Dall'eternità a qui

by Sean Carroll, Franco Ligabue (Translator)

L'approccio allo studio dell'universo delineato da Sean Carroll si fonda sulla caratteristica più immediata e insieme enigmatica del tempo: il fatto di avere una direzione. Per manifestarsi in tutta la sua problematicità, il dilemma della freccia del tempo, spiega Carroll, non necessita di telescopi giganteschi o potenti acceleratori di particelle. Si presenta in cucina ogni volta che rompiamo un uovo in padella: possiamo trasformare l'uovo in frittata, ma non viceversa. Eppure, nel bizzarro mondo quantistico, che soggiace agli stessi fenomeni, il tempo è reversibile e la catena degli eventi può essere percorsa a ritroso. La contraddizione insita in questa semplice, umile rottura di simmetria apre, sorprendentemente, uno spiraglio per comprendere i profondi misteri del nostro universo – e di altri ancora. La freccia del tempo puntata risolutamente dal passato al futuro deve infatti la sua esistenza a proprietà dello spazio-tempo preesistenti allo stesso big bang – proprietà ancora impensabili ai tempi di Einstein. A conclusione di un'ampia rassegna dei risultati e delle questioni ancora aperte della termodinamica, della relatività e della meccanica quantistica, Carroll suggerisce che il nostro universo faccia parte di un multiverso, una grande famiglia di mondi, in alcuni dei quali esseri simili a noi potrebbero sperimentare lo scorrere del tempo in direzione opposta. Nel presentare ai non specialisti temi come la dinamica dell'entropia, l'origine e la fine dell'universo, i viaggi nel tempo o il significato della vita, Carroll dà prova di rigore e insieme di un’esem­plare chiarezza, tanto che un fisico come Brian Greene, l'autore dell'Universo elegante, non ha esitato a definire "incantevole» questa "accessibile, coinvolgente esplorazione dei misteri del tempo».

Genres: Science, Nonfiction, Physics, Philosophy, Popular Science, Astronomy, Space

486 pages, Paperback

First published September 24, 2009

About the author

Sean Carroll is a physicist and philosopher at Johns Hopkins University. He received his Ph.D. from Harvard in 1993. His research focuses on spacetime, quantum mechanics, complexity, and emergence. His book The Particle at the End of the Universe won the prestigious Winton Prize for Science Books in 2013. Carroll lives in Baltimore with his wife, writer Jennifer Ouellette.

Reviews

[Manny · Rating 4 out of 5]

FREE From Eternity to Here COMPATIBILITY TEST

Read through the following dialogue between two people, A and B. Underline all the sentences which you can imagine saying yourself.
____________________________________________

A: What are you thinking?

B: Have you ever wondered why the future is different from the past?

A: What do you mean, different?

B: Well, you can remember the past, but you can't remember the future. Why?

A: Like déjà vu?

B: No, not déjà vu. Really remembering the future.

A: But the future doesn't exist yet. How can you remember something that hasn't happened?

B: On the other hand, the only reason you think the past exists is that you can remember it.

A: Hmm... maybe! But it would be very confusing to be able to remember the future. You'd have no reason to want to do anything.

B: It's not just memory, it's everything! For example, you have photographs of things that have happened in the past, but you can't have a photograph of something that's going to happen in the future.

A: But how could you have a photograph of something that's going to happen in the future?

B: A camera takes pictures of things that happened in the past. Why couldn't a future-camera take pictures of things that happen in the future?

A: Because you can't. It wouldn't work.

B: But why not?

A: What does that book say?

B: It says we can tell the difference between past and future because we're close to the Big Bang.

A: What's that got to do with it?

B: Well, it's a bit complicated. Imagine that the universe stopped expanding after a while and then collapsed again---

A: Is it going to do that??

B: No, it isn't. But imagine it did. Now would the result look like the Big Bang, only backwards?

A: I don't know.

B: Well, it wouldn't! That's very interesting, isn't it?

A: Let me have a look myself. Hm, hm, hm... so he's got this theory about baby universes that he developed with someone called Jennifer Chen. What's a baby universe?

B: Ah, first you have to imagine that our own universe continues for several zillion years---

A: Wait, he dedicated the book to Jennifer! "To Jennifer, for all time". So he developed this weird theory with his girlfriend? That's so romantic!

B: It might be a different Jennifer. Let me google it.

A: It can't be.

B: Oh, it is! His wife's also called Jennifer. Just a coincidence.

A: How disappointing!
____________________________________________

Now count the total number of As and Bs in your answers.

Mostly As: You are a normal person. Congratulations.

Mostly Bs: You spend too much time thinking about things that no one understands. You might enjoy this book.

Somewhere in between: Lower your entropy, goddammit.
____________________________________________

And so far we have...

Mostly As: notgettingenough, Kat, Warwick, Lynne, Ted, Shayan

Mostly Bs: Manny, Tatiana, Riku, Fionnuala, Matt, [Name Redacted], Ariel, Forrest, Shinjini, Saman, Kris, Samadrita, Sarah

Lower your entropy, goddammit: Zahro, Ivonne, Stuti, Joe, Jim, Cecily, Kalliope, Rakhi, Eti, Elham, Mike

Groot: Robert

[Jen · Rating 4.5 out of 5]

“When the Gods decided to point Percy at the porcelain of my face and thoroughly souse me with the urea of cosmic entropy, I did not flinch. Having precipitated out of a highly saturated parcel of associative network in my developing brain, I stood there grasping similarities among various phenomena of unidirectional character. Fragmentary memories schmoozing until their mass could no longer be suspended in the vapors of intuition, forming fat drops of golden tangibility which strafed my face with the hotness of Tiamat’s bodily interior. Golden lamellar flows breaking into errant sprays upon gooseflesh reefs, rivulets hugging the contours of my body and pulling the fabric of my white t-shirt taut against my skin with passionate viscosity, like some specimen trapped gasping in amber. “My tattoo artist interrupts here to admonish me, yet again, about the blood thinning properties of alcohol and how this can lead to compromises in the quality of the design, but the nerves of my bladder distract me throughout the lecture.

Returning from the bathroom and situating myself again, I continue, “Take for instance the time I traipsed through the neighbor’s house, forgetting, despite the evidence all around me, that they were in the process of remodeling. And so when I stepped from the rear portcullis and fell like a certain hubristic dramatis personae of Old Testament pseudepigrapha through a wooden chair, still clutching an entire pod of stolen bananas to my chest, it was not without some effort that I, responding to frantic interrogations about where I had absconded from with this ripe payload, groaned in exasperation at the overwhelming forensic evidence and replied, “South America.” It occurred to me then, at the age of eight, though obfuscated in part by the surge of fear and adrenaline, that if this scenario were to be played in reverse it would appear quite strange. With the lawn chair spontaneously reassembling itself and catapulting me back into the house. I knew of nothing from the atomistic view that would explain this asymmetry between fast forward and rewind, and yet it seemed deeply intuitive that events proceeded in one direction, but never the other. That’s what I’m trying to capture here with this equation you’re currently searing into my mons pubis.”

The artist, with furtive glances at my face to see if I’m having a giggle, clears his throat and asks how they relate. “This is the second law of thermodynamics. It is the mathematical formulation of what the fox, whilst chewing its own entrails, said in the movie Antichrist (i.e. an experimental psychological horror film written and directed by Lars von Tier, starring Willem Dafoe and his “confusingly large” penis erupting with geysers of blood proceeding savage ball obliteration); “Chaos Reigns.” To put it in statistical terms, there are vastly more ways to fuck something up than set it right. Take for instance this lovely showcase you have of your etchings. Arranged in a geometrically pleasing way with an eye for composition, the photorealistic vagina framed by a cartoon coffin with scroll reading; “Killed it.” Pulling the whole thing together. There may be various other permutations which could equal the aesthetic appeal of your current arrangement, but think for a moment of all the ways in which this whole barbwire diorama could be transfigured through time, or malevolent actors, to resemble nothing more than a hideous stew of tramp stamps and broken hearts. This is to say that disordered states are more numerous than their ordered counterparts, and the disparity is quite vast. Furthermore, it requires energy to order these things. How many Sausage Egg McMuffins do you think you’ve made fungible for Feng shui? Not just creating this fantastic setup but maintaining it? Ah, I can see from your face that you’ve apprehended the fact that the universe will asymptotically approach a state where all energy is evenly distributed.” I reach out to touch his face in solidarity only to be lectured, yet again, about attempting to touch the artist’s face in solidarity while being punctured at tremendous speeds by electromagnetic armature and indelible ink.

