177 of 196 people found the following review helpful
on January 7, 2010
The arrow of time is a central issue in fundamental physics, and one that remains an open question even in the age of quantum mechanics and general relativity. It is a tall task even to define the question properly, never mind to explain what some of the proposed resolutions are. Nevertheless, Carroll is one of the best writers of popular science working today, and in this book he tackles the topic beautifully, guiding the reader through the relevant ideas, many of which we all think we have an intuitive feel for, like entropy, and explaining their physical meanings, and how gravity complicates the story.
The book is worth reading for its expert descriptions of the background material alone, but the reader hungry for speculations of how physics at the frontier may provide an understanding of the arrow of time will not be disappointed. Carroll devotes ample space to the concepts of cosmic inflation, the role of quantum mechanics, baby universes, and the setting that string theory may provide for all of this. None of this is settled ground in physics yet, and the author makes that entirely clear. But it hard to read this account and not come away with a tangible sense of the excitement to be found in taking on these most fundamental of problems.
56 of 61 people found the following review helpful
on March 21, 2010
This is a wonderful book that would merit a second reading to understand it more fully. At a fundamental level physics consists of the Standard Model, General Relativity and the Big Bang Inflationary Model of the universe. However, in this model there is something unexplained and it is the Past Hypothesis, that is that the universe started in a low entropy configuration. However the author speculates that perhaps the Big Bang was neither the beginning of time nor a moment of low entropy, but a moment of lowest entropy and the entropy increases in both directions of time, towards the future of the Big Bang and towards its past (from our point of view). This would be the situation in a single connected universe, although string theory predicts a multiverse.
Trying to elucidate the meaning of time (perhaps "an emergent phenomenon rather than a necessary part of our ultimate description of the world") the author reviews special and general relativity, Boltzmann's entropy, black holes and the controversy about conservation of information, life, quantum mechanics, inflation and the multiverse. Generally speaking the book is written in an accessible style (eggs can be broken and turned into omelettes, but not the other way around to describe the Second Law), but you will need to reread some parts to make the most of it.
In the final chapter Sean Carroll faces the "search for meaning in a preposterous universe". I quote: "We find ourselves, not as a central player in the life of the cosmos, but as a tiny epiphenomenon, flourishing for a brief moment as we ride a wave of increasing entropy...Purpose and meaning are not to be found in the laws of nature, or in the plans of any external agent...it is our job to create them. One of those purposes -among many- stems from our urge to explain the world around us the best we can. If our lives are brief and undirected, at least we can take pride in our mutual courage as we struggle to understand things much greater than ourselves". I think he has a point. It is in our human nature to try to find meaning to things. The universe is meaningless. I agree and I think that Woody Allen would also.
102 of 118 people found the following review helpful
on January 7, 2010
I am not a physicist. I majored in English in college. I shouldn't be able to understand this book on any level. But I do. And it's fascinating. Illuminating. And just plain interesting as hell. That's Sean Carroll's greatest achievement in this page-turner about the TIME we live in. If you have any interest at all in getting your head around just what this elusive "time" we all experience is all about, you should read this book.
179 of 227 people found the following review helpful
on April 20, 2010
After reading Mr. Carroll's book, I am reminded of a conversation that supposedly occurred between an old Indian and a white man in the late 1800s. The white man was trying to impress upon the Indian how much more advanced white civilization was when compared to his. He drew a small circle in the sand and said, "This is what the Indian knows." He then drew a larger circle around the first one and said, "And this is what the white man knows." The old Indian thought about this for a moment and then proceeded to trace a much larger circle around this second circle and said, "And this is what the white man does not know." This is how I feel about the current state of theoretical physics and cosmology. There are more questions than answers. At the end of this book I came away with a feeling of profound futility. I lost count of how many times Carroll said something like, "More research needs to be done." or "We don't know the answer yet." or "It's a complete mystery." Every book of this type that I've read in the last ten years ends at the same place -- we're stuck and none of the current theories we have adequately explain any of the fundamental questions about the nature or origin of the universe. String theory? It could be correct, but there is no way to prove it one way or another. Is the universe comprised of 11 dimensions? Possibly. The jury is still out. Does time exist? Yes, but it may also be an illusion. Parallel universes? Very likely. But, we may never know for sure. Is time travel possible? In theory yes, but the universe doesn't seem to like it, so it may never be technologically feasible. How about quantum entanglement, is this a real effect or does it signify some deeper, hidden property of the universe, or is it more like Bohm's guide wave interpretation? Everybody seems to have a different view. Sometimes things seem to come down to personal likes or dislikes. Did the universe have a beginning or has it always existed? Was there a Big Bang or was there a phase state shift? Bump and grind branes anyone?
