Rating: 
Summary: Timewarps for dummies.
Review: After the brilliant book "About Time" I was really
looking forward to Paul Davies next book on
Time.
But as a followup to "About Time" "How to build
a timemachine" is a bit disappointing.
I had expected a fountain of new ideas on wormholes
in spacetime and their use for time travel. Plus all
sorts of other ideas like e.g. Tiplers rotating
cylinders with the possibility of global causality
violations and more.
Instead the book revisits some of the material covered
in "About Time" in a shortened format. Other parts
are covered more extensively in Richard Gotts
Timetravel in Einsteins universe.
So all in all I would only recommend this book
for someone who hadn't read anything about relativity,
timetravel and spacetime before. And weren't looking
for to much detail.
With that said the book is still pretty entertaining.-Simon
Rating: 
Summary: SPACEFOAM AND WORMHOLES ARE FUN
Review: An inventive little book, artfully designed and compactly
arranged into short sections, HOW TO BUILD A TIME MACHINE
explains the basic theories of time travel and then explores
what means are required to achieve it. Paul Davies makes the
strongest and, it would seem, irrefutable case for time travel
into the future. But such "travel," based on Einstein's special
theory of relativity, which distinguishes a "time-dilation
factor" between two bodies moving at different speeds, is more
like an exchange of times: a spaceship leaves Earth and
approaches the speed of light; then it returns and the crew find
that Earth has aged seven times faster than they have. They
have avoided the standard speed of time on Earth, but still have
aged by their spaceship time. Arriving, say, a hundred years
"in the future," they resume their normal rate of aging on
Earth, having turned themselves into visitors (or relics) from
the past. But they have not escaped time. The big question is whether they can go back--back to the year
of their departure. Davies thinks they can. The best of
conceivable methods, he determines, is the wormhole, a
theoretical entity that links one space/time in the universe
with another. Somehow he imagines that it could be managed by
human beings on Earth who want to travel from the present into
the past. He doesn't trouble much over such questions as where
one end of a natural wormhole would be, where the other, how
people would get to one, and where the hell would they be when
they came out the other, but rather embarks with great gusto
on drawing up plans for building a serviceable wormhole
right at home. Sliding cheerfully through "spacetime foam," "antigravity," "the
chronology horizon" and other such slippery concepts, he finally
focuses on the project of opening up the throat of his wormhole
in the interstices of space and keeping it open so that anyone
who enters it is not instantly "spaghettified" by a crushing
singularity. How this project differs from counting the number
of angels on the head of a pin is obvious: it is much more dif-
ficult and much more scientific. Davies pursues it in good
humor, and to his credit does not avoid the mechanical
difficulties. To open a wormhole, he calculates, you would need
either an accelerator as large as the solar system or so much
"negative energy" that it would take more time than the age of
the universe to produce it. No matter, he concludes, science
will get better and the job will someday get done. So much optimism, such high spirits! You can't dislike this
book! Sober reasoning, of course, reminds you that time is not
a thing that you can visit, like walking forward toward a
mountain or back toward town. Time is the relationship between
things that change. And so if you want to go back to things in
a previous state, all those things would have to reverse their
accrued changes simultaneously: water would rush back upstream,
corpses would rise out up of the ground, buildings would be
unbuilt. But Davies and other theorists of time travel do not
have such a past in mind. Rather, they assume that there is a
historical continuum, a sort of museum of history that preserves
every change in the universe in a long static hallway, and the
successful time traveller will be able to go back and visit any
room he chooses. How you get from our changing world to the
fixed continuum, historical museum or alternate universe is a
problem they never consider, because such a past does not exist.
And so they prefer to play with intellectual games like "the
twins paradox," "the mother paradox," and so on; even Stephen
Hawking indulges in them. Final verdict: If you want to take
pleasure in wormholes, go ahead: this is the perfect book.
It's when the scientists start to request millions of dollars to
build them that we should draw the line.