“What I’m saying here is that we perceive the arrow of time by virtue of the entropy all around us. That it is easy to mix our coffee with sugar and cream, but impossible to reconstitute those molecules from our morning brew and place them back in the scoop. Why we see eggs fall off tables (and idiots fall through chairs) but never the reverse. Why we remember the past but not the future. Why buildings collapse by never spontaneously assemble. Sean Carroll postulates that the linear nature of time may be the result of the low entropy initial conditions of the universe which continues now, however imperceptibly, to march towards a thermal equilibrium which nothing interesting is possible. A state where maximum entropy is achieved.”

[Anne · Rating 4 out of 5]

I'm boggled.



Granted, it doesn't take much to boggle me, but theoretical physicist Sean Carroll is working with ideas that made my head spin.
In a good way. <--don't want you getting the wrong idea.
95% of this went so far beyond anything that I understood, that I spent the vast majority of the book with a perpetual look of huh? on my face.
Checkerboards, disembodied brains, and black holes.
What? <--I'm assuming you asked.
I don't know. But it was educational in that broad-level crazypants way that made me feel smarter AND dumber at the same time.



But that's ok. Because 5% of what he was saying sank into my brain and just filled it up with incredible information and theories that it's fair to say I'll probably not retain, but were cool while they lasted.
On the flip side, the more I read about this stuff, the more I recognize ideas when I read the next book. There's that.
So while the title is catchy, the reality is that I understood very little of what was coming out of Carroll's mouth because I am old and have zero background in any of this. Don't think you're going to step into this book and really take anything solid away from it if you're not a science or math major. Carroll is talking to peers in this. Or if not peers, then not middle-aged folks who, like me, still count on their fingers and sing the alphabet song to put the letters in order.

But hey, if you got Nobel prizes for reading things outside of your comfort zone, I'd be collecting mine right now.
Recommendedish.

[Bradley · Rating 5 out of 5]

This is some very impressive stuff.

I've read a lot of nonfiction science books that sometimes had equations but mostly did not, but what I really wanted was a cohesive drive, an arrow to spear right through some of the biggest questions of our time... such as What Is Time.

Sean Carroll manages to keep things very sharp between what is perfectly understood and all of the theories that are somewhat understood, and the other Cosmology stuff that's mostly just baffling. :)

Any way you look at it, though, this is not a book that gets derailed or goes off into super strange directions. He lays out all the foundations, from the opening definitions of Time and what we think it means, from the average to the rather advanced notions of space-time and curvature, Einstein's energy equation, speed of light, diliation, moving all the way to Black Holes. This is very solid stuff.

Plus, we have a very coherent definition of Time as Entropy, showing us just how complicated it can get when time's arrow might just be the illusion that Hawking says it is. I really enjoyed that discussion.

Of course, we come up with lots of possibilities and digressions that are always explored in SF, too, but most of these are just bylines, moving quickly by the Grandfather paradox, etc, to get right back on the main track.

Yes. We have Equations. :) Fortunately, the author does a very good job about explaining them and even getting deeper into the extra areas that made this rather more interesting for me since I've read many science books and have heard most of this already.

I recommend this for anyone interested in Time. :) Not time management. Just Time. :) We do touch rather heavily upon Cosmology by the end, too, which was a blast and a half, getting into many-universes theory and string theory, to name a few. And he makes it clear! :)

Seriously. This was some sharp stuff. Very readable. It's not a general overview. You might say it's putting time's arrow right through the heart of a big question and staying on track all the way to the end.

[Aerin · Rating 4 out of 5]

In space, there should be no material difference between left and right, forward and backward, up and down. However, for us there is a substantive difference between up and down because we live in the spatial vicinity of a massive object - the earth - which exerts a gravitational pull on us.

And in spacetime, there should be no material difference between past and future. But for us there is a substantive difference because we live in the temporal vicinity of a massive event - the Big Bang - which gave rise to the arrow of time.

Or so Sean Carroll argues in From Eternity to Here The Quest for the Ultimate Theory of Time. He says that the reason we remember the past and not the future, the reason effects always follow causes and never vice versa, the reason we perceive time always moving in a particular direction, is because of entropy. For whatever reason (physicists collectively shrug their shoulders), the Big Bang was an extremely low-entropy event, and ever since entropy has been increasing and pulling the arrow of time with it. Even if the arrow were to somehow reverse and run time backwards, supposedly we would still remember the low-entropy direction as the "past", and feel we were moving toward the high-entropy "future".

Entropy itself is an interesting concept, one that most of us are taught in high school physics class is synonymous with "disorder". But that's not really what it is - entropy measures "the number of microstates corresponding to each macrostate". Basically, the more possible configurations of all the particles within a substance, the higher the entropy, and the more likely the substance is to eventually reach that state. After all, there are many more ways egg molecules can be situated in an omelet than in an unbroken egg, where all the yolk molecules must cluster together, and the albumin, and the shell and so forth. It's just statistics that any given egg is likely to end up in the high-entropy condition. (And another name for these statistics is the Second Law of Thermodynamics.)

Since everything is far more likely to be in a high-entropy condition than a low one, it remains a mystery why the universe currently has relatively low entropy, and apparently had even less in the past. Yet that seems to be the case, and so we need the Past Hypothesis: when considering the past, we ignore the statistically-preferred high-entropy condition, and assume that the closer we get in time to the Big Bang, the more the entropy decreases.

Why? Well, that's the question the book grapples with, because nobody has the faintest clue.

Carroll scoffs at popular concepts like the anthropic principle, irreversible temporal systems, and a symmetrical, entropy-decreasing Big Crunch, as irrelevant or inadequate to explain the mystery. Instead, he's fond of the idea of "baby universes", spawned spontaneously from the maximal-entropy atomic soup of impossibly ancient dead universes. The creation of the baby would have properties similar to the Big Bang - inflation, expansion, low entropy, an arrow of time, and so on. Maybe the reason we observe such crazily low levels of entropy is because our universe is just a toddler.

Admittedly, this is a cool idea, but it's not really science. It's not remotely empirical, testable, or falsifiable (which Carroll freely admits). And veering off into crazy metaphysical theories is fun and all, but I mean... we could all be brains in jars, too, or the elaborate fever dream of some minor god. Who knows?

I enjoyed this book, though. Its explanation of quantum mechanics was far more lucid and understandable than anything I'd read previously - and it uses cats, but more entertainingly (and humanely) than Schrödinger did. Its speculations on time travel and wormholes and multiple universes gave my inner sci fi geek plenty to chew on. And it introduced me to the concept of Boltzmann brains, which was worth the price of admission in and of itself.

But mostly, I just found Carroll's writing to be clear, funny, and affecting. As an example, the following paragraph, which encapsulates everything I consider beautiful and fulfilling about science:


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 from the Big Bang to the quiet emptiness of the future universe. Purpose and meaning are not to be found in the laws of nature, or in the plans of any external agent who made things that way; 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.

pg. 374

[Prerna · Rating 4 out of 5]

When it comes to foundational physics, we are apparently in a phase of stagnation. We have ginormous amounts of data, but we also have incomplete theories. Most physicists today don't work on foundational theories, we've apparently had a roughly exponential rise in the number of publications and authors, but most of them are within sub-fields of physics. And for someone who wants to work on foundational physics, this situation looks gloomy to say the least. And so I've been relying on books like From Eternity to Here. It might be too soon to say, but I reckon I couldn't have read this at a better time. Of course there's a stagnation and shitloads of bureaucracy, but there are also people asking all the right questions and sincerely looking for possible answers.