I think I've reached the point where I'll just stop reading this sort of book for the next ten years. Carroll and others of his ilk are clearly very bright people with a grasp of advanced mathematics that puts me to shame (college calculus was as far as I got), yet despite their intellectual accomplishments they have nothing new to say on these subjects at the moment. Apart from selling books and making a buck, I really wonder why we need more books like this. Until some new breakthroughs come along I'll bide my time and just hope I live long enough to know what dark matter and dark energy are.
13 of 14 people found the following review helpful
on May 3, 2011
Roughly the first 3/4 of the book is quite standard physics, and a few related fields, e.g. information theory. The last few chapters, where the author, Sean Carroll, suggests a possible answer to the puzzle, are much more speculative, something he makes very clear.
To me the book was quite interesting. A few equations are displayed, but there is no actual use of mathematics. I have an M.S. in Applied Physics, so I cannot really say how a reader with no technical background would cope with it. Carroll goes through a lot of material, and the sheer quantity of it might be overwhelming. Unfortunately, that is just the way things are. Nobody is going to cope with this without the willingness to do some hard thinking. Carroll does include a lot of pop culture references that readers can relate to, although one of those may not be in any future edition of the book.
A couple interesting (to me) notes:
The complexity of the universe is different from the entropy. Just after the Big Bang the universe was very simple--the same high energy subatomic soup every where. Right now the universe is very complicated: There are lots of galaxies, stars, planets, black holes, people, etc. However, the entropy of the universe has increased: The formation of all those objects is mostly due to gravitation, as matter coalesces together. This gravitational process increases the total entropy, more than offsetting the order in all the structure. Eventually all of this structure will fade away. Even black holes will decay by the Hawking process, leaving a very thin, cold, dark, and simple universe. So while the universe started in a simple state, evolved into a complex state, and will eventually decay into another simple state, the entropy is always increasing. See pages 199-201.
Long ago, as an undergraduate at Carleton, one of my professors talked about the total energy of the universe. The gravitational potential energy V between two bodies decreases as they approach, because gravitation is attractive. For computational purposes we usally set V = 0 when the distance between them is infinite, and he argued that this is the natural thing to do. Then the gravitational energy is always negative. Assuming a finite universe, you can add up all the positive energy of mass, kinetic energy, etc. and then offset it by the negative gravitational energy. Professor Titus suggested the total energy of the universe would be zero. Carroll mentions in passing that you can prove this in general relativity. See p. 358.
Now the universe appears to be infinite (this was not so clear back in 1972), so strictly speaking you cannot speak about its total energy. But the general concept still applies: Gravitational energy is still negative. Something can be created from nothing, if the something is offset by sufficient gravitational energy.
The whole book reminded me of what Sir Arthur Eddington wrote:
"The law that entropy always increases holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations--then so much the worse for Maxwell's equations. If it is found to be contradicted by observation--well these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation."
On a lighter note, all of this talk about time and the universe also made me think of Severn Darden's
_Metaphysics Lecture_, which begins:
"Now, why, you will ask me, have I chosen to speak on the Universe rather than some other topic. Well, it's very simple, heh. There isn't anything else!"
21 of 26 people found the following review helpful
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.
1. Schulman, L.S., Source of the observed thermodynamic arrow. [...], Nov 17, 2008
2. Gold T., Am. J. Phys. 30, 403-410 (1962)
3. The Road to Reality: A Complete Guide to the Laws of the Universe
4. Wald, R.M., The arrow of time and the initial conditions of the universe. [...], July 21, 2005
5. Time's Arrows and Quantum Measurement
19 of 24 people found the following review helpful
on November 18, 2010
Let me first say that I am a huge fan of Sean Carroll. I watched his Teaching Company Lectures on Dark Energy and Dark Matter, I read his graduate text on General Relativity (even after having taken the class and owning Hartle's book on GR), and I follow Carroll's papers/articles.
