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Saturday, August 5, 2017

Reading List: Time Travel

Gleick, James. Time Travel. New York: Pantheon Books, 2016. ISBN 978-0-307-90879-7.
In 1895, a young struggling writer who earned his precarious living by writing short humorous pieces for London magazines, often published without a byline, buckled down and penned his first long work, a longish novella of some 33,000 words. When published, H. G. Wells's The Time Machine would not only help to found a new literary genre—science fiction, but would introduce a entirely new concept to storytelling: time travel. Many of the themes of modern fiction can be traced to the myths of antiquity, but here was something entirely new: imagining a voyage to the future to see how current trends would develop, or back into the past, perhaps not just to observe history unfold and resolve its persistent mysteries, but possibly to change the past, opening the door to paradoxes which have been the subject not only of a multitude of subsequent stories but theories and speculation by serious scientists. So new was the concept of travel through time that the phrase “time travel” first appeared in the English language only in 1914, in a reference to Wells's story.

For much of human history, there was little concept of a linear progression of time. People lived lives much the same as those of their ancestors, and expected their descendants to inhabit much the same kind of world. Their lives seemed to be governed by a series of cycles: day and night, the phases of the Moon, the seasons, planting and harvesting, and successive generations of humans, rather than the ticking of an inexorable clock. Even great disruptive events such as wars, plagues, and natural disasters seemed to recur over time, even if not on a regular, predictable schedule. This led to the philosophical view of “eternal return”, which appears in many ancient cultures and in Western philosophy from Pythagoras to Neitzsche. In mathematics, the Poincaré recurrence theorem formally demonstrated that an isolated finite system will eventually (although possibly only after a time much longer than the age of the universe), return to a given state and repeat its evolution an infinite number of times.

But nobody (except perhaps a philosopher) who had lived through the 19th century in Britain could really believe that. Over the space of a human lifetime, the world and the human condition had changed radically and seemed to be careening into a future difficult to envision. Steam power, railroads, industrialisation of manufacturing, the telegraph and telephone, electricity and the electric light, anaesthesia, antiseptics, steamships and global commerce, submarine cables and near-instantaneous international communications, had all remade the world. The idea of progress was not just an abstract concept of the Enlightenment, but something anybody could see all around them.

But progress through what? In the fin de siècle milieu that Wells inhabited, through time: a scroll of history being written continually by new ideas, inventions, creative works, and the social changes flowing from these events which changed the future in profound and often unknowable ways. The intellectual landscape was fertile for utopian ideas, many of which Wells championed. Among the intellectual élite, the fourth dimension was much in vogue, often a fourth spatial dimension but also the concept of time as a dimension comparable to those of space. This concept first appears in the work of Edgar Allan Poe in 1848, but was fully fleshed out by Wells in The Time Machine: “ ‘Clearly,’ the Time Traveller proceeded, ‘any real body must have extension in four dimensions: it must have Length, Breadth, Thickness, and—Duration.’ ” But if we can move freely through the three spatial directions (although less so in the vertical in Wells's day than the present), why cannot we also move back and forth in time, unshackling our consciousness and will from the tyranny of the timepiece just as the railroad, steamship, and telegraph had loosened the constraints of locality?

Just ten years after The Time Machine, Einstein's special theory of relativity resolved puzzles in electrodynamics and mechanics by demonstrating that time and space mixed depending upon the relative states of motion of observers. In 1908, Hermann Minkowski reformulated Einstein's theory in terms of a four dimensional space-time. He declared, “Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.” (Einstein was, initially, less than impressed with this view, calling it “überflüssige Gelehrsamkeit”: superfluous learnedness, but eventually accepted the perspective and made it central to his 1915 theory of gravitation.) But further, embedded within special relativity, was time travel—at least into the future.