One of the biggest clues we have about the underlying principles of reality is the arrow of time - the fact (in our part of the universe at least) that entropy always increases and so we remember the past but never the future. What we also know is that the initial conditions of the universe were of low entropy - but this is where our existing theories fail. We do not currently have the physics to talk about this, to describe this. Sean Carroll here tries to examine our past, present and future all on the basis of entropy. What does the principle of increase of entropy mean for the further evolution and expansion of the universe? Why did it all start in a low entropy state? Are we just in a local region of increasing entropy - in a universe within a multiverse of constant entropy? What has entropy and time meant for our very subjective human experience? And most importantly, why can't we unscramble eggs or unmix coffee? Because we'd all like to do that.

The writing here is clearly well-researched, accessible and eloquent. I ought to buy myself a hardcopy because I know I'll return to it whenever I'm confused or pessimistic about physics.

[David Rubenstein · Rating 5 out of 5]

This is a very well-written, and entertaining book on our understanding of the arrow of time. Entropy is a key concept, which deserves (and gets) lots of attention. The second law of thermodynamics states that, in a closed system, entropy can either stay the same or increase--it cannot decrease. Sean Carroll shows why the reversibility of physics at the particle level gives rise to a seeming paradox; if the physics of particles is just as correct with time switched backwards, why can't entropy decrease? Carroll discusses this issue at length.

I have often wondered, if the universe were to start contracting at some point (for example, if dark energy did not exist, causing inflation to accelerate), wouldn't entropy start decreasing then? Just as I was getting annoyed that the answer wasn't forthcoming, Carroll started addressing this very issue! He explained that a calculation of entropy requires including gravitational fields, especially in the realms of "lumpy" matter. For example, the single supermassive black hole in the center of our galaxy contains more entropy than all of the rest of the universe combined, excluding other black holes!

Toward the end of the book, Carroll starts to wander into speculation; he clearly states that his opinion is a minority opinion, but that it is just as well supported as other ideas. The central question that he raises, is this; if the entropy of the universe is still relatively low, but always increases, then at an earlier stage the entropy of the universe must have been much much lower still. So, how did the entropy get that way, so very very low compared to what it would be if the universe were in "equilibrium". It is a very important question in cosmology, but I won't "spoil" your reading by giving away the answer here. Read the book, and discover a truly interesting, but highly speculative explanation of this conundrum.

[Kristin · Rating 3 out of 5]

If I could, I would give 5 stars to the first half of this book and 2 to the second. For the first 500 pages or so (in the iBook version), I was enthralled by Carroll's exceptional ability to lay down the fundamentals of physics and cosmology in clear, straightforward language with simple but highly effective illustrations. This book has by far the best description of general relativity (especially the equivalence principle) for the non-physicist that I have ever encountered. His explanation of dark matter and vacuum energy is also brilliant--I was all mixed up on those concepts before I read this book but now I feel I have at least some clarity (though in reality I'm probably as confused as ever). I also loved the discussion on black holes, white holes, and Hawking's black hole radiation. The chapter on quantum mechanics, the collapse of the quantum wave function, and the wave function of the universe (is this stuff for real?), and Miss Kitty is also superb.

However, the book isn't really meant to be an introduction to the fundamentals of physics and cosmology. It is about the arrow of time (i.e., why we experience time as flowing from past to present to future) and the author's view that it connects not only to the origin of our universe but to a much broader cosmology. We're talking multi-verses, baby universes, and all sorts of other wild visions here. Carroll believes that they key to understanding time, the universe, and everything is...entropy. I would try to summarize entropy here but, quite honestly, I'm now terrified at the thought. Carroll spends about 1/3-2/3 of the book explaining and re-explaining it. It all started with some engineers in the 19th century who were determined to build more effective steam engines. But, in this book, entropy's reach extends far, far, FAR beyond that. And, whether you are convinced by Carroll's application of entropy/thermodynamics will determine your final views on this book.

In the end, I wasn't totally convinced. Entropy is a slippery, fuzzy statistical concept, and it seemed to me that Carroll had no trouble defining and re-defining it on a whim to suit his argument. At times, this practice has completely ridiculous and nonsensical consequences (Boltzmann brains, for instance, which the author seems to believe are "out there"). And, I felt that Carroll spent way too much time chasing down dead ends, seemingly for its own sake.

I did, however, find the end game--once we finally got there--to be really intriguing and imaginative. It will be interesting to see if his model--of our universe being one of many "baby universes" that bubbled up from an infinite, static, high-entropy background universe--is ever backed up by real data. What is it they like to say in physics? "Shut up and calculate?"

[Max · Rating 5 out of 5]

Carroll offers a provocative explanation for the arrow of time. I found it fascinating, but those without a strong interest in physics may not. Most of the book is spent explaining the basic concepts he uses to make his case. I learned a great deal from these science lessons. In the last few chapters Carroll steps outside the mainstream. He carefully identifies what is speculative. His unconventional ideas true or not were thought provoking.

Carroll defines time through its use to label and locate events and its perception as something we move through. Unlike space, time has only one direction. Carroll explores this taking care to clarify difficult concepts such as the space of states, microstates, macrostates, time translation invariance, reversibility of the laws of physics and the conservation of information. We get brief but helpful discussions of special relativity, general relativity, quantum mechanics, light cones, black holes, and white holes all of which play into Carroll’s theme.

Much of the book is devoted to entropy. The reason we proceed from past to future is entropy, the relentless decrease of order and useful energy. This comes from the second law of thermodynamics. Ultimately the universe will likely stabilize in a high entropy state. Carroll examines Boltzmann’s equation defining entropy and gives us many simple examples to help us understand it. Carroll explains the scientific meaning of entropy as disorder. Scientists do not use disorder in the general sense.

That the universe is always going from lower to higher entropy indicates it started in a low entropy state. This is not a law of physics, but an assumption, a boundary condition called the Past Hypothesis. The Past Hypothesis is essential to construct a credible story of the universe. It is often construed to be due to the Big Bang. Perhaps but Carroll has other ideas. Carroll’s point of departure into the speculative part of the book is the question of how the universe began in a low entropy state.

Carroll is searching for a credible theory of quantum gravity. He gives an approving nod to string theory admitting it is a work in progress. In one respect Carroll seems to be in line with some popular string theorists. He believes in the multiverse. He posits that universes are created by quantum fluctuations in empty space. These form bubbles that become baby universes that bud off from the universe that formed them, a process without beginning or end. This answers the anthropic principle, the idea that the universe is finely tuned for life, and the problem of first cause. Carroll takes care to craft his solution so that there are fixed space of states with reversible laws of physics and conservation of information. Most important to Carroll is that this is a way the universe could have started with low entropy.

Throughout Carroll points out common assumptions that we may forget are assumptions such as the Past Hypothesis. Another is that the universe we can’t see is like the universe we can. There is no reason to believe that. We don’t know what lies outside our small visible slice. Also while there is no evidence of unusual influence from the unobservable universe on our slice, the observable universe is not a closed system. The second Law of thermodynamics only holds true in a closed system.

Another common assumption is that the Big Bang model means a singularity was the beginning of time and space. Carroll views the Big Bang model working well as we look backwards until we reach the singularity at which point the laws of general relativity break down. Thus the conditions present in a singularity are unknown. Carroll accepts the theory of inflation because it is consistent with what we believe about the early universe, but as he notes that does not prove it is true. Obviously that can be said of Carroll’s multiverse; just because it can explain conditions such as low entropy doesn’t make it true. As Carroll points out the multiverse is not even a theory since it is not falsifiable.

I enjoyed Carroll’s wild ride into the multiverse at the end of the book. It makes just as much sense to me to have multiple universes as it does to have only one, but then I am not a physicist. Carroll writes with enthusiasm and purpose. The basic physics he takes pains to explain he ties nicely together to support his ideas. I learned some worthwhile science, whether or not there is a multiverse. Recommended highly for those who were wondering why we always wake up tomorrow instead of yesterday.

[Sense of History]

A fascinating book, about a subject that interests me greatly: why does time exist and why does it always flow in one direction? Of course, we end up with the inevitable second law of thermodynamics and the concept of entropy. Carroll really does his best to explain the consequences of that strange concept, and especially the mysteries that linger around it. But for the time being, science does not succeed in formulating a truly satisfactory answer, and so doesn’t Carroll. See my review in my general account Marc: https://www.goodreads.com/review/show....

