If you're suprised by the variable flow of time you can't have a Physics doctorate, so the author has setup his own logical paradox: If this is a book for the knowlegable enthusiast it's too lightweight, but if its aimed at the novice it addresses the interesting questions too late (you'll have given up before you get to the "Back to the future" discussion - assuming linear casuality).

Aside from the uneven tone, this is a fun book in a handy size with friendly line-drawings on every other page. Clearly, it's not meant to be taken too seriously. Still, it is a shame that the middle (pseudo hard-science) portion isn't at the end, where it can be safely ignored. An alternative to consider is Kip Thorne's "Black Holes and Time Warps: Einstein's Outrageous Legacy" which is suprisingly readable, and only four times the length.

However Paul Davies makes this subject easier to understand because after the inevitable hard to grasp idea or impossible word, he explains it with a simple analogy.

The only criticism i would have is that it does slightly begin to fall away in the middle and become tedious and hard to read. This however is the nature of the beast that is astrophysics and it then turns around in the last few chapters to being thoroughly absorbing again.

In my opinion this a fantastic book for those with a casual interest in time travel etc, and very much worth a read.

-Simon

How to Build A Time Machine should sell by the time-machineful. The title sounds like an archetypal mad scientist's manual; chiming with this theme, the cover looks like an archetypal mad scientist's manual with its bold, confident sans-serif font, and it's written in a breezy, accessible style that leaves one with perhaps misplaced confidence that one understands the niceties of spacetime and quantum mechanics.

In his introduction, Paul Davies quotes JBS Haldane's dictum "the world is not only queerer than we think, it is queerer than we can think" and the whole book acts as proof. Particularly fascinating are the early chapters "How to visit the future" and "How to visit the past." The wondrous implications of Einstein's Special and General Theories of Relativity are explicated clearly for the laity; time is elastic, and by simply moving about the exact duration of time between two definite events is lessened; by flying from London to Cape Town and back, physicists demonstrated in 1971, ultra-accurate atomic clocks lost 59 nanoseconds relative to identical clocks that stayed in London. Gravity's slowing effect on time effects even the relative times on the bottom and top of a building; in 1959 in Harvard it was found that this timewarp factor in a tower 22.5 metres high resulted in a slowing effect of 0.000000000000257 percent.

So in other words there is a miniscule time difference between the top and bottom deck of the Number 10 bus. And furthermore due to the Number 10's motion a timeshift effect relative to the stationary observer occurs. Something else to ponder as the bus chugs its way homewards.

The difficulty for the general reader in accepting all this moved Einstein to write in a letter that "the distinction between past, present and future is only an illusion, even if a stubborn one" - this observation, running counter to instinctive wisdom about the immutability of the past and inaccessibility of the future, is the key to the book.

Davies shows the theoretical possibility of time travel via wormholes, essentially a black hole with an exit as well as an entry (apologies to those readers who properly understand the physics for my blunt summarising, and those who don't are referred to the book), and doesn't gloss over the enormous technical obstacles to any attempt to tame these awesome cosmic forces and build a nice shiny Tardis.

The actual chapter "How to Build A Time Machine" is slightly disappointing, particularly if you buy the book hoping to knock one together in the back yard over a bank holiday weekend. It's a little more complicated than that. Davies' proposed time machine design consists of a collider ("the first step in delivering the required energy to the spacetime foam" [to pluck out a 'virtual' wormhole and make it a bigger, permanent one]), an imploder (to further compress the spacetime foam in order to boost the temperature to create a wormhole), an inflator (to enlarge the wormhole), and a differentiator (to create a permanent time difference between both ends of the wormhole) This section is full of sentences like "with perfect separation of positive and negative energies from a million terawatt lasers running flat out and continuously, it would still take far longer than the age of the universe to build up that much negative energy" - indicating that Davies is quite far into the realm of the speculative and far from the realm of the practical.

In the final chapter, "Making sense of it all", Davies addresses the manifold paradoxes raised by the idea of time travel; as exemplified by Marty McFly's Oedipal tangles in "Back to the Future." Davies differentiates between these apparent paradoxes on the basis that some are self-consistent causal loops; for example "imagine a rich venture capitalist whose vast inherited wealth derives from a mysterious benefactor who befriended his great grandmother a century before. He finances a time machine project, and then uses the prototype machine to go back and discover the source of his wealth. He can't resist proving his time travel credentials by taking a newspaper with him, which he duly presents to his young great grandmother. Being an enterprising soul, the lady scans the newspaper's stock prices and makes some shrewd investments. These investments are the source of her, and her great grandson's, immense fortune, and the time traveller himself is the mysterious benefactor. No paradox ensues here because the causal loop is self-consistent, and everything fits together neatly." I can't help thinking that there must be a paradox here, but maybe I'm all too wedded to Einstein's stubborn illusion.

In recent years, Davies writes, time travel has gone from a parlour game for theoretical physicists to a fertile source of the thought experiments that are the mainstay of the field. Davies' book, like all the best popular science books, allows the non-specialist the comforting feeling of expertise. "How to Build A Time Machine" will beguile away a winter evening, and possibly act as an entry for the uninitiated into the frontiers of modern physics.