### Quantum: Einstein, Bohr and the Great Debate About the Nature of Reality by Manjit Kumar

*Quantum*is an excellent historical overview of the development of quantum theory and the personalities and people involved in addition to providing useful explanations of complicated concepts involved in quantum physics.

The book is generally accessible but it would likely be beneficial to have read a quantum primer, prior to reading

*Quantum*, to get more out of the work (and perhaps the rest of this brief review).
At the heart of the book, and emphasized in the title, is the debate between Niels Bohr and Albert Einstein about the nature of (quantum) reality. Bohr did not really believe in a quantum world per se, but frame quantum theory as an abstract description of reality - particles do not have properties until they are examined and forced to have properties (i.e., the Copenhagen interpretation). Bohr had his theory drive his philosophical position.

Alternatively, Einstein had a deep-seated belief in a causal, observer-independent reality. As such, he disliked quantum mechanics and sought to undermine it in some way. He first attempted to demonstrate it was inconsistent through ingenious thought experiments that taxed Bohr and friends for days, weeks or months. When that failed, Einstein attempted to show that quantum mechanics was not a full description of reality (i.e., Einstein would concede it was 'correct' but not that it was 'complete').

While

*Quantum*centers around Bohr and Einstein, it provides sufficient detail on the usual suspects such that one learns of great rivalries, like between Heisenberg and Schrödinger, and even conflict within individuals as Bohr was initially reluctant to follow quantum theory down its rabbit hole of peculiarities, but then became one its greatest proponents.
What else? Maxwell, Planck, Born, Einstein, Bohr, Heisenberg, Schrödinger, Dirac, EPR paper, Bell's Theorem, Von Neumann, Young's double-slit, particle-wave duality, entanglement vs. the speed of light (locality violations?), Germany and German scientists affected by the wars, the difficulty of a demarcation line between the micro and macro worlds, how it took painstaking years of math and hard work to figure any of this out and how everything is so extremely complicated. It was fascinating to think of all these exceedingly brilliant people disagreeing with each other, and further, to hear of one having a mathematical insight that the others could not have had (e.g., the maths Schrödinger and Dirac were critical).

Additionally, I appreciated learning that the Copenhagen interpretation became dogma for decades, likely because Bohr and his students spread out over the world and advanced their interpretation (and many younger physicists thought the matter was settled and it was waste of time to revisit philosophical musing that are not easily resolved). Yet, the newer generation doesn't feel the same as the pioneers and the Copenhagen interpretation doesn't have the same majority support. What does? Perhaps no one interpretation but several and many simply saying they do not know or are unsure. The point: one of the most verified and useful theories of science does not resist on a generally agreed upon interpretation of reality. Isn't that interesting?

Anyone with a curiousity in the historical, scientific, philosophical and personal issues surrounding quantum theory will enjoy this book.

Highly recommended.

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