I was somewhat disappointed by the lack of a clear answer to that basic question about time. But nevertheless Carroll in this book introduces a number of lines of thought that stimulated me in my current reading program about time experience and temporality. For example, he refers to an interesting remark in John Pirsigs' cult book Zen and the Art of Motorcycle Maintenance: An Inquiry Into Values, namely that the ancient Greeks apparently “saw the future as something that came upon them from behind their backs, with the past receding away before their eyes”. I do not know where Pirsig got this from (Plato/Aristotle?), but it is a view that intrigues because it goes against our intuitive feeling in western modernity that a person always is facing the future and has the past in his back. Interesting to ponder about that one.

Another notion that comes up in this book is our intuitive sense about the past as fixed, unchangeable, as opposed to the future that seems like open to many possibilities. Theoretical physics seriously challenges this view: in quantum physics cause and effect do not appear to be unambiguously linked to each other and - at least in theory - can also occur in reverse order. That uncertainty actually fits in with the post-modern historiography of the last decades, which places a great deal of emphasis on perception and representation. In an extreme version of that current the past seems to be open to infinite variability. I don't agree with that extreme way of looking at the past, but you can't just wipe it off the table: every glance at the past is coloured by the present and the context of the observer, and the 'represented' past can therefore undergo considerable change over time.

The observation that we are living in a 4-dimensional universe, where spatial aspects take on 3 dimensions, but time is limited to only 1, also stimulates the imagination. I understand from Carroll that (in connection with the issue on entropy) scientists still cannot really give a clear explanation for that one time dimension (there are theoretical models that take into account multiple temporal dimensions, but frankly this line of thinking seems very much like pure science fiction). And so we arrive at what may be the best quote that this book contains: "But the question remains: "what is time?" The response from the American physicist John Wheeler is worth remembering: "Time is nature's way of keeping everything from happening at once.” It may seem a very laconic way of representing things (not taking into account the quantum mechanical issues), but this statement certainly is unequivocal.

[Marc Lamot · Rating 2 out of 5]

I’ll be honest: this was a tough job. Sean Carroll really does his best to explain the basic issues of theoretical physics in a comprehensible and pleasant way. Certainly in the beginning he uses a lot of humour, nice literary references and a very patient style to successfully present the ins and outs of classical mechanics (from Newton to Einstein) and quantum mechanics (lots of cats involved).
But halfway through he starts the real topic of this book when it comes to time as a physical phenomenon: entropy. And from then on this book slowly becomes a difficult read. From as many as 20-something different perspectives, Carroll tries to answer the question why entropy exists, how it defines the one-directional arrow of time, and why in our universe it runs from past to future (yep, apparently that is not an obvious question).

The answer to that question seems to come down to the initial state of our universe, namely in low entropy. But why was it actually that low back then? From a logical point of view this seems to make no sense. To explain this Carroll jumps from one scenario and theory to another, and every time new issues pop up that make the proposed hypotheses unlikely. It has been cited in some reviews: this book looks like an infinite set of Russian dolls placed one inside another. But in the end Carroll has to conclude: we know too little (especially on quantum gravity) to give a definite answer on this time- and entropy question.

But then, Carroll ventures into his own hypothesis (he calls it a prediction, not a theory), which is inspired by the multiversum theory (charming little baby universes are part of it). He admits that this is all very speculative, but he also defends that approach, because to him that is simply the way in which science advances. This was a very interesting book, which ultimately didn't provide a satisfactory answer to the question why time exists as it exists. But it does provide an honest insight into how theoretical science works, although at the cost of a mind boggling reading experience. (2.5 stars)

[WarpDrive · Rating 5 out of 5]

This is a very good book about some fundamental modern physics concepts (such as arrow of time, entropy, symmetry, time-reversibility, complexity, theory of information) and their complex inter-relationships.
I found that this book has one of the best explanations of entropy for the layman, and the treatment of potentially complex areas such as symmetry, quantum mechanics and relativity, is very good - deeper than in most popular science books.
Overall, a very enjoyable read, which would appeal to a wide audience, from the mathematically challenged (as maths is very minimal), to individuals looking for a deeper treatment of modern physics than what found in most popular science books.
Just a couple of areas where I found areas for improvement:
- in the treatment of quantum mechanics, the collapse of the wave function is correctly regarded by the author as a potential problem of the so-called "Copenhagen interpretation": this is correct, but there are other interpretations (in particular, the DeBroglie-Bohm theory) that overcome this problem (yes they are non-local, but the author himself actually tends to agree with the non-locality position of this and similar approaches) - so a even brief mention of these alternatives would have made the book much more complete
- whilst the concept of symmetry is very nicely illustrated and explained, unfortunately the very important concept of symmetry-breaking is not explained - which I think is baffling in a book which is supposed to deal with time reversibility and the arrow of time.

Apart from these minor points, a very enjoyable and rewarding read.

[D · Rating 5 out of 5]

A fascinating course in physics: from Newton and statistical mechanics to special and general relativity and, of course, quantum mechanics, black holes evaporating and the kitchen sink. The author is a master in explaining things intuitively, there's hardly an equation in sight. Especially quantum mechanics is presented in such a way that you 'grok' it without noticing. He makes a strong case that time points to states with more entropy and then comes up with a sketch of a theory that ingeniously combines things to end up with the appearance of low entropy baby verses, each expanding (and increasing in entropy) like our own. Recommended.

[Cheenu · Rating 1 out of 5]

I have read 3 of Sean Carroll's books and all 3 have been rated 5 stars. However, I just couldn't really get through this book.

Maybe it's because I can't wrap my head around the concept of Spacetime. Or maybe this book is one of his misses, where he has failed to make this book accessible to the non-physicist.

Either way, read The Big Picture: On the Origins of Life, Meaning, and the Universe Itself instead. That is an exceptional read.

[Pavle · Rating 5 out of 5]

Šon Kerol je jedno veliko dete zaglavljeno u odelu teoretskog fizičara – a ima li šta bolje od entuzijastičnog prvoklasnog naučnika?

Ovo je jedna divna, fascinantna knjiga koja služi istovremeno kao polemika o značenju „strele vremena“ ali i kao podsetnik na trojstvo oba relativiteta i kvantne (i kao bonus i statističke) mehanike. Entropija kao čekić, vreme kao ekser, drvo kao uni, ili možda pre multiverzum. Dosta je spekulativna, mora se priznati, ali to ni ne krije, već to nosi ponosno i izrazito racionalno (ali ovo drugo se podrazumeva). A i to je samo jedno poglavlje pred kraj. Sve u svemu, Kerol je razborit čovek sa svežim smislom za humor i prosto pleni sopstvenom radošću koju oseća prema svemu onome što nas okružuje, počevši od onog malog, kvantnog, pa do onog najvećeg, kosmološkog. A ta radost ume da bude infektivna.

5

[Mohamed al-Jamri · Rating 2 out of 5]

This book asks and attempts to solve several important questions such as what is time? Why is there an arrow of time? Was there time before the Big Bang? Why did our part of the universe start in a low entropy state? Is time eternal? Is time travel possible?

The author explains several cosmological and physical stuff such as the Big Bang, the Steady State model, special and general relativity. He does so using easy to understand language. However as I finished the first third, the book started to become hard to follow and I eventually stopped reading by the middle of it.

[Merilee · Rating 4 out of 5]

OK, I cry uncle! I've read 373 out of 470 pages and I am lost. Life's too short. It's not Sean Carroll's fault...I just cannot conceive of multiverses and quantum gravity...

[Sabin · Rating 5 out of 5]

Very, very impressed with the author’s treatment of his subject material. The book is about time. Not about what we do with our time, not about how to manage our time more efficiently, not even about the history of timekeeping, time-travel, or other things that we can do in time and with time. Ok, it is a bit about each of these subjects, but it is essentially about why we have this concept. Why do we perceive time in the way we do and why is time such a difficult concept to grasp.