According to the equations of special relativity, which have been experimentally verified as precisely as anything in science and are fundamental to the operation of everyday technologies such as the Global Positioning System, a moving observer will measure time to flow more slowly than a stationary observer. We don't observe this effect in everyday life because the phenomenon only becomes pronounced at velocities which are a substantial fraction of the speed of light, but even at the modest velocity of orbiting satellites, it cannot be neglected. Due to this effect of time dilation, if you had a space ship able to accelerate at a constant rate of one Earth gravity (people on board would experience the same gravity as they do while standing on the Earth's surface), you would be able to travel from the Earth to the Andromeda galaxy and back to Earth, a distance of around four million light years, in a time, measured by the ship's clock and your own subjective and biological perception of time, in less than six and a half years. But when you arrived back at the Earth, you'd discover that in its reference frame, more than four million years of time would have elapsed. What wonders would our descendants have accomplished in that distant future, or would they be digging for grubs with blunt sticks while living in a sustainable utopia having finally thrown off the shackles of race, class, and gender which make our present civilisation a living Hell?

This is genuine time travel into the future and, although it's far beyond our present technological capabilities, it violates no law of physics and, to a more modest yet still measurable degree, happens every time you travel in an automobile or airplane. But what about travel into the past? Travel into the future doesn't pose any potential paradoxes. It's entirely equivalent to going into hibernation and awaking after a long sleep—indeed, this is a frequently-used literary device in fiction depicting the future. Travel into the past is another thing entirely. For example, consider the grandfather paradox: suppose you have a time machine able to transport you into the past. You go back in time and kill your own grandfather (it's never the grandmother—beats me). Then who are you, and how did you come into existence in the first place? The grandfather paradox exists whenever altering an event in the past changes conditions in the future so as to be inconsistent with the alteration of that event.

Or consider the bootstrap paradox or causal loop. An elderly mathematician (say, age 39), having struggled for years and finally succeeded in proving a difficult theorem, travels back in time and provides a key hint to his twenty year old self to set him on the path to the proof—the same hint he remembers finding on his desk that morning so many years before. Where did the idea come from? In 1991, physicist David Deutsch demonstrated that a computer incorporating travel back in time (formally, a closed timelike curve) could solve NP problems in polynomial time. I wonder where he got that idea….

All of this would be academic were time travel into the past just a figment of fictioneers' imagination. This has been the view of many scientists, and the chronology protection conjecture asserts that the laws of physics conspire to prevent travel to the past which, in the words of a 1992 paper by Stephen Hawking, “makes the universe safe for historians.” But the laws of physics, as we understand them today, do not rule out travel into the past! Einstein's 1915 general theory of relativity, which so far has withstood every experimental test for over a century, admits solutions, such as the Gödel metric, discovered in 1949 by Einstein's friend and colleague Kurt Gödel, which contain closed timelike curves. In the Gödel universe, which consists of a homogeneous sea of dust particles, rotating around a centre point and with a nonzero cosmological constant, it is possible, by travelling on a closed path and never reaching or exceeding the speed of light, to return to a point in one's own past. Now, the Gödel solution is highly contrived, and there is no evidence that it describes the universe we actually inhabit, but the existence of such a solution leaves the door open that somewhere in the other exotica of general relativity such as spinning black holes, wormholes, naked singularities, or cosmic strings, there may be a loophole which allows travel into the past. If you discover one, could you please pop back and send me an E-mail about it before I finish this review?

This book is far more about the literary and cultural history of time travel than scientific explorations of its possibility and consequences. Thinking about time travel forces one to confront questions which can usually be swept under the rug: is the future ours to change, or do we inhabit a block universe where our perception of time is just a delusion as the cursor of our consciousness sweeps out a path in a space-time whose future is entirely determined by its past? If we have free will, where does it come from, when according to the laws of physics the future can be computed entirely from the past? If we can change the future, why not the past? If we changed the past, would it change the present for those living in it, or create a fork in the time line along which a different history would develop? All of these speculations are rich veins to be mined in literature and drama, and are explored here. Many technical topics are discussed only briefly, if at all, for example the Wheeler-Feynman absorber theory, which resolves a mystery in electrodynamics by positing a symmetrical solution to Maxwell's equations in which the future influences the past just as the present influences the future. Gleick doesn't go anywhere near my own experiments with retrocausality or the “presponse” experiments of investigators such as Dick Bierman and Dean Radin. I get it—pop culture beats woo-woo on the bestseller list.

The question of time has puzzled people for millennia. Only recently have we thought seriously about travel in time and its implications for our place in the universe. Time travel has been, and will doubtless continue to be the source of speculation and entertainment, and this book is an excellent survey of its short history as a genre of fiction and the science upon which its founded.

Posted at August 5, 2017 19:33