This book is garbage. I give it 3 stars out of respect for the author and I think that someone who is picking up a popular physics book for the first time will find some value in it. However, the book is poorly written, in my opinion. One would be better served purchasing books by Michio Kaku, Paul Davies, Stephen Hawking and any other writer of popular physics who has demonstrated some kind of connection to the layman.
It is important to mention that this book doesn't even come close to developing a complete theory of time. in fact, there is no new contribution herein made on the subject. Furthermore, I think Carroll bit off more philosophy than he could chew. He claims that entropy gives a necessary arrow of time, but entropy only demonstrates time asymmetry (see "Asymmetry of Time" by PWC Davies and "Direction of Time" by Hans Reichenbach). Also, Carroll fails to achieve awareness of the philosophical import of some of the statements he makes, such as the 2nd Law of Thermodynamics acts as the arrow of time, necessarily moving events forward. The problem here is that the current description of this natural phenomenon is statistical, which means that the local ordering of microstates will occur. What does this mean according to Carroll's interpretation is that time can locally reverse and, one would expect at somepoint, that time would run backwards for no reason sometimes. The necessary addition to the interpretation, as Carroll explains, is that there must exist some underlying physical or metaphysical structure that extends beyond the current mathematical description, a result Carroll would not like given his perspective.
Carroll fails to give us the "why" or the "what" of time. What he does give us is a subpar history of physics and its association with time, as well as a 375 page book that could have been written in 150 pages.
5 of 5 people found the following review helpful
on December 27, 2010
In his book "From Eternity to Here," Sean Carroll attempts to guide the reader beyond relativity and quantum mechanics toward a new paradigm of thinking about time. He uses examples from popular culture to illustrate his points. In the novella and movie, "The Curious Case of Benjamin Button," the protagonist is born as an old man and progressively becomes younger. In his "Through the Looking Glass" (sequel to "Alice in Wonderland"), 19th century novelist Lewis Carroll tells of a White Queen who experiences an effect before its cause takes place. And in Martin Amis' "Time's Arrow," the narrator is a disembodied consciousness living inside of an Auschwitz doctor. For the narrator, life's scenes move backward, rendering an atrocity less detestable. In all three cases, the arrow of time reverses.
This concept need not be dismissed. As Copernicus dethroned humanity with his Heliocentrism, Darwin with his Natural Selection, and others (like Freud) since then, we are now confronted with the possibility that even our universe may not be unique (and time in these other universes may not resemble time as we think we know it). Imagine being just another evolved creature from a backwater solar system in a backwater galaxy in a backwater universe? Talk about an ego bust! Maybe that's the way it is, to paraphrase the late Walter Cronkite. It still doesn't disallow God's interactions with humanity.
Carroll tiptoes gingerly through the God/intelligent design issue. After all, there is no known empirical test for God as an external agent creating the universe(s) out of nothing. Even Belgian priest Georges Lemaitre, originator of the Big Bang model, refused to enlist it for theological purposes: "As far as I can see, such a theory remains entirely outside of any metaphysical or religious question."
Another scientific thinker who did not rely on a God hypothesis was Pierre-Simon Laplace (1749-1827). Using mathematics, Laplace instead invoked an intelligence (called Laplace's Demon) that could exactly foretell the universe's "predetermined" future. Later in the 19th century, Scottish physicist James Clerk Maxwell (1831-1879) also concocted a "demon." Maxwell's Demon wreaked havoc on conventional thinking about the Second Law of Thermodynamics and entropy by interfering with the normal kinetic process in a closed system. No, the Second Law was not violated after all, for the Demon siphoned entropy away from the box to himself, entropy that would have increased within an untampered closed system.
At the end of the day, Carroll leaves the reader with a lot of unanswered questions, but enlightened nonetheless by his yeoman's effort to bring us further along in our quest to understand the cosmos. Fr. Dennis
10 of 12 people found the following review helpful
on January 12, 2011
I'm fascinated by modern cosmology and modern physics; I've read several other popularizations. I have some math and a little physics background.. Also I often read and enjoy Sean Carol's blog, Cosmic Variance. So I expected to enjoy and benefit from this book.
