The reader's biggest challenge is to understand the complex material. If you're a lay person who's curious about time and the universe and also willing to accept that you're unlikely to comprehend all of the highly technical concepts being presented, then this book is probably for you. I enjoyed it and am glad I bought it.
If you're a hard-core scientific type who believes that scientists have no right to engage in speculative thought about the ultimate nature of the universe, then the predictions in the last few chapters may rub you the wrong way. Traditional scientific wisdom treats speculation unsupported by data as belonging to the domain of naïve amateurs. Accepted scientific thought is that such speculation should be avoided at all costs by 'real' scientists. To my mind though, the author (Dr. Sean Michael Carroll, Ph.D.) is to be commended for engaging in just this type of speculation. To a lay person like myself, it is intriguing to hear a well-respected theoretical physicist make specific "predictions" about the ultimate nature of our universe. ("Predictions" is a word Dr. Carroll chooses carefully.)
Even though Dr. Carroll does a fantastic job of formulating his explanations using everyday language rather than mathematical formulas, the challenge to the reader's intellect is still substantial. These include concepts such as Boltzmann brains, Maxwell's demon, de Sitter space, white holes, the Planck length, Schrodinger's Cat, the Heisenberg uncertainty principle, Poincaré's recurrence theorem and many, many more. Thought experiments are high on Dr. Carroll's list of epistemological techniques.
One of the most rewarding aspects of Dr. Carroll's writing is that he goes to great lengths to distinguish between what is known fact, what is unproven theory and what is speculation. He is equally at ease admitting what is not known about our universe as he is with explaining what is known. The book also follows a very orderly progression of thought processes. Entropy and its entanglement with the arrow of time is introduced early on and remains the central theme throughout the book. Initially entropy is explained in the context of the second law of classical thermodynamics. The reversibility or irreversibility of physical processes is considered next. Then comes general relativity, quantum mechanics and quantum gravity.
If this book were a work of non-fiction, this paragraph would probably be labeled as a 'spoiler'. So consider yourself warned. Throughout the book, Dr. Carroll shows how the arrow of time always points in the direction of increasing entropy. He leaves the reader with the idea that this is the number one clue to the ultimate nature of our universe. Likewise, he leaves the reader with the feeling that the number two clue is the unlikely fact that our currently observable universe is in a low entropy state that was even lower at the 'big bang'. Using these two observations as stepping stones, he goes on to predict that our observable universe is merely a bubble, or "baby universe" that has pinched off from a parent universe. Collectively, Dr. Carroll calls the parent universe and all the baby universes a "multiverse". In this context, he describes how the multiverse is compatible with all the principles of entropy, thermodynamics, relativity, quantum mechanics ad quantum gravity that he has already described in the rest of the book.
In summary, this book is not for the faint of heart. If you're not prepared for a cerebral workout, don't buy it. If you like an intellectual challenge and enjoy musing about the nature of time and space, then this book is probably for you.
Two closing observations: On the 'upside', the narration by Erik Synnestvedt is very well done. On the 'downside' the narrative often refers to graphs and tables that are not accessible from the audio book file. This can be rather frustrating. The graphics should be bundled into the audio file download or a web page containing these images should be created for people who have purchased the audiobook.
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