Well it turns out that according to the reversible laws of physics in effect around us, time is just a construct by convention: it helps us order events, measure the duration between them and there’s also the fact that we see time as something which flows past us. But just like Terry Pratchett’s “atoms of justice” or “molecules of mercy” there is no fundamental particle of time. The physical process which makes us imagine time is the decay of useful energy, or the increase of entropy. It quite straightforward actually, you notice it as you age: you get weaker, internal organs perform worse, there’s an increasing chance that something will fail, and so on. So the evolution from a low-entropy state towards a higher entropy state is the direction of the arrow of time.

And now comes the interesting question. “Why?” Why should there be an arrow of time, a preferred direction, from a low-entropy past to a high-entropy future. Caroll states from the outset that we don’t actually know the answer, but that there are some things we do know. And Caroll hopes they can help point us in the right direction. Instead of an answer to that question, what we get in this book is a very thorough exploration of the concept of entropy in thermodynamics, information theory, black hole cosmology and how it ties together with quantum mechanics.

This is not another way to look at the world around us, but a way to challenge the reader to think seriously about an issue with almost no practical implications to our day to day life. The nature of time is right there, on the same level with the beginning of life, consciousness and the nature of reality: we could function very well without knowing any of the details. But by thinking about such issues, a person sees oneself as part of a privileged group, the human species, and their personal existence as a chance event in the great scheme of things. To me it feels like a kind of spiritual liberation.

A book of non-fiction always runs the risk of requiring too much specialised knowledge from the reader to properly understand the arguments therein, but, on the other extreme, it can get really dull if it spends too much time on basic information. The middle ground is hard to find and it depends heavily on the reader’s familiarity with the topic. For my part, this book was almost perfect. It filled a lot of holes in my sketchy understanding of entropy and showed me how to think about relativity past its basic implications, but the quantum mechanics part was a bit sketchy. I could follow his arguments, but I was left hungry for more information.

Caroll discusses the concept of entropy in depth because it is central to his theory, then gives a whirlwind tour of the main statements in special and general relativity, and also of the most popular interpretations of quantum mechanics. He concludes that the nature of time is still an open issue, without much hope of being solved with our current understanding of the physical laws of the universe, at least until quantum gravity is finally integrated into a unified system of laws. It seems that since we get different configurations of matter when we account for entropy at first without and then with gravity, we will need to understand what happens inside of black holes and on the other side of their event horizon, where quantum gravity becomes important. Since 2010, when Caroll wrote the book, we have made a few more discoveries concerning the nature of gravity, but from what I can gather there is still no definitive theory to integrate the theory of gravity from general relativity with quantum physics. So the answer may still lie out there.

I guess one person’s enjoyment of a science book comes down to how much they can actually follow the author’s line of reasoning and understand the author’s conclusions. So for light reading or an introduction and background information you could do worse than read something by James Gleick, but if you feel ready to go to the next level I definitely recommend Caroll. It is demanding, frustrating at times, but well worth the time invested, and has a great bibliography, with suggested reading material both for the specialist and for the reader of popular non-fiction.

[Jenn "JR" · Rating 4 out of 5]

I enjoyed this as an audio book and spent a lot of time playing back some sections - and then - I got to the last chapter where the author nicely summarizes all the preceding chapters. It was then that I realized: I should have started with the last chapter but don't always approach audio books the way that I do paper / e-books: I read the table of contents, the indices, references, review the sections - intros and summaries. I then read the book chapters in order (usually).

It strikes me that I may have to re-read this book in paper some day - but ultimately - my takeaways are that the arrow of time goes in one direction (entropy). There is no such thing as "balance" - objects are always going from one state to another: ice melts into water. Water freezes into ice. The creamer blending into the coffee but not separating (ok, he's not talking about almond milk which sort of does separate when left standing).

I thought it was really interesting that the author mentions how there were so many brilliant thinkers coming up with theories -- from a particular place/time. Are they, perhaps, self validating? Do monopoles exist or not?

In the last chapter, the author mentions that science is always finding new ways to look at the world around us and we can pretty much guarantee that at some point, we'll learn some new information that will help us better understand how our world came to be. He also says that time has always existed - but what if it hasn't? How can we possibly know if our existence is marked by time/entropy -- whether there are some extra-temporal states would we even be capable of perceiving them?

The author wants everyone to be able to casually discuss the laws of thermodynamics with anyone in their lives - the information should not be seen as inaccessible. While a self study course might take a really long time, even pursuing a degree in physics might not even afford the opportunity to discuss string theory or quantum mechanics. I guess I'll keep up my course of self-study and one day, maybe, I'll be able to discuss some of these concepts.

[Nicholas · Rating 5 out of 5]

Simply the best popular physics book I've ever read. Carroll is amazingly lucid, practical and totally excited about the subject while being conscious of the problems inherent in understanding something that is so fundamental to our existence that we take it for granted: time.

What is time? Does it exist naturally, or is it emergent from some other property of the universe? These are the grand questions that drive From Eternity to Here. In fact, the questions are so grand, and so monumental, Carroll can't help but comment on the scope of modern physics from classical mechanics and relativity to the absurd realities of quantum mechanics. Within From Eternity the lay reader will find a candid and clear discussion of pretty much anything physics related a non-specialist would want to know (and perhaps a few things you didn't).

The book is divided into roughly four parts - each dealing with an aspect that explains several key features about time. Of primary focus to Carroll is the arrow of time - a fundamental asymmetry in the flow of time. How is it that we can remember the past, but not the future? It sounds like a nonsensical question at first glance, but when you really think about it, there is no logical reason for why it should be so. The first part of the book attempts to nail down what we even mean by time, and to elucidate some of its properties so that we can better understand what it is we're looking for when constructing a theory of time. Next, Carroll takes us on a whirlwind tour of Einstein's theories of relativity and the way that they distort our common sense notion of what time is and how it's supposed to function. In particular, Carroll emphasizes that subjective nature of time, not in the figurative sense we're all used to (This class feels like it's taking forever!), but in a very literal sense through the exploration of light cones and the time bending effects of black holes.

Perhaps the most interesting part of the book follows thereafter. Carroll ties the evolution of time and its direction to the laws of thermodynamics, in particular to statistical mechanic descriptions of the second law: entropy always tends to increase. Here we see asymmetry manifest and perhaps the first clue as to why we perceive time "flowing" in a single direction, or even existing at all. The tendency of any closed system is to always be more and more entropic (here we can insert the word disorganized, but very carefully). In common parlance, any system tends toward the most likely state we should find it in - mostly equilibrium. Think of a box of gas that initially has all of the gas restricted to one side in its initial condition. If we watched such a box over time, what would we find? Most likely, we'd find that in the future, the gas would have dispersed until it reached equilibrium throughout the box, at which point it stops evolving. So goes the universe according to Carroll. For some reason that we're not one hundred percent clear on, the universe just after the Big Bang contained very low entropy. Our experience of change and time rests on the natural working of that system toward a more natural high entropy condition.

The last section is highly speculative, but exciting to think about. Here Carroll expands upon a number of theories about the evolution of the universe, its origins and its ultimate destination. Admittedly, there is little evidence from any of the proposals, but they're all fantastic to think about.

Carroll seems to succeed more than anything in making you feel smaller and more insignificant than you thought possible. This is an impressive feat given the series of substantial widenings in perspective given to us by scientists from Galileo to Hubble. But it's not a bad thing. On the contrary, there's a wonderful sense of order and determinism in the evolution of the universe as presented by Carroll that's mind-blowing to think about. As a reader, you really can't help but be infected by Carroll's own excitement and optimism that science will ultimately unlock even the deepest mysteries of where we came from and where we're going.

Probably not for people who have no concept of physics whatsoever, but if you've read any popular physics books by Greene or Kaku, you'll find that Carroll is much more down to earth. The stuff in the latter section tends toward the esoteric, but it's presented clearly and with as much caution as possible. Pick it up. It's worth your while.

[Terence · Rating 3 out of 5]

Unlike my usual practice since joining GoodReads, I very deliberately did not take notes while reading From Eternity to Here. I wanted to enjoy myself with an interesting topic (cosmology) and not be overly concerned with learning anything - the nonfiction analog of the fictional brain candy I read.

