initial idea of entanglement and used Bell's ideas to construct practical experiments to test one of the strangest ideas in quantum mechanics.

Einstein thought the non-local and fundamentally unpredictable nature of quantum mechanics was too weird to be true, but when scientists finally managed to test entangled particles, they found that they really do behave just as quantum physics said they would. Quantum mechanics really *is* weird, but only because it describes the universe, and the universe is weird.

This is a nice little book that does a good job of weaving the history with the science. I thoroughly enjoyed reading it, and highly recommend it.

Excellent history showing how much of the quantum weirdness was discovered. I really enjoyed the personal stories of the scientists that made these discoveries. However, not enough detail was given when it came to EPR and how the results pointed to non-local reality. Otherwise, it was a great read and well worth the money!

I should begin by saying that I was expecting (or hoping for) a different book, though perhaps from the other book by Aczel that I have read (Mystery of the Aleph), my expectations were probably misplaced. The book that I was hoping for would have been much more technical, though given the fact that only a handful of equations appeared in the book at all, this would not be difficult), and one that would explain what this entanglement thing is, or at least provide arguments for some of the prevailing theories.

What this book did provide, though, was a brief account of the history of entanglement as a controversial physical concept. I first encountered entanglement while doing some studies in quantum computation, and my studies were on the computer science/mathematical side, which basically meant that entanglement was a given, and it never really occurred to me that there would have been much controversy --- in retrospect, this was quite naive of me. By going through the breakthroughs made by many physicists over the passed century, Aczel was able to bring light to the fact that while science textbooks state principles as undeniable truths, doing science and interpreting science are more akin to a somewhat political struggle. For this reason, there is much to commend this book.

However, a great shortcomming is the length. The book is divided into 20 chapters with an average length of about 12 short pages. Most chapters have a two-fold purpose --- to introduce and give a brief biographical sketch (leaning more towards intellectual development) of someone involved in the history of entanglement, and also to explain briefly what that person did. Due to the length, it is impossible to provide much detail of either the person(s) introduced or how the result fits into the overall development of our understanding of the quantum world. The only results that seemed to permeate the book were the paper by Einstein, Podolsky and Rosen which introduced the concept as an argument against quantum physics, and John Bell's theorem which provided a theoretical mechanism to determine whether Einstein or quantum physics is correct.

After reading this book, I am looking forward to going through more books listed in the References, in the hopes of finding the book I want.