Rating: 
Summary: A very good discussion of time travel, one error of omission
Review: As I have always been fascinated by the idea of time travel, I very much enjoyed its discussion both in 'strictly scientific' terms and from a philosophical, literary, and, essential, pop-culture perspective.
Sadly, Nahin completely ignores one aspect that features prominently in many modern time travel narratives: the idea of alternate universes / alternate realities and, tied to that, the narrative perspective of sequentiality, which follows the POV of the protagonist of a narrative and projects his continuity against the alterations his actions cause. As a result, a number of time travel stories are missing, while others show flaws/inconsistencies in their interpretations. For instance, Isaac Asimov's The End of Eternity (1955), which deals with the idea of realities literally being 'engineered' through minimum manipulations in the course of history, thus weeding out 'mishaps' like world wars and famines but at the same time stalling natural progress, remains completely unmentioned. On the other hand, Nahin points to the 'flaw' in the end of Back to the Future I that the shopping mall should always have been "Lone Pine Mall" and never "Twin Pines Mall" as it was called in the beginning because taken from a timeline perspective, Marty had already been to 1955 and run over one of the twin pines with the DeLorean when the temporal experiment #1 took place in the parking lot in 1985. However, taken from this perspective, Marty's family should have been healthy and wealthy from the beginning on as well, which would undermine the whole plot idea of changing history (which is, added to that, even expressedly discussed in detail in the also unmentioned Back to the Future II).
Nevertheless, Nahin gives a lot of food for thought on the idea of time travel, and the rather extensive bibliography supplies a very good reference for further individual exploration.
Rating:
Summary: A good discussion of time travel, with one error of omission
Review: As I have always been fascinated by the idea of time travel, I very much enjoyed its discussion both in 'strictly scientific' terms and from a philosophical, literary, and, essentially, pop-culture perspective.
Sadly, Nahin completely ignores one aspect that features prominently in many modern time travel narratives: the idea of alternate universes / alternate realities and, tied to that, the narrative perspective of sequentiality, which follows the POV of the protagonist and projects his continuity against the alterations his actions cause. As a result, a number of time travel stories are missing, while others show flaws/inconsistencies in their interpretations. For instance, Isaac Asimov's "The End of Eternity" (1955), which deals with the idea of realities literally being 'engineered' through minimum manipulations in the course of history, thus weeding out 'mishaps' like world wars and famines but at the same time stalling natural progress, remains completely unmentioned. On the other hand, Nahin points to the 'flaw' in the end of "Back to the Future" that the shopping mall should always have been 'Lone Pine Mall' and never 'Twin Pines Mall' as it was called in the beginning because taken from a timeline perspective, Marty had already been to 1955 and run over one of the twin pines with the DeLorean when the temporal experiment #1 took place in the parking lot in 1985. However, taken from this perspective, Marty's family should have been healthy and wealthy from the beginning on as well, which would undermine the whole plot idea of changing history (which is, added to that, even expressedly discussed in detail in the also unmentioned "Back to the Future II").
Nevertheless, Nahin gives a lot of food for thought on the idea of time travel, and the rather extensive bibliography supplies a very good reference for further individual study.
Rating: 
Summary: time for a trip to the passsed
Review: I am no scientist, that's for certain.This book is a wonderful blend of science and science fiction. It is perfect for people like me, who are fascinated by the idea of time travel but can't understand math or physics for anything. The first few chapters are basically a literature review. An extremely comprehensive literature review. It'll be enough to make you run to your library or bookstore (or computer) in search of these books and short stories. Nahin also discusses the reality behind time travel with relatively little math. Most of the math is tucked away in the "Tech Notes in the back of the book. Nice technique to sucker in the math-scaredys like me. What I really loved about the book, though is Nahin's enthusiasm. He is obviously just as nuts (or more) as I am about this outlandish subject of time travel, which makes the book, in my opinion, stand-out.
