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

The behavior of matter (or energy) in space and time is described by the laws of physics, but the puzzling thing about physical reality is that space and time behave differently. Space is the same in all directions and it never changes, but time has preferred direction; past to future and the cause-effect relationship runs parallel to this. There is no such thing as special place (space) in the universe but there is a special time. This is a mystery because physical laws governing the fundamental particles are mostly time-symmetric (it can function thermodynamically or anti-thermodynamically), but the time-asymmetry observed in many macromolecular processes is thermodynamic and it has an arrow of time. Examples include, a glass bottle breaking into pieces or hot water becoming cold are attributed to the second law of thermodynamics which seem to set this arrow of time. Thus the physical reality is not only governed by laws of quantum physics and relativity, but also by the second law of thermodynamics which requires that the entropy (a measure of disorderliness) of a closed system, such as this universe, increase with time. This implies that the past has more order than future, hence the state of orderliness was probably the highest (or the entropy was the lowest) at the origin of the universe (big bang). The problem of justifying this arrow is not so much showing that the entropy of isolated systems increased, but explaining why there was low entropy in the past. While inflationary theory proposed by Alan Guth explains many key features of the early universe but it doesn't explain low entropy.

In this book, the author looks for clues in several areas such as, properties of black hole; information-loss paradox and Hawking radiation, string theory, inflationary epoch, multiverse cosmology and baby universes. He argues that a classical de Sitter background (mother space-time where vacuum energy is positive) does not fluctuate, but the space would be expanding and quantum fields will be fluctuating in a classical fashion. But if quantum gravity is taken into consideration then de Sitter space is itself susceptible to quantum fluctuations and this result in not only stretching and bending of spacetime as required by general relativity but also they could splice into multiple pieces. These pieces first appear as bubbles of spacetime, and then they grow and splice off to form baby universes. The baby universe created in a background de Sitter space is inclined both towards its past and to the future, but each baby universe starts in a dense low entropy state and exhibits a local arrow of time as it expands and cools. The baby universes born in the past have an arrow of time pointing in the opposite direction to those in the future, but for each universe, the time is directed towards increasing entropy and the multiverse manifests overall time symmetry. The author's hypothesis sharply contrasts the idea that big bang represents the boundary to space and time, and it dispels the notion that space and time were created at this time. He distances himself from other physicists like Larry Schulman who suggests that the universe switched to a highly ordered state at about 380,000 years when the universe became transparent to light (1, 2). The essential features of thermodynamics in the arrow of time are discussed by others which include mathematical physicist Roger Penrose (3), physicists Robert Wald (4), and Larry Schulman (5).

This is an excellent review of the concept of time in terms of physics, cosmology and philosophy. You need to have basic knowledge of physics to understand and appreciate the core ideas of the author. Chapters 12-15 are most interesting and the author discusses certain aspects of cosmology and black holes that are not relevant to physics of time but his discussions are well presented and it is very interesting to read. The main hypothesis of the author, about the arrow of time presented in chapter 15 is largely speculative and it is unlikely that physicists are convinced with his argument, however the debate will continue.

This is a book that explores the nature of time and in so doing takes the reader in some unexpected directions. It is also quite a hard book.

Although Sean Carroll throws in quips and touches of levity here and there, he is no Bill Bryson writing `A Short History of Almost Everything'. `From Eternity to Here' is a book you have to work at - but I found the effort worthwhile.

The author links the idea of time closely to that of entropy, the quality of orderliness in matter and in particular the Second Law of Thermodynamics: "The entropy of a closed system never decreases". Crude examples to illustrate the basic concept are the omelette (high entropy) that cannot be put back in the egg (low entropy), or the cream and coffee (low entropy) which when stirred together (high entropy) cannot be separated.

Extending this concept to the universe, it is believed to have started as a tiny something (low entropy) and has evolved to today's observable universe of stars and galaxies (higher entropy). This one-way process is seen as a analogue to time: it goes one way. We know about the past but we do not know about the future: it goes one way.

However this description of entropy I found counter-intuitive. Surely in the past the universe was high entropy with a primeval soup of basic particles and energy? While today is it not low entropy with stars, solar systems, galaxies and a sense of colossal order? It was not until page 166 that this paradox was explained.

"The culprit in this case is gravity. We're going to have a lot to say about how gravity wreaks havoc with our everyday notions of entropy..."

Aha! Now we begin to make progress. Dr Caroll's constant refrain is the question as to why the universe is relatively low entropy. He thinks a `natural' state would be space with an even spread of particles and energy with high entropy. Instead we have an organised universe as we have seen which is low entropy.
Dr Carroll takes the reader down some very strange meanderings to seek answers. To be fair he is attempting to explain complex issues by resorting to simple analogies and examples. Yet I found many of them so trite that I would have accepted the original complex argument on its face value without the having the sweat of relating analogy to argument.

Having given the Second Law a good run out, Carroll then turns to quantum field theory, quantum mechanics and much later on to quantum gravity. In the meantime we meet Stephen Hawking, the world's greatest expert on black holes. His discovery that black holes emit radiation transformed the understanding of how the universe could evolve. As black holes, of which there are believed to millions, including very large examples in the centre of each galaxy, suck in matter they become larger and suck in more. One theory is that eventually (trillions of years) all matter will have been absorbed into black holes. But as black holes are emitting radiation, then after even greater lengths of time, they will steadily dissipate into energy.

The end of the universe will simply be a vast energy field empty of all matter.
But there could be an epilogue. Because of how energy and matter behaves at the quantum level, random fluctuation might create every million or ten million years a node or bubble of false vacuum. It could split off from the main field and then:
"Now we have a baby universe...all set to undergo inflation and expand to a huge size. If the properties are just right...the energy will eventually be converted into ordinary matter and radiation, and we'll have a universe that evolves according to the inflation-plus-Big Bang story".

So finally we get to the concept of the multiverse, something so beyond possible demonstration or discovery that it will forever be a mystery. Yet it would resolve the issue of a low entropy origin - it was spawned from a high entropy field. And that's how time started.

Read the book and be absorbed.

Although I'm an engineer with my feet firmly on the ground, I like to ponder on the wider aspects of science and was attracted by the terrific title of this book. Unfortunately, I found its content a little disappointing.

The author's treatment of Schrödinger's cat, quantum entanglement and the basic concept of entropy I thought were very instructive. I wasn't completely unaware of entropy as the 2nd Law of Thermodynamics is part of basic engineering. But I had no idea that it was so important when considering the arrow of time. So this was somewhat revealing.

The problem is that a large part of the book deals specifically with entropy in its various forms. So much so, that after a while I found myself ploughing through the book, forcing myself to finish it. To me there are a few chapters in the middle of the book that are just not interesting enough. The book ends with some speculation about cosmology which I found more interesting even if very near the edge scientifically.

I've read a great many science books, many of which I haven't been able to put down and some of which I've read again. But I won't be doing that with this one, despite its attractive title!