But this is the post-GoodReads era of my life so am compelled to offer some note to the interested reader. Thus:

The problem under discussion here is the "arrow of time" - why, unlike the physical principles of space, do the principles of time appear irreversible? It turns out we simply don't know. We know enough about quantum mechanics and classical gravity to make reasonable guesses but absent a theory of quantum gravity we're pretty much fumbling around in a very dark room.

Carroll has his own favored answer - which has to do with the multiverse, de Sitter spaces, infinite entropy, low-entropy states and baby universes, and preserves the reversibility of time (though not within a particular universe (sorry, "Dr. Who" fans) - but he's honest enough to say it's unverifiable with the present state of knowledge and lays out other promising competitors.

I thought Carroll padded the laying out of the problem (and why it is a problem), and he's another author who has a tendency to make cute, folksy, annoying asides but if you're interested in cosmology this is a book worthy of your unrecoverable time.

(In an aside - Reading this book brought to mind one of the worst books I've ever read - John Horgan's The End of Science Facing the Limits of Knowledge in the Twilight of the Scientific Age Helix Books, where the author argues that we've pretty much come to the end of scientific advance and are only mopping up a few loose ends here and there. This is one of the few books that truly disgusted me. Most of my one-stars are simply books that I didn't like but this one made me feel intellectually assaulted. In the light of what we're discovering in space, consciousness and genetics (among the sciences that I'm interested in), Horgan's short-sightedness appears especially appalling.)

[Tony Heyl · Rating 5 out of 5]

Time is both a simple and yet complicated scientific question. I looked up books about time after seeing one of the Through the Wormhole shows on Science Channel. This is actually a really well put together book. Most of the book is about entropy and the evolution of the universe, so it makes sense that the content itself goes from the very simple to the very complicated, bring you along the way without making you feel like an idiot. The equations and explanations are also done in a way to really tease out how they came to be formulated so that you understand the timeline of understanding time and can put things into context. The way that charts and examples are done is good too because it's not like you have to move to the next page to understand what the text is saying. By the end of the book, not only did I have a much better picture of time, but also of how a multiverse would work, and it really does make sense. Not the best science book I've read because it is complex at times, but I definitely think it is quality.

[Dave Courtney · Rating 3 out of 5]

Sometimes the every day person who's not a scientist or a physicist or anything such related wants a science book that can help explain the most simple and basic questions relating to the human experience. After all, the world we observe is also the world we experience, and it could be said that the latter is what truly matters (although Carroll thinks the former matters more, but I'll get to that).

In those moments, its always great to find someone who can distill big and complicated ideas that people like me could never otherwise understand, and break it down into language and ideas that can make sense. Usually that involves taking the observations and placing them within the human experience. Carroll does precisely this in the first half of this book. One of the reasons I picked this book up is because I just finished Kieran Fox's spiritual biography of Einstien (I am Part of Infinity). This book felt like a helpful pairing, and truth be told, if you want something that can distill things like Einstein's theory of relativity into the most clear and basic possible picture, this book is worth it.

I will admit though, by the time I got to the end my brain was hurting. It's not that I felt completely stranded (although I won't pretend like I understood it all), its that there is such a sharp shift between the first half when he is dealing with practical and shared observations and the second where he suddenly leaps off the cliff into the real reason for his writing this book- into his predictions about the arrow of time and the question of what happened (or what is) before the big bang. Others have noted that this is where his admitted extreme minority position ultimately ends up unconvincing, and I would share that sentiment. I would simply attach that sentiment to my intuition. What I'm really interested in is, how does this idea make sense of the way we experience the world in the here and now, in this minute space that we call our minds and our bodies. What he posits is very clearly a thought exercise that appears so eager to get to his prediction that he keeps covering up hypothesis after hypothesis in order to make it coherent.

Now, one of the best parts of this book is his observations about the scientific process. He acknolwedges an elephant in the room right off the start, which are his disenters. So right off the bat we are left with this sense that the reason he wrote this was to respond to the criticisms of his present conclusions. Fair enough. It's just important to keep that in full view. One of his main rebuttles is that science cannot be distilled merely to information. Predictions, which are made in relationship to working theories which can be falsified, are equally necessary. Even if the entire prediction proves wrong, that part of the process is an essential part of getting us to where we are going, which is to a fuller understanding of the world we experience. I think he's right, and hits on a core issue within modern science.

This is a side tangent, and it will speak to my own concerns that I bring with me into this book. I am both a person of faith and someone who has a layperson's interest in certain subjects and ideas that exist far beyond my own expertise. In the epilogue, Carroll makes the categorical mistake of defining God in terms of the common notion of "the gaps." What's more odd is that he uses this as a slight jab against his disenters whom might see him appealing to an argument from the gaps. In one fell swoop, he wants to distance himself from what he sees as the common move of the religious while slyly suggesting that it's actually his disenters that are enabling that view, not him. The fact that he is willing to place equal emphasis on "predictors" in dialoging about what is stands as the essential proof that his approach actually trusts the notion that "we" (by which he means the human enterprise of science) will one day arrive at the proper answers and figure it out.

Which of course, from my own vantage point, is something that I think he misrepresents and takes too far. Given that his argument centers around spacetime as an eternal state without a true beginning, he sees this as the definitive point that sets him apart. And yet, the only thing he is offering is more funcitonal information about how things work in spacetime. The illusion he is creating is that somehow he has bypassed the necessary questions about beginnings or existence by placing a universe with, as he states, clear boundaries, within cycles. All this does is reshape the information, it doesn't change any of the quesitons or the intuitions. On the question of God, no matter how much he positions himself in such a cosmic portrait, none of this changes what we find in our experience of this world as creatures occupying existence in spacetime. He hasn't rid the world of God, God is still present in the fabric of these experiences.

Which is not to say that he must conclude the existence of a God. It is simply to say, I think his giddiness for, ironically, distilling all things to "information," which he seems to think is what truly matters and should be a source of human pride and exceptionalism as "we struggle to understand things much greater than ourselves," makes him forget how it is we know anything at all- through our experience of this world. With all due respect, I simply do not buy that we "create purpose and meaning" by seeking information that explains our world away in reductionist terms. That's simply not true to our experience. Its true for a worldview that adheres to scientism, but not to the world we both observe and experience.

And with all due respect, seeking God is not seeking something "outside of the universe." But of course one would appeal to such a descriptive if one wants to keep God out of sight out of mind. Rather, it is about what the nature of the world reveals itself to be. One in which God exists, or one in which God does not. And what's ironic about my experience of this book, and perhaps more ironic given Einstein's own spiritual convictions, the simple fact that reality has an eternal shape evokes an eternal nature. For me, his predictions are not doing away with God, they are finding in our present theories a nature that points to God.

So, what does all of his prediction, ultimately defined by appeals to the multiverse (which is not a working theory, as he rightly points out) and baby universes that grow and die (or expand and detract), come down to? Entropy. Time is entropy. The natural boundary of how a universe, or anything, emerges relates to that initial low entropy state that moves to a high entropy state, or from order to disorder. Entropy increases but always wants to come back to that initial low state. Where time is concerned relates to that which is irreversable. Here is prediction of baby universes takes root by suggesting that these universes make sense of what we might call the incoherence of the seeming arrow of time precisely by acounting for the loss of that emergence that connects to high entropy states. Once this conracts or reverses, it acts more like a child all its own rather than a singularity. It's not coming back into the womb in other words.

