We are all only too familiar with the idea of time passing. In our lives, time is linear: we remember the past but not the future, and despite our best efforts, we age rather than regain our youth. This pattern seems true of the Universe, too, which began life in a Big Bang and has cooled, expanded and aged irreversibly ever since.
But although the arrow of time seems to be a fundamental feature of the Universe, it is one we don't yet understand. Investigating it demands an exploration of some of the most mind-bending subjects in physics - quantum mechanics, gravity, black holes and time travel. Happily, From Eternity to Here, written by a theoretical physicist at the forefront of cosmological research, offers a fascinating account of what we currently think the nature of time might be.
For something so familiar, time is a complete nightmare to understand. In physics, we think of it as a coordinate, a direction in space that we can locate things by; or as something that clocks measure; or as something mysterious that flows from the past to the future. Sean Carroll starts his exploration of the nature of time gently, by discussing ways we might think of it that would allow us to quantify it.
It is not long before we are introduced to entropy, a measure of disorder and the one concept that embeds a direction of time in physics, through the second law of thermodynamics and its requirement that entropy must always increase. It is hard to picture what entropy is, even for a physicist, but Carroll's descriptions are lucid and clear.
The discussion then turns to what we know about the cosmos: that it started in a Big Bang; that space is still expanding now, driven by mysterious "dark energy" (although Carroll is careful to point out that this isn't understood very well); and what we think the possible ultimate fate of the Universe might be, namely, stars burning up and galaxies collapsing into black holes, and then finally an endless void.
So far, we have covered the birth and death of the Universe and everything in between. But there is still the small question of why any of this should happen, and it is the exploration of why (a question that the author admits has taken up an impressive number of brain-hours over the course of history) that occupies the rest of the book.
Knitting together the evolution of the Universe, entropy and time demands some mind-bending thinking. Luckily, Carroll is an able guide, describing the deep structure of the Universe, space-time, how Albert Einstein linked it to gravity in his general theory of relativity, and the implications this has for travelling forwards and backwards and generally manipulating time. Wormholes? Simple. There are even straightforward pictures that show you how to think of them.
There's more to learn about the second law of thermodynamics, too. It turns out that entropy is intimately linked not only to the idea of disorder and complexity, but also to information. The arrow of time, measured by entropy, is subtler than it appears at first sight, and even plays a role in interpreting what happens in quantum mechanics when a measurement is made. Carroll takes us through quantum mechanical wave functions, Schrodinger's cat, entanglement and decoherence without skipping a beat. It makes sense, at least while you are reading it. And where it doesn't make sense, because our understanding of the underlying physics is still a work in progress, we are given a good overview of the problems theorists are trying to solve, and what sort of solutions we might find.
We revisit black holes, learning how they are related to gravity and entropy; string theory makes an appearance; and we learn more about current ideas of the ways a universe can be produced, or crunch, or bounce.
It is so easy to suspend your disbelief in following Carroll's argument that you don't notice that this book has stealthily started to bend your grey matter into the shape of a theoretical physicist's. By this stage, your brain will be able to seriously contemplate ideas on the frontiers of our knowledge, from multiple universes to baby universes (and Carroll is honest enough to point out that the latter is his favourite idea, rather than an accepted theory).
This is a privileged tour through not only the Universe, but also the absolute state-of-the-art thinking about it. The blurb on the back of the book claims that this is the "radical new history of time". So don't buy the brief version; get this one instead.
From Eternity to Here: The Quest for the Ultimate Theory of Time
By Sean Carroll
Oneworld, 448pp, £12.99
Published 1 March 2011