Rating:
Summary: time for a trip to the passsed
Review: I am no scientist, that's for certain. This book is a wonderful blend of science and science fiction. It is perfect for people like me, who are fascinated by the idea of time travel but can't understand math or physics for anything. The first few chapters are basically a literature review. An extremely comprehensive literature review. It'll be enough to make you run to your library or bookstore (or computer) in search of these books and short stories. Nahin also discusses the reality behind time travel with relatively little math. Most of the math is tucked away in the "Tech Notes in the back of the book. Nice technique to sucker in the math-scaredys like me. What I really loved about the book, though is Nahin's enthusiasm. He is obviously just as nuts (or more) as I am about this outlandish subject of time travel, which makes the book, in my opinion, stand-out.
Rating: 
Summary: Messy writing ruins a good reference
Review: I found Paul J. Nahin's Time Machines to be a messy book. I expected to learn all I needed to learn about time and the potential for time machines, but didn't. There is information there, but Mr. Nahin manages to break it and hide it very well. Any discussion of physics is quickly broken up by comments from physicists and philosophers and discussions of how writers handled it. The idea of showing the effects of time travel thought on philosophy and literature (and vice versa) is good, but I felt that a clearer separation would have made the book much easier to read. An explanation of ideas followed by discussion of how they were handled in literature would have been great, but Mr. Nahin mixes them all together, often repeating previously made points, and without creating a logical path of thought. The tech notes aren't all that great, either. They did introduce me to concepts of physics that relate to time, which allowed me to do further research, but I found much better and more comprehensive (and not any longer) explanations on the web. It seems that either Mr. Nahin doesn't fully understand the physics and the questions it raises, and so just quotes things without explaining them, or just doesn't have the gift of providing clear explanations in an orderly manner. For example, Tech Note 2 explains time dilation in the context of an observer watching another clock move by. The note implies that time is actually shortened for the mover (it's not just an observed effect). Tech Note 3, on the other hand, mentions as an afterthought that when one object moves quickly relative to the other, the same thing is seen from the point of view of the other, and both will see the other's time dilated exactly the same. Which leads to an awful lot of question, which lead to the "twins paradox". However, the paradox isn't mentioned until Tech Note 5, where the "paradoxical" nature of it is glanced upon, instead of linking it to the obvious questions raised by the previous notes (and possibly mentioning this note when the paradox is first raised in the reader's mind). The book isn't completely useless, though. It offers many references, and its explanations can serve as a starting point for further research. As such it is a valuable resource on the matter of time travel in thought, literature and physics. With some more engaging or at least more orderly writing (and content pages that start at the beginning instead of 30 pages into it would also have been nice), this could have been a really good book.
Rating: 
Summary: Awfully weak
Review: I got the second edition (1999) of this book. I was hoping for something interesting. After all, there was even a forward by none other than Kip Thorne!
But this book was an awful disappointment. First, Nahin did his readers no favor by utterly failing to understand the writings of Hospers. On pages 289-290 and elsewhere, Nahin criticizes Hospers strongly, but Nahin is seriously in error. For one thing, Nahin claims that Hospers says that one can not go back in time while Nahin says it is possible. In fact, the reverse is true. Hospers says that time travel is possible and Nahin actually is arguing against it. To put it mildly, Nahin is an intellectual lightweight who has become totally confused by the plethora of time-travel paradoxes.
Nahin is so sure that there is only one worldline and so sure that anyone who disagrees with him does not understand this that he simply refuses to read anything which hints at multiple world lines. However, if he wants to disagree with those who discuss the concept of multiple world lines, he ought to read what they say first and even try to understand it!
On top of this, I was amazed at the amount of material (some of it interesting and some of it nonsense) written about people travelling to the past, as opposed to people merely sending signals to the past. A fundamental scenario would be to send a book to a time 50 years earlier, thereby "creating" information (the author, having already received the book, might no longer need to write it). Or perhaps sending instructions to the past on how to build a camera, so that various famous historical events could be filmed and sent back to the "future." I think topics such as these would have merited some discussion.
