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.