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the writer is a well informed physicist and cosmologist. He breaks down complex theories, yet discusses these concepts with the reader, on equal terms. I came away, understanding intriguing concepts about time and space, feeling a bit wiser and all the better for it.
No, not really. I haven't managed to build a time machine yet, hence 1 star off right away.
The 2nd star has been removed because the author, like many who write at this level, suddenly make vast leaps of imagination right where your own non PHD understanding on the subject starts to fail... so they basically cover the stuff you know well and then suddenly just blurt out something without any explanation.
For instance, ageing one end of a worm hole near a dense body, and then being able to travel into the past by traversing the worm hole. Really? Does that even work? I don't doubt time passes more slowly near dense bodies, but does the actual exit of the worm hole exist in the past? Or is it merely younger? I just felt some explanation around such suggestions would have been valuable as that is exactly where I struggle with these concepts. There must be maths to support it, but no mention of it at all is made etc.
A great idea, but in my view, just like Brian Cox on TV, they tell you all the stuff you possibly know already with lots of nice analogies, but then when it really matters, they quickly gloss over things in a single sentence.
Dies ist eine kurze Abhandlung zum Phänomen Zeit und der Frage, ob man in der Zeit reisen kann. Super zu lesen, gut zu verstehen und hochgradig interessant. Ja, es gibt Wormholes und ja man könnte – rein theoretisch durch sie reisen. Dazu braucht man allerding einige Vorkehrungen, Antimaterie und alle Energie des Universums. Was soll’s, allein die Auseinandersetzung mit dem Thema ist spannend und Davis der Autor dafür, das Geschehen einem näher zu bringen.
When the mathematician Rudolf Ruker* asked Godel, who told him he believed man would one day travel freely in time, about the Grandfather paradox Godel, a man of few words, only replied "There will be no paradox". Davies presents Godel's model of the universe along with many other plausible physical approaches to time travel.
Though there are many Philosophical problems with time travel, which Davies lucidly present, yet it has not been possible to find plausible physical constraints other than by ad hoc paraphrasing of prohibitions. Davies lucidly presents many of these constraints thereby also introducing his readers to many areas of physics. He appears to believe that surely one of these ad hoc principles will eventually acquire some physical plausibility.
On the other hand to appreciate what Godel, the man of few words, might have had in mind consider the following. Godel's universe does not allow for a universal time coordinate. The set of events only allows partial time orderings i.e. ordering along the path of any observer. To appreciate the consequences we may consider the following parable.
Our time traveler wishing for whatever reason to painlessly end his life and being an avid science fiction reader, aware of the Grandfather paradox but not wanting to harm his brother or sister, he decides to return to the time of his childhood to shoot his childhood self. However, the only effect of the shot is a small recoil since Locally, even in Godel's universe, Newtonian physics holds. I.e. there is no Newtonian way for him to disappear.
Unfortunately, Nature's Laws aren't the only Laws applicable here. Society's Laws dictate that our traveler will spend the rest of his life behind bars for killing a child and, thus, unable to return to the 21st Century as Godel's universe would allow.
We will assume that our time traveler presents a negligible perturbertion to the large scale structure of the Godel universe. Though these events our traveler encounters on his trip share Godel coordinates with events he recalls from his past they are New Events. To our traveler they occur in the 21st Century. To those whom our traveler has met on his journey the events occur in the 20th Century. ( Likewise, if our traveler were to return to an earlier time in the 21st Century than when he left, his memories of the intervening period will not be shared by those greeting him; nor would he share the memories of those who saw him off on his journey. ) Only events along a particular path are relevant. Such events are time ordered w.r.t. the particular path. As Godel said, there is No Paradox!
* R. Ruker, "Infinity and the Mind", pp.170 - 181, Bantum Books, 1983 ISBN 0-553-23433-1