From Eternity to Here: The Quest for the Ultimate Theory of Time ,

New York
, 2010. $26.95 (438 pp.). ISBN 978-0-525-95133-9

Many cosmologists seek theories in which the early universe’s properties emerge naturally or generically from the dynamical laws that govern cosmic evolution. Inflation is often held up as an exemplar of such a theory. However, the thermodynamic arrow of time tells us that the entropy of the early universe was lower than the entropy today, which suggests that the state of the early universe was not dynamically typical or natural. So how does one reconcile inflation theory with the second law of thermodynamics?

That is a defining question of From Eternity to Here: The Quest for the Ultimate Theory of Time by Caltech theoretical cosmologist Sean Carroll. He asserts, and I agree wholeheartedly, that this reconciliation lies at the core of modern cosmology research and should be addressed head on. He also recognizes the wide appeal of the inflation theory-thermodynamics puzzle and ambitiously reaches out to a very broad audience—much broader than the readership of Physics Today.

From Eternity to Here covers a comprehensive range of topics related to the arrow of time and the universe: Poincaré recurrences, entropy, ergodicity, coarse graining, decoherence, closed timelike curves, holography, cosmic inflation, eternal inflation, and many more. Carroll assumes his readers have no prior knowledge of those focuses and provides introductory material on special and general relativity, quantum mechanics, quantum field theory, and other topics. His writing style is relaxed and conversational, with quotes from the movies Annie Hall and Ghostbusters and chatty reflections on the historical progression of science. The arrow of time is a topic susceptible to purely philosophical musings; Carroll runs them through the “does it really matter?” filter that is familiar and reassuring to physicists.

Most readers will need to make compromises to enjoy this book. Advanced physicists will want to skim past much of the introductory material and may wish they could trade the conversational reflections for the dry and efficient style of professional physics writing. They may also wish for more technical depth on the topics they’re most interested in. Less expert readers will appreciate the introductory material but may not have the appetite for the many technical topics that follow. Still, Carroll gives some wonderfully intuitive discussions of difficult material, and the nonexpert will benefit much more from this book than one might guess considering the large number of challenging topics treated. The book may work well for a first-year graduate student: Its relaxed and intellectually adventurous style would provide a welcome contrast to the demanding technical problem-solving of the curriculum, and the student would learn about interesting areas of physics not generally covered in graduate courses.

Carroll keeps the reader aware of connections between the topics he chooses and current cosmology research, but his discussion of contemporary research is rather brief. Most of the book considers well-established material. Research on the arrow of time currently has more unresolved questions than concrete results; Carroll faithfully reports that fact and argues that it is therefore most important to treat such fundamental ingredients as Poincaré recurrences or decoherence, which are traditionally absent from the physics curriculum. On that point, I agree. Cosmologists will find places where they disagree with Carroll on modern research topics, but that is a natural consequence of problems being unresolved. For example, I regard Carroll’s willingness to dismiss the finite-universe scenario as a mistake. In many ways that scenario, I believe, offers the most promising path to reconciliation.

The relationship between Carroll’s book and others on related topics is addressed quite nicely in a paragraph preceding the extensive bibliography. Publicity material on the jacket sleeve and press release contrasts From Eternity to Here with Stephen Hawking’s A Brief History of Time (Bantam, 1998). Those are indeed two very different books. Carroll’s emphasis is almost entirely on the thermodynamic arrow of time in cosmology, whereas Hawking gives much less emphasis to that topic, and removes the corresponding chapter entirely in his newer A Briefer History of Time (Bantam, 2008). Still, many will bristle, as I did, at the suggestion in From Eternity to Here’s publicity material that Hawking, in contrast to Carroll, does not appreciate the importance of asking what was before the Big Bang.

From Eternity to Here is an important book that explores topics related to the arrow of time and explains why they are essential to current cosmology research. Carroll’s ambitious effort to reach and challenge both a lay and technical audience can feel awkward in places. Still, I expect many Physics Today readers will find the book both provocative and rewarding and will be glad to recommend it to less technically trained individuals who hunger for a window on exciting problems in science.