He spends a good deal of time in the first half of the book trying to explain how spacetime acts and functions as a coherent thing. The question is not about time itself, but about where something exists in relationship to another thing. It is only because we have a reliable and consistent measure that time is a thing. Otherwise what we have is not past, present and future, but entropy changing things from one state to another. There is no "similtaneous far-away events." Rather, what we have is a surface of constant time that spreads throughout the universe, in which every point is the same relative to its vantage point (the big bang occurs and all things move apart, thus where we might stand in spacetime would not be at a particular point but the same point with a different vantage. As he states, "Time can be measured along each individual world line, but it's not a built in feature of the entire spacetime. " p80

So much of this has to do with things in motion. One interesting point here is the notion that freefall is the nature state, gravity is not. What we experience in gravity is the push of things trying to get to its nature state against a contrary force. From the perspective of a free fall, any measure of spacetime becomes equally incoherent, which is precisely what applies to the movement of something like our earth. Or the fact that things like dark energy and black holes are universals where as objects are not (which is to say, not all things are equal to this end). What we experience as time is really understood as a personal vantage point

And yet, any ultimate theory still includes life. He spends some time talking about life in the second half of his book. Some brain breaking theories regarding the expansion of the universe, and how all things will eventually disappear into empty space. And yet, he brings up pertinant questions about why we don't just see empty space. Why, he asks, do we find ourselves in a universe evolving gradually from a state of incredibly low entropy (order), rather than being isolated creatures that recently fluctuated from the surrounding chaos?" (p314). A problem, he states, that needs us to look at the arrow of time. The cold and forbidding emptiness is overhwelmingly likely. Not only that, but an early universe "suffused with dark energy" (inflation theory) causes the acceleration precisely to this end. Why we find ourselves where we are is because we are very close to the big bang.

How does this apply to our experience of this world? I'm not sure. I don't think Carroll is entirely sure. One place I disagree with Carroll is on the notion that theories ever arrive. They don't. Theories are always overturned, and will continue to be overturned for all of time. That's how it works. I think he overplays his hand on that front. Information stays true. Experience stays even more true. On this front I agree with Carroll when he posits that theory is best explored in the present (which of course doesn't really exist), given that what happens now should be a measure for what happens at all times. I would narrow that down even further to two things- what is the nature of our experience of this world, and what is the nature of the world we are experiencing. On that front, I think there is lots of fascinating ideas here to transport on to both of those things. After all, what we do with our time is probably one of the most important questions facing the human experience. We shape our lives around this. And if a shaped life is relating to a specific reality or world, understanding the nature of that which we relate to is also a helpful part of that. That might apply to the changing of seasons. It can apply to the contrast between the seeming wideness expanse of space and our minute slice of if where we are. All conceptions that matter to a lived life. Here is where I would suggest that, for me, a world which seems to contain both an eternal nature and the experience of finite time seems to make sense of what we find in how it is we (an emergent universe) relate to God (an eternal presence). After all, if life is disorder awaiting transformation to a truly ordered state, wouldn't that make sense of those inate desires interwoven into all things? If it truly is the case that this is compatible with the notion of distinct things persisting apart from that ordered state (although Carroll believes such things just disappear), all of a sudden there is language to imagine how it is one thing might exist within a singular, eternal whole. That's where my brain tends to go anyways, and this was a good place to kind of spark that kind of thought process.

[Charles Daney · Rating 5 out of 5]

I'm giving this a top rating, because the book is truly excellent. But it should be said that to get the most out of it, a reader should have at least some intuitive understanding of scientific topics like elementary physics, cosmology, relativity, black holes, quantum theory, probability, statistical mechanics, thermodynamics, and entropy. These all play a major role in the story, and there's not much basic material on most of them. Very little math is required, however.

Although the book is subtitled "The Quest for the Ultimate Theory of Time", it's not actually so much about time as it is about how our notion of "time" arises out of what's known scientifically about the universe. Much of the story is speculative, because there's still so much unknown, yet it's grounded in well-established science. This is in contrast with what countless philosophers have attempted to say about the subject. There is the well-known remark of the philosopher Augustine: "What is time? If no one asks me, I know. If I wish to explain it to one that asketh, I know not." That's a candid admission that while philosophers have said a great deal about the experience and psychology of time, they have little useful to say about what time really "is".

The funny thing about time is that there's - seemingly at least - a sharp difference between the past and the future. We generally think we know a lot about the past, both from our own experiences and from history recorded by others. We have knowledge, albeit hardly complete, of the remote past from the work of geologists, paleontologists, and archaeologists. Things like dinosaurs, ancient human societies, and the U. S. Civil War actually existed, even though many details are sketchy. But there's very little of the future - especially more than a few years out - that we can claim to have good knowledge of. This striking asymmetry is referred to as the "arrow of time". Time seems inevitably to point in only one direction.

From a scientific point of view this is paradoxical, because fundamental laws of physics are reversible in a specific sense. If we could know in sufficient detail the present position and momentum of any physical object, physical laws could determine those data just as accurately in both the past and the future. Given the necessary detail, the position of the Earth relative to the Sun and other planets could be calculated just as accurately for a million years either in the past or the future. (Of course, knowledge would also be required of other things that might reach the Solar System in that time period.) One aspect of this that's explored extensively in the book is that it should be impossible for information to be lost. That's because physical laws seem to be reversible. So calculation (with a fast enough computer) of information about the past state of a system should be possible if given sufficient information about the system's present state.

And yet there is a phenomenon called "entropy" that seems to contradict the idea that information can't be lost. What is entropy? As much of the book tries to explain, entropy is what accounts for the "arrow of time". It is the reason that we feel we can know the past much better than the future. But it's a very difficult concept to get a handle on. Much of the book is devoted to that effort.

You can get some idea of what "entropy" is by thinking of your own home. Although your place might not be as neat and tidy as it could be, there's still a lot of order in it. Most of what you need for preparing and eating meals is in the kitchen. You have specific closets, cabinets, drawers, and boxes for storing stuff you need for all the other things you do. But if your home is burglarized, some of the stuff will be gone, and much of the rest will be scattered all over. So there's much less order. And if your home burns down, there may not be much left of your stuff except for smoke and ashes - extremely less order. Most or all of your books, pictures, movies, electronic devices, etc. will be gone. The information they contained acutally will be lost, along with all the other order in your home.

Entropy is simply the process in which order turns into disorder. Although it can't in practice be measured in the same way as velocity or temperature, it can be conceptualized as a measure of disorder. It's why living things and people and whole civilizations inevitably die and disappear. The Second Law of Thermodynamics says, essentially, that entropy always increases. So information, apparently, can be lost and cannot be reconstructed from information about the present. And this is the basic reason an "arrow of time" exists and points in the direction of increasing entropy.

The concept of entropy was formalized by Ludwig Boltzman, who expressed it in the simple formula S = k × log(W), where S is the entropy, k is a constant (Boltzmann's constant), and W is the number of possible microscopic-scale system states (which, in practice, can't be enumerated). The science of cosmology now generally regards the universe to have started in what's called the Big Bang. During the very earliest seconds nothing existed except quarks and gluons of a small number of types. Since each type of particle was indistinguishable from others of the same type, the entropy was very low.

However, since the earliest times, entropy has always increased, as additional different types of particles formed, followed eventually by innumerable distinct objects like stars, planets, and galaxies. The initial very low entropy is a major mystery, since for almost all times after the earliest the universe has been in a vastly higher state of entropy. Yet ever-increasing entropy is precisely why time itself even exists. The final chapters of Sean Carroll's book give a plausible but admittedly speculative explanation of how to resolve the enigma of very low entropy at the beginning.

The subject is likely to remain speculative for quite a while, because there are important unknowns that may long resist understanding. Among the most important is how general relativity and quantum mechanics fit together, even though they are very well established in their own realms. Even more remote from the possibility of empirical research are cosmological questions such as how the Big Bang itself came about. Perhaps it was a routine fluctuation in the quantum gravitational field - if we even had some idea what that was.

Although this book was first published in 2009, it seems unlikely that most issues covered will be better understood in the near future. Consequently, little of the book's material should become outdated anytime soon.

