In the 1930s and 1940s, and then in the 1970s, one mathematician and three physicists made tentative inquiries into the subject of pastward time travel. The work of each was duly referenced and duly forgotten. The investigation began again in earnest in the late 1980s, but not before overcoming a formidable obstacle: doubts about its respectability. Scientists have professional concerns: dissertation and tenure committees to please, department chairs and college deans to impress, and reputations to tend. Moreover, their livelihoods are in many ways contingent on other people. In addition to holding lectures and writing papers, university scientists are expected to raise a substantial part of the funds necessary to support their research. The committees allocating those funds, in turn, are composed of people also concerned with their own reputations; and a committee does not want to become known as naïve, spendthrift, or an easy mark. For all these reasons, physicists investigating the possibility of time travel in the 1960s and 1970s worried that such interests might be regarded as trivial. This is not to say that they resisted the subject entirely. In fact, they broached it with each other in conversation, and a few of them published papers that alluded to it, at least obliquely. But the conversations were informal, and the papers disguised their content (at least to a lay reader) with opaque phrases like “causality violation” and “closed timelike curves.”
In 1988, much changed. In September of that year, Physical Review Letters published a three-page piece with a fairly sensational title: “Wormholes, Time Machines and the Weak Energy Condition.” The paper made it clear that the subject was determinedly theoretical, and that no one was likely construct a time machine anytime soon. To do so would require, among other things, enough energy to move masses the size of several suns at relativistic speeds, or an ability to pull submicroscopic wormholes from the quantum foam and inflate them by a factor of 1035 (that is, a 10 followed by 35 zeros)—and in either case, to create and manipulate a kind of matter that may not exist at all. Nonetheless, the paper’s publication marked a fairly significant moment. It was the first time anyone had proposed how one might undertake pastward time travel in the universe that we know. It was also the moment when the idea of time machines crossed from the realm of science fiction into the realm of science.
The paper sent a small shock wave through the community of theoretical physicists. The subject of time machines had suddenly been made respectable. Almost immediately, what had been a trickle of publications grew into a steady stream, and time machines became the subject of panels at professional meetings. Those driving this inquiry were larger-than-life figures: among them were Frank Tipler, Stephen Hawking, and Caltech’s Kip Thorne.
In 1992 a workshop devoted largely to the subject was held at the Aspen Center for Physics in Aspen, Colorado. There, physicists were given the leisure to converse at length, and the freedom to consider the more philosophical aspects of time travel: the causal paradoxes that might result from changing the past, questions of free will, alternate universes, and the nature of a civilization with the power to send signals into the past. In the years following, these ideas were more fully shaped and appeared in print. By that time the conversation had branched in many directions. Some papers discussed the problems that time travel presented to logic; others discussed the challenges it presented to physics. Still others revisited ideas of the nature of spacetime, and bizarre concepts of branching pasts and futures. As this book goes to press, the most respected professional journals in physics have published well over two hundred papers on the subject of time machines, and the physicists themselves have produced ten or twelve related books.
The scientists described in these pages have expanded the conversation that Wells imagined, broadening it to include a variety of time machines and deepening it to call upon not merely geometry but relativity, quantum mechanics, and (at least tentatively) quantum gravity. They have also made the conversation more rigorous, proposing and testing hypotheses (not in laboratories but, in the manner of theoretical physicists, through “thought experiments”), then dismissing them outright or accepting them and building further hypotheses upon them. Through all this, the ambience of that gaslit Victorian drawing room has survived. The Time Traveller’s guests would not feel out of place in the oak-paneled seminar rooms of Cambridge or Princeton, or those of Caltech’s Bridge Laboratory. They might, though, be somewhat disoriented at the other locales: the paths through the woods near the Institute for Advanced Study in Princeton, New Jersey; and laboratories where subatomic particles are accelerated to velocities approaching the speed of light. Indeed, the physicists who discuss the subject do so everywhere. They meet incidentally in hallways outside classrooms, and more deliberately over coffee or tea. They attend professional conferences where they sit on panels and deliver presentations, and listen to presentations delivered by others. In recent years they have come to telephone and e-mail each other almost daily, discussing their own work and asking about the work of others. They write, most often in collaboration. They show early drafts of papers to colleagues, asking for suggestions and objections. Some post their work online before it appears in print and revise in light of comments from colleagues who read it there. Finally, they examine each other’s published work with care, and when they perceive an error they respond with a passion that might surprise a nonphysicist.
* * *
Is time travel possible? Special and general relativity allow a kind of one-way time travel into the future. But what we usually mean by time travel—that is, travel into the past or “round-trip” travel into the future and back to the present—is quite another matter. As to the viability of those endeavors, the preponderance of opinion is determinedly agnostic. We simply do not know. Furthermore, most researchers agree that we cannot know until we achieve a better understanding of quantum gravity, and such knowledge may lie a decade or more in our future.
Does this mean that we must wait to think seriously about time travel? The figures in this book have not waited. As historians of science like to remind us, scientists are human and so possess human weaknesses. It is unreasonable to expect them always to work in a methodical, step-by-step fashion—proposing a theory to explain a phenomenon, devising tests for the theory, implementing the tests, and so on. We would ignore human nature by expecting them never to reorder the steps, never to get ahead of themselves. In fact, it may be argued that many advances occurred precisely because steps were taken out of sequence. The French mathematician Henri Poincaré remarked, “It is possible to contemplate the spectacle of the starry universe without wondering how it was formed: perhaps we ought to wait, and not look for a solution until we have patiently assembled the elements . . . but if we were so reasonable, if we were curious without impatience, it is probable we would never have created Science and we would always have been content with a trivial existence.”
Sometime in the near future, physicists will have a better understanding of quantum gravity. At that time perhaps there will appear another book about time travel and it will offer more definite answers to the questions raised here. It will show that certain lines of inquiry have proven viable, it will discard others, and it will introduce still others as yet unimagined. In the meantime, it is not too soon to recount what is known already, for that is a great deal. This is a story of talented and inquisitive men and women who have thought seriously and deeply about a subject most of us left behind as children, in the process producing a discussion shot through with startling insights. In some ways, as they will admit, they may be getting ahead of themselves. But this is part of their appeal. Indeed, they would please Poincaré, for each, in some fashion, is unreasonable, curious, and impatient.
Their story is the subject of this book, and its telling has three purposes: first, to trace of the idea of time travel over several decades; second, to offer a view into the lives and work of a fascinating group of thinkers; and finally, to make long overdue payment on a debt owed a twelve-year-old boy—and, I hope, many others like him.
Copyright 2007 by David Toomey
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