Rating:
Summary: a valuable starting point
Review: Davies has attempted a very difficult task: how to write a brief and easily understood account of the physics and mathematics underpinning the idea of time travel. In the main part Davies succeeds admirably. It is clearly not that simple to condense highly complex mathematical equations, and concepts of and about quantum physics to a level that the ordinary person will not only grasp but perhaps think about. The book is effectively divided into two parts. The first is a synopsis of the theories underpinning time travel. In this section Davies provides a speedy overview of the history of thinking about time travel, the development of the theories, and he attempts to at least conceptually work through the possibilities and problems associated with the main theories that hold currency. In the second part, Davies sets out the mechanics of how to build a time machine. The content contained in is part, not matter how hard Davies tries, and despite its brevity, was very difficult for this reader to understand. Having no background in pure physics, I became a bit lost in places. But struggling through out, it is still informative and challenging. Overall, the book is a valuable starting point for the general person to get a grip on this though provoking topic.
Rating: 
Summary: What a crock!
Review: I followed the plans of this book, and even had my dad double check my work, and all I could do was go FORWARD in time! And very slowly at that! They should retitle this book "How to Build a Time Machine THAT ONLY GOES FORWARD IN TIME! AND VERY SLOWLY AT THAT!" Oh, yeah. That's right, I went there. So if you have a couple of extra blenders, some plywood and a spare flux capasitor, you might find some use for this book. Although I did not!
Rating: 
Summary: finally writing as shallow as the theories truly are
Review: I normally hate "scientific" writing about topics like this, because the authors often make their concepts sound more complicated than they truly are. In this case, there is nothing of the sort. This book stated everything flat out, and let us see how ridiculous some of the ideas are. I was amazed at how often Davies would appear to contradict himself within the same sentence. He would give an example of how time is relative, and then state that the opposite had happened than what was shown within the example. And what in the world was this with "Timeline" at the beginning of the book. That novel had nothing to do with real time travel, and some of the characters even stated this flat out. One of the scientists in the novel mentioned that time travel is a pure impossibility. Either Davies hates believing that someone disagrees with him, or he cannot understand anyone else's writing. If you want to be given a flat, straight forward statement of what many "scientists" (or "philosophers", as I would call them) believe, then read this book. If you want a realistic look at whether or not time travel is a possibility, "Timeline" would actually be a better bet.
Rating: 
Summary: Thinking about Time
Review: If you have the time and can make the considerable effort needed to really understand it, I recommend Brian Greene's "The Fabric of the Cosmos" as the best, most up to date introduction to modern Physics and Cosmology available for the general reader.
What if you can't spare the time and effort?
Well, you could read this book: it is a short and light (but not lightweight) introduction to the Theory of Relativity (including the very difficult General Relativity). This book even discusses problems of Quantum Field Theory and Quantum Gravity.
The discussion is centered on the problem of time travel. Time travel to the future (even remote future) is certainly possible, as the author explains. Time travel to the past is much more difficult.
The author discusses in a simple, conceptual way the technologies needed to produce a time machine. He can only give an example of a machine that can travel to a time after the time-machine itself was initially built: traveling to the remote past using his scheme would require finding a time machine built in the remote past by the "Ancients".
If no Ancient-built time machine is available, this solves the time-tourist problem: if time machines will be built starting, say in the year 3000 AD, nobody will ever travel back to the present!
The author argues that the kind of thought experiments he is playing with has an illustrious past: "Significant advances in technology have now enabled many thought experiments to be performed as real experiments." (This is a quote from the book, and very good point!)
The book ends with a discussion of Hawking's "chronology protection hypothesis", according to which a successful "theory of everything" will make time travel (to the past) impossible even in principle.
