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4.0 out of 5 stars The Arrow of Time, 24 Feb. 2012
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This review is from: From Eternity to Here: The Quest for the Ultimate Theory of Time (Paperback)
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.
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Initial post: 28 Jun 2014 04:43:37 BDT
Miketang says:
But surely the big bang starting everything is the ultimate low entropy= closed system where the entropy never decreases? it's started from its maximum state of stability and entropy is winding it down - the creation of stars, planets etc is just a function of this. According to one theory everything will eventually get colder and colder and the universe will be unsustainable.
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