I got some benefit; there were two or three tangential topics I felt I understood better after reading them here. But I didn't enjoy it; it was a slog to finish. Others have stated the problems well so I'll quickly summarize. He has a few points he needs to make for his arguments, and we need to understand them to follow him to his conclusion. But he's too much the academic here, constantly going off on tangential issues to debunk alternatives or remind us of things he's already established (to his satisfaction, at least), so it was hard for me to keep attentive to what was important in a section. And he tries to exhaustively consider, and dispose of, all the alternatives to his conclusion. But there are a LOT of those and in many cases his arguments against them just aren't that convincing.
I like where he got to, but I'd have liked a much leaner book or a book with more real content and a lot less re-hashing, a lot more.
4 of 4 people found the following review helpful
on August 30, 2011
If you think you are going to settle in and read about time and "what it means" from a physics perspective, be prepared for a long diversion first or to put it another way, be prepared to think about time differently. Carrol starts out with some brief observations about time and how the belief that the universe was at low entropy prior to now (maybe when it began, if you agree that it did begin and wasn't always here . . .) and is moving towards higher entropy and thus time moves forward as well (he refers to this as the arrow of time throughout the book). He states this in a much more complicated way so I'm not sure that I'm even right, but it's something like this. Then he gets into physics, first the classical stuff and then the quantum. But, most of this was far more complicated than I could understand and unlike some of the other physics books I've read, hard for me to wade through. I don't know if it was the writing style or the esoteric concepts or that it seemed to be drifting so far away from explaining a theory of time, but I would find myself just wishing the chapter would be over. But, from my perspective the book is ultimately a discussion of entropy and understanding how/why it is that the universe was at a low entropy state prior to the big bang and is moving towards a higher entropy state.
There are lots of explanations of physics in here in far more depth than most introductory books I've read, so if you are looking for discussion of Hawking radiation, black holes, the Copenhagen explanation, etc, this will provide that level of detail.
Most chapters start with a problem and Carroll attempts different hypothesis to solve the problem eventually coming to the hypothesis that is most accepted (if there is one; some problems do not have even an accepted hypothesis to solve them, according to Carroll, like speculation about many universes and a theory of quantum gravity). While I like the idea of this organization and Carroll explains historically how we've come to the current explanation it seems unnecessary to his topic. Why not tell us the accepted hypothesis and move on? I can only speculate that Carroll, one wants to provide us with the history of the idea so it is more convincing; and two, is being careful because many of these ideas are still debated (like does a black hole emit radiation; and what happens to information in a black hole?). Carroll states in the epilogue that he spent so much time on the historical and theoretical background because "we can state the problem clearly but have only a few vague ideas of what the answer might be" (p. 367).
But for the beginning reader, this level of historic detail was just overwhelming. Another stylistic convention that Carroll uses is to say, "You may be wondering why . . ." and then posit something that would seem contradictory to his conclusion. I almost never wondered whatever he was saying because I was so confused, but there were a few times when I did (like when he says our universe is heading to a high entropy state of empty space) and I was thinking, how can high entropy be empty space. He nicely answered that question.
Ultimately, Carroll does not provide an answer to the question why is the initial state of the universe at low entropy. After reviewing the available possibilities, he suggests one possible hypothesis: that maybe the universe is eternal (there is no beginning and there is no end) but just gradually and forever increasing entropy, that reaches de sitter space (empty space with positive vacuum energy) but then continues on (I hesitate to even attempt to summarize his explanation because of my level of understanding, but I think this is right). This explanation assumes the existence of baby universes that continually appear. There is no single trajectory, but space goes on forever in all directions so that the past in one is the future in another. He does address one major counter argument against this theory: there is no way to falsify the theory of the multiverse. Carroll suggests that it isn't a theory but a prediction and what we should be working on falsifying are the theories that predict the multiverse. This seems like somewhat of a copout, but maybe not.
I found this a very difficult book to get through, especially the middle. But I'm glad I read it. I imagine that as I read more books of this type, this book will add to the conversation.