Take just a minute to consider...

In theory, travel across the ages is viable, but Paul Davies is keen to know how

Time is the most elementary aspect of our experience. We feel it passing within our very being in a manner quite unlike our impressions of space or mass. This personal aspect of time makes the subject fascinating and perplexing, and it explains the huge popularity of fictional and non-fictional books about the nature of time. Unfortunately, a lot of nonsense has been published on the subject. Michael Lockwood's book, by contrast, is a model of balance and clarity. He weaves together the many disparate strands of time's story, from quantum physics to cosmology, from the origin of life to the mystery of conscious awareness.

The common thread that runs through his treatment is the search for an answer to a seemingly trivial question: does time pass? In daily life, we experience a special moment of conscious awareness - "now" - that progressively advances as "time flows". The present moment is our window on the universe and is sharply distinct from the future, which does not yet exist, or the past, which has slipped away into a shadowy realm of memory and records.

All this may seem obvious, except that physicists can find no evidence at all that time actually flows or moves in any way. Furthermore, philosophers are hard put to make sense of the very concept. Ask the simple question "How fast does time flow?" and you get the vacuous tautological answer:

"One second per second."

Einstein expressed the physicist's position succinctly when he wrote: "The past, present and future are only illusions." His special theory of relativity denies any absolute significance to the notion of simultaneity: there can be no universal "now". To say that two events in separate locations happen "at the same moment" is a purely relative statement and will not be true in the reference frame of observers who move differently.

The conclusion is that past and future events are just as real as the present. Viewed in this way, events do not "happen" - rather, there is a static "timescape" containing all events. Some events are correlated with observer experiences, each labelled as "now" by the respective correlated mind state. Somehow from this static sequence of physical-mental correlations emerges the sensation of temporal flux, even though no such flux, it seems, truly exists. Lockwood examines this conundrum with care and deliberation, giving generous attention to the conceptual issues and to the scientific ideas.

Denying the objective reality of the moving present is not to deny that past and future have no meaning. Take a movie of a familiar scene and play it in reverse; it looks preposterous, because sequences of events generally possess a well-defined directionality, or arrow of time. The origin and nature of time's arrow has a long and disputatious history. The most pervasive asymmetry comes from thermodynamics, and there is no better example than the fact that the sun is burning up its nuclear fuel and irreversibly dispersing the released energy around the cosmos. The entire universe resembles a gigantic clock slowly unwinding towards what 19th-century physicists called a heat death - a state of thermodynamic equilibrium, or maximum entropy, after which little of interest would occur. Still unresolved is how the universe got wound up in the first place or why the Big Bang created a universe with less-than-maximum entropy.

These bewildering puzzles are thrown into sharp relief by the topic of time travel. The special theory of relativity predicts that travel into the future can be achieved by moving close to the speed of light, and this so-called time dilation effect has been confirmed many times in experiments. Leaping forward in time in this manner is, however, a one-way journey. Going backwards in time is far more problematic, not least because it opens up a Pandora's box of paradoxes familiar to all Dr Who fans. What happens if a time traveller visits the past and tries to change history, for example, by murdering his mother before he was born?

Lockwood sifts through the various paradoxes and reviews the many muddled discussions that have been published in an attempt to resolve them, including all sorts of hand-wringing about free will. He points out that there is no real paradox so long as the scenario is restricted to a self-consistent narrative. A time traveller may participate in past events, so long as the effects are consistent with the future from which he or she hails. For example, a time traveller may visit the past and save his future mother from murder. But, as Lockwood remarks, the requirement of self-consistency will restrict where and when they may go, and what they may do.

Possibility is one thing, actuality another. Is a visit to the past permitted by the laws of physics? Einstein's general theory of relativity, which incorporates the effects of gravitation, does not forbid an object or observer from looping back in time. There are even some proposals for how to accomplish it. The best-known model of a two-way time machine is the wormhole that links two points in space to create a shortcut. Physicists who dislike time travel focus their attention on why something might go wrong - for example, the quantum vacuum might create a surge in energy that would wreck the wormhole before a time difference could be established between the two ends.

I have just touched on a handful of the many topics covered in this wide-ranging book. It is a gentle introduction for readers seeking a lucid guide to basic physics and cosmology, and especially ideas about black holes, event horizons and causality. Even string theory and loop quantum gravity get a mention towards the end. Lockwood's style is clear and straightforward, even if the reader is left with mind reeling and a sense that, for all the intellectual power that has been brought to bear on this vexed topic, something crucial is still missing.

Paul Davies is professor of natural philosophy, Australian Centre for Astrobiology, Macquarie University, Sydney, Australia. His latest book is How to Build a Time Machine .