The New Time Travelers: A Journey to the Frontiers of Physics
2007 W W Norton
£15.99/$25.95 hb 320pp
Upon receiving this book I was somewhat sceptical to see that the author, David Toomey, is not a physicist but instead has a PhD in English literature and teaches technical and non-fiction writing at the University of Massachusetts in the US. As I began to read it, however, my attitude changed. To many physicists, a book about time travel would be virtually indistinguishable from a piece of science fiction, particularly when there are no equations or experimental results provided. To have any chance of presenting the work in a convincing manner, therefore, any author of such a book must be quite skilled. Toomey not only accomplishes this, but also makes the book an enjoyable read.
It is well known that special relativity allows time travel into the future; it is travel into the past that is the problem. The book is a well-balanced mix of physics, science fiction and biographical details of the “new time travellers” — the physicists who, in recent years, have been publishing serious articles about the physics of time travel. They include people such as Kip Thorne, Igor Novikov, Richard Gott, Stephen Hawking, Matt Visser and David Deutsch. Thorne started thinking seriously about the possibility of time travel in 1985 following a request from Carl Sagan, who was writing a science-fiction novel and wanted the physics to be as accurate as possible. Serious publications in respected physics journals by Thorne and others followed.
The potential time machines described in Toomey’s book all rely on distorting space–time in some manner, with the standard device being the wormhole. The time traveller enters one end of the wormhole at a particular time and emerges at the other end at an earlier time. As with most time machines, this raises the possibility of closed timelike curves, or causal loops. These can lead to paradoxes, such as the well-known “grandfather paradox”: what happens if you go back in time and kill your grandfather before he has children? If you succeed, then you will not exist so you cannot have killed him, in which case you will exist and so on.
The simplest way to avoid such situations is to adopt Hawking’s “chronology protection conjecture”, which states that the laws of physics prevent the formation of closed timelike curves, and therefore rules out time travel. Visser, a mathematician at the Victoria University of Wellington in New Zealand, has added what he dubs the “boring physics” conjecture; he feels that in dismissing time machines physicists are also dismissing a large set of interesting questions.
A second way of avoiding the grandfather paradox is to assume that the physical laws in some way force all causal loops to be self-consistent — in other words, they ensure that the probability of an event that would give rise to a paradox occurring is zero. This assumption has a very long history and is indeed the substance of many science-fiction stories. It has also been discussed in serious scientific literature since at least the middle of the last century. The late philosopher David Lewis treated the problem as one of logic and pointed out that every event in a closed causal loop is both in the past and in the future of every other event in the loop. Thus because you did fail to kill your grandfather, you will fail to kill your grandfather.
Novikov’s work also gave some support for self-consistency in terms of the principle of least action, which states that nature behaves in the way that minimizes action. For some particular cases he found that of all the possible paths along a closed timelike curve, those that were self-consistent were the most economical in terms of action. This idea was not universal accepted, however. Visser, for example, called it the “Novikov consistency conspiracy” because nature somehow seems to conspire to prevent you from killing your grandfather despite your every intention to do so. Presumably, however, just as quantum mechanics dictates that the probability amplitudes for paths not near the path of least action must cancel, the probability amplitudes for inconsistent loops would also cancel, thus leaving only consistent loops with a non-zero probability of occurring.
A third way of avoiding causalloop paradoxes involves the Everett–DeWitt multiverse, or the many-universes interpretation of quantum mechanics. In a 1991 article in Physical Review D Deutsch describes a scenario in which a time traveller, in attempting to force a paradox, removes himself entirely from one universe and adds his presence to another universe. In other words, when you travel back and kill your grandfather, you remove yourself from the universe in which you were born and remain in a universe in which you were never born. In this way, a time machine could be a gateway between universes and permit a number of interesting and remarkable circumstances. There is certainly no boring physics here.
The multiverse model also addresses the problem of a “jinn” — something created from nothing. In order to write a book review without much labour, I travel into the future, collect the memory stick containing the completed review, bring it back with me, submit a copy of the review and leave the memory stick where I can find it in the future. In the multiverse model, when I collect the stick, I will have travelled into a universe in which I did read the book and write the review. In returning to my past I will have transported the review from a universe where it was created into one where it was not created.
Overall, I found this book to be skilfully written and, indeed, quite thought provoking. It held my interest throughout. I can certainly recommend it to physicists with wider interests than their own specialities, to students, to science-fiction readers and to general readers with some interest in physics.