All in all, I found this book a bit too speculative for my tastes. I like my science books to be grounded on accepted facts, so for me this was only a two-star book, but perhaps not for you. In the author's own words (on page 243), "The developments described in Part II in chapters 11 through 18 are not yet fact and do not yet amount to a coherent theory." As such, I had to wonder if it was a bit premature to present ideas that are not yet coherent. I had the feeling that in a decade or so there might be enough data to determine which of the very many ideas contained in this book would be developed into a coherent theory and which would be discarded. Also, this book is replete with theories and ideas, so many that after a while I came to the conclusion that Professor Smolin was covering too much and that the book was not sufficiently focused. While not to my taste, I do recommend this book to those who liked "The Life of the Cosmos", and to those who like to be mystified by all the possible ways in which our universe can be explained, even if none of them are proven science. This book would also appeal to those most interested in the philosophy of scientific ideas, as opposed to science as a descriptor of experimental observations. Those, like myself, who like their science to be coherent and based on actual data would likely not find this book to their liking.
What is in the book -
Time in the context of this book is more philosophical than the parameter found in many of the equations utilized in physics, such as that which defines velocity. In this context, time is (or is not) something that transcends this operational definition, and the unreality of time refers to the idea that time itself is an illusion and is not real, not defining past and future, which may also be illusions. Professor Smolin's believes, and much of this book is aimed at showing, that time itself is real and that the rebirth of the title refers to his refutation of the idea that time is not real. My background and training (as an engineer and materials scientist) puts me squarely in the camp that treats time as real and integral to describing the world, so I never took much stock in the philosophical idea that time was somehow unreal, or dead and in need of rebirth. I found the philosophical aspects of the book, while somewhat interesting, were not my cup of tea.
As near as I can figure, the contention that time is not real stems from several ideas, which have been posited by numerous physicists. Two of these ideas are as follows: (1) Newton's laws describe a universe ruled his equations that describe the effects of a force on the movement of a body. With knowledge of all the initial conditions, Newton's laws enable one to determine the future evolution of a system and somehow this is interpreted as meaning that time in irrelevant, since the evolution of the system is preordained by these equations and the initial conditions. Furthermore, these equations are assumed to be timeless and thus time has no meaning because it is governed by something (the equation) that exists outside of time, in spite of the fact that time is one of the variables used in these equations. (2) Any system can be described in terms of configuration space, with the system moving from position to position, independent of time, in spite of the fact that time is one of the dimensions of this space and that time should enter as the rate at which one goes from position to position. These ideas are refuted in Part II of the book.
The meat of this book is contained in Part II, which occupies 2/3 of the total book. In essence, this book builds on the author's previous book "The Life of the Cosmos". He espouses the idea that, to quote the book, "Our universe is thus a descendant of another universe, born in one of its black holes, and every black hole in our universe is the seed of a new universe." This concept dispenses with the need to have very specific initial conditions for the universe as we know it to have been established. Instead of requiring specific initial conditions and specific laws, the conditions are allowed to evolve through successive black hole generations. The approach (Cosmological Natural Selection) eliminates many of the dilemmas posed by theories such as those employing an inflationary multiverse. Furthermore, Professor Smolin contends that this Cosmological Natural Selection approach is scientific in that it is capable of being proven wrong (the property of falsifiability), but so far has not been proven to be wrong (nor has it been proven to be correct). While wholly speculative, I did find some of these ideas interesting.
This part of the book also extends his ideas to the conundrums of quantum mechanics and he proposes a non-local hidden variable theory to explain them. I found this to be the most interesting part of the book. John Bell proved that hidden variable theories were not possible in quantum mechanics, except if they were non-local, so Smolin's theory is consistent with Bell's analysis. (If you are lost by my reference to non-locality and Bell's theorem, this may not be a book for you.) The book also discusses the second law of thermodynamics and entropy as "time's arrow", and why this proves that time is real.
Most of Part II is devoted to arguments as to why time is real, but that space many not be. This unreality of space is, among other things, used to explain how entangled particles are able to communicate faster than the speed of light in EPR experiments. I also found this to be an interesting part of the book as it gave an explanation (although not a proven one) for the experimental results that illustrate that entangled particles do indeed communicate with each other instantaneously, i.e. faster than the speed of light.