[Andrés Astudillo · Rating 5 out of 5]

Todo libro de cosmología siempre es imprescindible. En este caso particular, Carroll plantea diversas preguntas, todas relacionadas con el espacio tiempo. Sin embargo, de lo que empezó como la simple explicación de mecánica estadística con el que nos presenta formalmente al concepto de "entropía", el concepto es adaptado hasta llegar a las grandes preguntas cosmológicas y por tanto, a los modelos teóricos del universo.
Fue fascinante saber que estuvo relacionado alguna vez con Hawking, menciona a Sagan, a Wheeler, a Feynman, a Hoyle, Thorne, a Alan Guth, y a muchos físicos de renombre, por lo que es un libro sumamente agradable para quién ama al Universo y a la divulgación científica.
Considerando que vivo en Ecuador, y que al igual que cualquier otro país neoliberal de la triste Latinoamérica, es necesario indicar que vivimos en un -cono de luz- del pasado; esto es, un desfase de divulgación científica de unos 20, 30 años, Carroll logra aterrizar a los conceptos más recientes de la fìsica teórica, tales como la 1) paradoja de la conservación de la información, la 2) física cuántica de campos, 3) gravedad cuántica, los 4) universos de De Sitter (energía oscura) y sus consecuencias que resultan en los 5) multiversos.
Por tanto, si tienes una base de cosmología, el libro va a proporcionarte nuevos rumbos sobre los que puedes dirigir tu enfoque. Sumamente claro, lenguaje amigable cuando debe, pero no debe de ser tomado a la ligera, ya que requiere de la comprensión de algunos conceptos que guardan estrecha relación con relatividad general y la clásica función de onda de la mecánica cuántica.

[Bjorn Bakker · Rating 5 out of 5]

A joy to read, and a triumph of popular science writing on the nature of time.

[Alex Zakharov · Rating 4 out of 5]

Great read, under proper expectations. The central theme is the puzzle of macroscopic irreversibility which we are all intuitively familiar with - eggs don’t spontaneously unbreak from the frying pan and the milk doesn’t unmix from your morning latte. The puzzle is that at the microscopic level the fundamental laws of physics are perfectly reversible, so why does the Second Law of thermodynamics hold in every situation that we know of? More specifically, Carroll seeks the explanation for why the time flows in one direction only.

Frankly, nobody knows the answer, and even in order to ask the question meaningfully Carroll is forced to cover the basics of relativity, gravity, entropy, thermodynamics, evolution of early universe and quantum field theory. This is a lot of ground to cover and the man refuses to use any equations! Sure, intuition is often useful, but “the unreasonable effectiveness of mathematics in the natural sciences” is not accidental and avoiding equations altogether is a disservice to your audience, no matter what your publishers tell you.

And so, the chapters are really hit and miss. For example, the overview of special and general relativity were disappointing (Brian Greene’s and Jeffrey Bennett’s pop introductions are better). Similarly, for clear overview of primordial nucleosynthesis, plasma, and recombination epoch I would go with Max Tegmark.

Let’s move on to entropy – the subject is notoriously confusing so I won’t hold too much grudge against the author. In fact he covers Boltzmann entropy better than most, but once he moves out of thermodynamic equilibrium into Shannon, Gibbs and free energy the absence of any equations muddies up the “intuitive” explanation.

On the plus side there are number of areas where Carroll is fantastic - black hole thermodynamics, Hawkins radiation and microscopic reversibility were all excellent. The various flavors of Boltzmann brain arguments and the havoc they wreak on the inviolability of arrow of time are by themselves worth the price of the book. And in the end Carroll does make a convincing case for the necessity and arbitrariness of the “Past Hypothesis”. In other words, we fully concede that we cannot derive macroscopic irreversibility from the fundamental laws of physics, and we simply must assume low-entropy early universe as the boundary condition. And it is that boundary condition that sets up the directionality of the Second Law and One-way Arrow of Time.

The “Past Hypothesis” seems to be the mainstream view today, in a sense that we have to assume it until we find a better explanation. Naturally, there is no dearth of principled suggestions that attempt to avoid the necessity of assumption (e.g. bubble universes, cyclical time, string-theoretic “branes”), but clearly the question is still open.

Carroll himself has a speculative theory (the Carrol-Chen model) where there is a background de Sitter space, in equilibrium and without arrow of time. Against this background, quantum fluctuations occasionally give rise to “baby universes” that start out in a low-entropy state and have local arrows of time (backwards or forward!). These baby universes eventually dilute back into the background, resulting in an overall time-symmetric multiverse. Yep, a little crazy, but wait till you get to the next paragraph.

The book was published in 2010 and since then there were further developments in the field. I particularly liked the tasty 2014 marriage of quantum computing and spacetime, where perhaps for the first time in history computer scientists contributed to theoretical physics. Yes, I’m referring to the whole ER=EPR conjecture, and the dissolution of the “Firewall Paradox” with the help of computational complexity. In that context Leonard Susskind explicitly relates space to quantum entanglement, and time to the growth of quantum computational complexity. If this plays out then “Past Hypothesis” won’t be necessary, but till then I will leave you with this thought:

“Those who think of metaphysics as the most unconstrained or speculative of disciplines are misinformed; compared with cosmology, metaphysics is pedestrian and unimaginative.” —Stephen Toulmin

[Nick Black · Rating 3 out of 5]

not as good as Greene's books on similar topics, especially The Fabric of the Cosmos.

[Hamid · Rating 5 out of 5]

Figures such as Galileo, Newton, and Einstein are celebrated for proposing laws of physics that hadn’t previously been appreciated. But their accomplishments also share a common theme: They illuminate the universality of Nature. What happens here happens everywhere. Galileo showed that the heavens were messy and ever changing, just like conditions here on Earth; Newton understood that the same laws of gravity that accounted for falling apples could explain the motions of the planets; and Einstein realized that space and time were different aspects of a single unified spacetime, and that the curvature of spacetime underlies the dynamics of the Solar System and the birth of the universe. Likewise, the rules governing entropy and time are common to our everyday lives and to the farthest stretches of the multiverse. We don’t yet know all the answers, but we’re on the threshold of making progress on some big questions.

Over the course of this book, Sean Carroll investigates what we know about how time works, both in the smooth deterministic context of relativity and spacetime, and in the messy probabilistic world of statistical mechanics. He finally arrives at cosmology, and explores how our best theories of the universe fall embarrassingly short when confronted with the universe’s most obvious feature: the difference in entropy between early times and late times.

Understanding a deeply puzzling feature of the natural world is a process that can go through many stages—we may be utterly clueless, we may understand how to state the problem but not have any good ideas about the answer, we may have several reasonable answers at our disposal but not know which (if any) are right, or we may have it all figured out. The arrow of time falls in between the second and third of these options—we can state the problem very clearly but have only a few vague ideas of what the answer might be.

In such a situation, it’s appropriate to dwell on understanding the problem, and not become too wedded to any of the prospective solutions. A century from now, most everything Carroll covers in the first three parts of this book should remain standing. Relativity is on firm ground, as is quantum mechanics, and the framework of statistical mechanics. We are even confident in our understanding of the basic evolution of the universe, at least from a minute or so after the Big Bang up to today. But our current ideas about quantum gravity, the multiverse, and what happened at the Big Bang are still very speculative. They may grow into a robust understanding, but many of them may be completely abandoned. At this point it’s more important to understand the map of the territory than to squabble over what is the best route to take through it.

[Todd Martin · Rating 2 out of 5]

Science and journalism are two very different disciplines and require a different set of skill sets. Those with skill in one area may or may not have talent in the other. I’m not sure about Sean Carroll’s skills as a scientist, but I have little praise for his talents as a writer.

Carroll seems incapable of explaining things clearly. He attempts to illustrate points throughout the book with convoluted examples that appear to be intended to confuse rather than illuminate. He even manages to make very simple topics (ice cooling water in a glass, or the uncertainty principle for example) a confusing, over-complicated muddle. Couple the above with a dry, meandering writing style that goes on for close to 500 pages and you are left with a reading experience that can best be described as pure drudgery.

In the end, those that already understand the concepts covered will be able to follow Carroll’s tortuous examples, but are unlikely to learn anything new. Those not familiar with the subject matter will probably not be able to follow Carroll’s examples, and are also unlikely to learn anything new. ‘Time’ is therefore better spent elsewhere (let me suggest Brian Greene’s “Fabric of the Cosmos”).

So what is the ‘ultimate theory of time’? Time is an inherent property of the universe, it goes forwards and not backwards because entropy would decrease if it did (a violation of the 2nd law of thermodynamics). That’s it.