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

  • Sean Carroll
Dutton: 2010. 448 pp. $26.95 9780525951339 | ISBN: 978-0-5259-5133-9

Why does time run forwards, not backwards? In his much-anticipated book, theoretical physicist Sean Carroll offers an explanation that unifies thermodynamics and cosmology.

Carroll is a good citizen of the scientific blogosphere. A regular Internet pundit — writing first on his own blog, The Preposterous Universe, but more recently on Cosmic Variance, a collective blog hosted by Discover magazine — he opines on topics from physics and philosophy to religion, poker, baseball and coffee. Carroll's easy and engaging style has attracted a mass of followers, and opened up a lively forum for debate about science.

Carroll suggests that universes such as ours continually pop into existence within the multiverse.

Carroll's research focuses on the arrow of time. The Austrian physicist Ludwig Boltzmann supplied the standard explanation that time's advance is due to the second law of thermodynamics and the growth of disorder, or entropy. As Carroll points out, this merely shifts the problem. The question becomes: why did the entropy of the Universe begin so low? The answer to that involves gravity, space-time and multiverses, and Carroll brings these jigsaw pieces together to explain it.

The narrative is colloquial and jolly, but From Eternity to Here reads more like an extended essay than a popular physics book. It is largely devoid of anecdotes and potted biographies of famous scientists, and the pages convey the high density of material that you would find in a Scientific American article. Carroll has a point to make, and must cover a lot of concepts to make it. The book is a modern incarnation of the sort of works that twentieth-century physicists such as Arthur Eddington, Erwin Schrödinger and Werner Heisenberg wrote late in their lives — venerable, erudite accounts with a new, possibly esoteric idea to put across.

Time is a clever unifying theme. Using its role in relativity, quantum mechanics, philosophy and thermodynamics to pull together many developments of modern physics, Carroll brings the reader back to focus on his big question. This trick gives the book a sense of purpose and prevents the prose from becoming too glutinous. Carroll's excellent description of Boltzmann's towering achievements and the limitations of his proof is sharp. The explanation of attempts to study time travel in general relativity is on a par with Kip Thorne's masterful Black Holes and Time Warps (W. W. Norton, 1994).

Carroll's solution for the arrow of time invokes the multiverse, a controversial concept that is a current battleground in theoretical physics. Arguably a prediction from cosmology and string theory, and to some extent quantum mechanics, the multiverse idea supposes that the Universe that we perceive is but one of a countless collection, each of which can be in a different physical state. A case can be made that our Universe looks the way it does because it is the only one of the many possible universes that can harbour us. For example, in another universe in which the constants of physics were slightly different from those in ours, life would not be possible.

Going further, Carroll adapts the multiverse idea to explain the directionality of time. Universes such as ours, he suggests, continually pop into existence within the multiverse, which is itself in thermal stasis, with no sense of time. We just happen to live in one of these baby universes, which started off in a low-entropy state and has an entropic arrow of time.

Carroll rightly relegates his speculative proposal to the final few pages. It remains to be seen how his provocative conclusion will be received within the physics community, but multiverse explanations are fashionable so it will garner interest. As a device to end the book's digression from relativity to the quantum, it works. From Eternity to Here is an engrossing, well-crafted introduction to the Universe and the foundations of modern physics.