Rating:
Summary: Too much compilation, too little synthesis
Review: Mr. Nahin obviously is very interested in the topic of time travel. He has read tons of sci-fi stories, has spoken with many physicists and/or read their books.
But he is a journalist, not a physicist. And he makes little or even no effort to synthesise.
As a result the book reads mostly like a list of everything that has been written by sci-fi writers, scientists and philosophers about the subject. But not like a book by someone who truly understands what is going on - provided it is possible to understand.
In my view this is a good book because of all the material in it, all the references. But it is not a good book per se. I gave it 3 out of 5, including 1 point for all the references.
Rating:
Summary: interesting, though often tendentious
Review: Nahin has a lot of interesting things to say with regard to time travel, but he has a tendency to set up as straw men viewpoints not in agreement with his and let viewpoints that do agree with his slide by when, perhaps, they warrant deeper critical examination. Special derision is reserved for various philosophers and their "fairy tales" and such. The chief respect in which I really enjoyed this book was in the many and varied discussions Nahin engages in with regard to time travel in literature. What he has to say is almost always lucid, interesting and fair, and the examples he rescues from obscurity are invariably intriguing. It's unfortunate that he didn't show such equanimity, again, with regard to opposing philosophically-oriented viewpoints. I have mixed feelings about the book because the recurring tendentiousness can wear on one, but I think the discussion of time travel in literature makes it worth the while.
Rating:
Summary: Paul Nahin has written an excellent book for the layperson.
Review: People have long been fascinated with the idea of time travel. The possibilities are exciting -- you could go back in time and experience firsthand all the wonders of history. You could actually get a second chance to correct mistakes in your own life. To some these possibilities are frightening -- if you really can change the past, what does this do to our sense of continuity? To history? What if you went back and killed your own father before he even met your mother?
Paul J. Nahin discusses both sides of this issue in his thought provoking book Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction. He begins with an overview of time travel, from scientific possibility of it, to popular conjecture about it. He goes on to discuss the nature of time itself, and then ends with an in-depth analysis of paradoxes created by the possibility of time travel. He assures us that we do not have to worry about changing history, because the past cannot be changed.
Nahin has written an excellent book for the layperson. He includes many references to popular works of science fiction, including many stories and movies the reader is probably familiar with. This helps illustrate many of his points. The text is clear and well written. Anyone without a background in physics can understand this book. For those with a more technical bent Nahin includes a few "Tech Notes" at the end of the book to explain certain phenomena he discusses. Time Machines is an exciting book for anyone who has ever sat outside on a long summer night and wondered "what if."
Rating:
Summary: Man Must Conquer Time!
Review: This book is a good introduction to some of the science fiction and science fact speculations concerning the possibility of time travel. It considers time travel from early science fiction speculations (e.g. H. G. Wells' _Time Machine_) to modern science speculations involving quantum mechanics, faster-than-light tachyons, and wormholes. From the classic speculations involving General Relativity of Kurt Godel and his rotating universe, to the modern speculations of Kip Thorne involving the use of wormholes to travel backwards in time, the science of time travel is made clear to the lay person. Philosophical speculation with regards to the metaphysics of time is dealt with fully. The apparent paradoxes of time travel (backwards in time) (e.g. the "grandfather paradox") are considered and possible resolutions to them are proposed. In the end, the reader is left to decide for himself whether time travel is: 1. possible, and 2. feasible (at whatever level of technological advancement). According to Stephen Hawking, the fact that we haven't been visited by time travelling tourists is evidence against the possibility of backwards in time, time travel. I myself do not believe this to be the case and think that there is some other reason for the apparent absence of time travellers. For those of us who boldly wonder about the possibilities of man's future evolution, future technological progress, and future civilization, the issue of time travel is an unavoidable and a tempting one. If man is to ever conquer the galaxy, he must conquer time first. It must be possible - it will be possible! This book is an appeal to dreamers and speculative philosophers to examine fully the issues, paradoxes, and proposed methods of time travel.
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