This book is a masterpiece of biographical writing and one that will stay with you long after you read it. Its lasting impact is not just because of Heisenberg's ground-breaking contributions to physics but because Cassidy fully engages the reader in the personal and moral issues surrounding Heisenberg's decision to stay in Nazi Germany and his eventual work on nuclear fission for Hitler's regime. There were times when I was in awe of Heisenberg's genius and times when I wanted to slam my fist down in frustration because of Heisenberg's self-induced isolation from the horrors around him. Heisenberg wanted to "preserve" theoretical physics in Germany and argued that science and politics were totally separate domains. Yet he wanted to do everything he could to help Germany win the war. Then, after the war, he formed numerous rationalizations and half-truths about how he was keeping atomic weapons away from Hitler. None of the evidence supports that claim and Cassidy does a thorough and very fair job of laying it all out. Cassidy's in-depth understanding of Heisenberg's personal history prevents him from making sweeping judgments about the man but he clearly and frequently points out Heisenberg's compromises and the often dubious reasons he gave for doing them. He shows in detail how Germany as a nation and as a culture became so deeply imbued in Heisenberg from youth on that he simply could not bring himself to leave or actively protest. And, as Cassidy shows, once Heisenberg initially accepted the regime and the first expulsion of Jewish colleagues from their jobs and their homes in 1933-34, it became just a little bit easier the next time to compromise again. In many ways the book is a study in self-deception but done with a profound and caring understanding of the subject. One reason this book is so powerful is that it is not simply a value judgment on Heisenberg by the "winning side" but a well-written study of the development of a person's character by an author who cares about his subject.
In 545 pages of text Cassidy spells out in detail the life of one of the most famous people in physics. As expected, a good deal of the book deals with Heisenberg's physics - both his own contributions and his relationships with other great physicists of the time. Almost all of the physics material is clear and understandable. I would estimate about 15-20% of the material on quantum mechanics is technical, including some of the most famous equations in physics. I skipped a few paragraphs but never lost the flow of the book. I don't think this should be held against the author since, as the book is a major resource on Heisenberg, Cassidy had an obligation to include some of the technical details for those with a background in physics. I never found these sections intrusive. Other than the sections involving the equations themselves, as a non-scientist I found Cassidy's explanations excellent. The book is a journey through the origins of quantum mechanics.
This large and detailed book is absolutely worth reading for anyone interested in the history of science in the 20th century. It also is worth the time for anyone interested in a careful character study of a brilliant human being who lived through one of the most repressive and destructive political regimes in human history. What Heisenberg did to compromise and survive in that environment, and why he did it, would make a fascinating book in itself. This book does all that and does it well while at the same time fleshing out the foundations of much of modern physics. I recommend this book in the highest terms.