Rating:
Summary: Good Book, But Somewhat Derivative of His Earlier Work
Review: Paul Davies book "How to Build a Time Machine" is a good book with an entertaining thesis: he is of the opinion that it is theoretically possible to build both a forward and backward time machine. The engineering problems, however, are of an aboslutely staggering scale, and the project is not, by current standards, a practical one. Nevertheless, Davies has fun with the concepts. Davies also gives a good overview of the various theories of how time travel might be accomplished, and the book is very useful in showing the layperson that time is, indeed, relative. My only complaint with the book is that it is somewhat derivative of his earlier work, About Time. Part of this is not Davies' fault -- the concept of the flexibility of time is usually so shocking for people that he feels the need to explain this first, and so both books discuss some similar ideas, such as the Paradox of the Twins, etc. I recommend About Time to people (usually who don't believe me when I explain that time is relative), and those who read it come away stunned; the book is a marvelous explanation, in layman's terms, of how Einstein's theories work with regard to time. If you have not read About Time, read this one first as it will familiarize you with the concepts Davies' discusses here. This book is a good follow up to About Time in that it turns to the "engineering" problems involved in building a time machine.
Rating:
Summary: Good, concise book
Review: Really enjoyed this book. Very clear, very good job of making the pretty complex concepts involved understandable to the general public. This being said and imho it tries so much to be quick and to the point and to pre-highlight and to include only the simplest drawings, that one feels one would have liked some more depth after all - since Mr. Davies makes such a good job of explaining complex science.
Rating:
Summary: A Very Interesting Book
Review: This book is about all the different theories of time travel. It also tells different ways that time machines "could" be made but they are highly unlikely. Paul Davies is Professor of Natural Philosophy in the Australian Centre for Astrobiology at Macquarie University. Davies is interested in the nature of time, high-energy particle physics, the foundations of quantum mechanics, the origin of life and the nature of consciousness. Davies is well known as an author, broadcaster and public lecturer. Paul writes regularly for newspapers, journals and magazines in several countries, both about science and the political and social aspects of science and technology. In 1991 Davies won the ABC Eureka Prize for the promotion of science in Australia. In 1992 he won the University of New South Wales Press Eureka Prize for his book The Mind of God, and in 1993 he was presented with an Advance Australia Award for outstanding contributions to science. This book turned out a lot different than I thought, but I really liked it. A friend gave me the book to read and he liked it also. The book is about all the different theories of time travel and possible ways that you could make a time machine. This book was interesting, a little hard to understand, but it was never boring. I thought that this book was going to be more of a story but it turned out being an informational book about time travel. I actually did learn a lot about traveling through time and all the different theories that people like Einstein had. It also proved why time travel wouldn't be possible because of the size that the time machine would have to be. This book was a good length; it wasn't to long but it explained things enough for you to
Rating:
Summary: A Very Interesting Book
Review: This book is about all the different theories of time travel. It also tells different ways that time machines "could" be made but they are highly unlikely. Paul Davies is Professor of Natural Philosophy in the Australian Centre for Astrobiology at Macquarie University. Davies is interested in the nature of time, high-energy particle physics, the foundations of quantum mechanics, the origin of life and the nature of consciousness. Davies is well known as an author, broadcaster and public lecturer. Paul writes regularly for newspapers, journals and magazines in several countries, both about science and the political and social aspects of science and technology. In 1991 Davies won the ABC Eureka Prize for the promotion of science in Australia. In 1992 he won the University of New South Wales Press Eureka Prize for his book The Mind of God, and in 1993 he was presented with an Advance Australia Award for outstanding contributions to science. This book turned out a lot different than I thought, but I really liked it. A friend gave me the book to read and he liked it also. The book is about all the different theories of time travel and possible ways that you could make a time machine. This book was interesting, a little hard to understand, but it was never boring. I thought that this book was going to be more of a story but it turned out being an informational book about time travel. I actually did learn a lot about traveling through time and all the different theories that people like Einstein had. It also proved why time travel wouldn't be possible because of the size that the time machine would have to be. This book was a good length; it wasn't to long but it explained things enough for you to
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