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books & arts Tinkering with THE NEW TIME TRAVELERS: A JOURNEY TO THE On our FRONTIERS OF BY DAVID TOOMEY bookshelf W. W. Norton & Co.: 2007. 320 pp. $25.95

Years ago, David Toomey picked up H. G. Wells’ moving clocks tick slowly, making The Time Machine and couldn’t put it down. to the future possible. We now know a number The Mathematics of He was most interested in the drawing-room of solutions to Einstein’s equations of general Egypt, Mesopotamia, discussion between the time traveller and his relativity (1915) that are sufficiently twisted to China, India, and Islam: friends in which time as a fourth dimension was allow time travel to the past: Kurt Gödel’s 1949 A Sourcebook discussed, but wanted to know more about just rotating ; the Morris–Thorne–Yurtsever edited by Victor J. Katz how that time machine might work. Toomey was (1988); the Tipler–van Stockum infinite therefore delighted to learn that that drawing- rotating cylinder, moving cosmic strings (myself), Princeton Univ. Press: room conversation continues — this time the rotating black-hole interior (), 2007. 685 pp. $75 among physicists. a of (Matt Visser), the We’re aware that Toomey captures well the personalities Everett–Alcubierre , my and Li–Xin Li’s the ancient cultures of the ‘new time travelers’ — those physicists self-creating universe; Amos Ori’s torus; and were mathematically interested in whether time travel to the past is others. The book discusses all of these. advanced. Now possible — from ’s sense of But can a time machine really be constructed? translations of early texts humour to ’s penchant for placing Hawking, like one of the time traveller’s sceptical from five key regions are scientific bets. Toomey has researched the history friends in Wells’ drawing-room, put forward the available together for the of the field extremely thoroughly. You will learn of chronology protection conjecture — that the first time, and put into British mathematician Charles Howard Hinton, laws of physics will somehow conspire to prevent context by experts. who thought of world lines, which he called time travel to the past. As Toomey describes, ‘filamentary atoms’, before Hermann Minkowski. Hawking gathered a number of arguments. If a You will learn that Carl Friedrich Gauss time machine is created where there was none considered two-dimensional creatures, which he before, there must be a Cauchy horizon separating called ‘bookworms’ or ‘flatworms’, confined to live the no-time-travel and time-travel regions and on an infinitely thin sheet of paper — well before the quantum vacuum might blow up as one Edwin Abbott’s book . approached the Cauchy horizon. Secondly, if the A highlight of the book is its lucid discussion time machine is made of ordinary positive-energy- of time-travel paradoxes. What if you kill your density stuff, the solution is unstable approaching grandmother as a young girl? Igor Novikov, the Cauchy horizon and singularities may occur. Thorne and their colleagues favour the idea that Yet, there are loopholes. Alternative quantum the four-dimensional must be self vacuum states can be found in some cases that consistent — time travellers never change the do not blow up at the Cauchy horizon, and past, being always part of it: you can’t kill your wormholes avoid instability by using exotic Quantum Theory Cannot grandmother, because you didn’t. Using extensive (the Casimir vacuum). Alternatively, Hurt You: A Guide to thought experiments with billiard balls going stable singularity-free solutions can be found if the Universe through wormholes, no matter how hard they the time loop occurs at the very beginning of the by Marcus Chown tried to produce paradoxes, they could always universe. To understand whether it is possible to Faber and Faber: 2007. find self-consistent solutions. David Deutsch has construct any of these time machines, the author 200 pp. £12.99 a different view. In the many-worlds theory of rightly concludes we may need to understand the , if you kill your grandmother laws of quantum — how gravity behaves Quantum theory and in the past, you just make the universe branch off on microscopic scales — one reason physicists Einstein’s general onto a parallel universe with a time traveller and a working on this problem find it so fascinating. dead grandmother. The original parallel universe made simple? To be where the time traveller was born still exists. J. Richard Gott read in one morning? When Wells wrote his book in 1895, the J. R. Gott is in the Department of Astrophysical Chown shares his laws of physics (Newton’s laws) allowed no time Sciences, Princeton University, Princeton, New amazement at “how travel to either the future or the past. But in 1905, Jersey 08544, USA, and is the author of Time Travel much stranger science is ’s theory of showed in Einstein’s Universe (Mariner Books, 2002). than science fiction”. physics | VOL 3 | NOVEMBER 2007 | www.nature.com/naturephysics 751 © 2007 Nature Publishing Group