Paul Johnson's popular history of the twentieth century opens thus: "The Modern World began on 29 May 1919 when photographs of a solar eclipse, taken on the island of Principe off West Africa and at Sobral in Brazil, confirmed the truth of a new theory of the universe."1 The new theory of the universe was Einstein's General Relativity, a radical, mysterious, new explanation of gravity, destined to replace the more intuitive and accessible theory of Isaac Newton that had inspired and sustained the Enlightenment.

The emergence of the new theory also signaled the arrival of what cultural anthropologist Chris Toumey has called "Old Testament Science," a science "respected without comprehension" and revered because of "awesome signs," like the God of the Old Testament.2 This kind of science could not be popularized in a way that could be understood by the educated person. General Relativity was a highly mathematical description of space, time, matter, and their interactions. Time and space lost their straightforward Newtonian identities and dissolved into an abstraction called "spacetime." What we used to think of as time became another dimension, somehow similar to the more familiar dimensions of space, and perhaps something we could travel through instead of merely ride along. The formerly clear-cut notions of "space" and "time" were replaced by "spacelike intervals" and "timelike intervals," themselves abstractions from the physical structure known as "spacetime."

Spacetime is a four-dimensional matrix that can be twisted and distorted by massive bodies like stars in ways that are highly counterintuitive and even bizarre. Light, for example, no longer travels in "straight" lines; a planet in an orbit close to a star, like Mercury, does does not retrace its path each time around like its more distant companions; clocks run at different rates depending on the strength of the local gravitational field. Perhaps the most bizarre element of the theory is the prediction of black holes—collapsed stars with such intense gravitational fields that nothing, not even light, can escape them.

But that is not all. Certain mathematical solutions to the equations of relativity imply that spacetime can be warped in such a way that widely separated regions can actually connect to each other, much the same way that a steel rod might be bent into a circle so that the "ends"—initially as far from each other as possible—actually become adjacent, making what used to be the longest possible journey for an ant crawling along the rod the shortest. These different parts of spacetime are connected via "wormholes" which are "tunnels" with each end of the tunnel in a different part of "space" and "time."

But this suggests that something, say a spaceship filled with people who really wanted to hear Chopin perform live, could at least conceivably travel through such a tunnel to the past (or the future) and maybe even come back. And this in turn raises all the conventional paradoxes of time travel—like what would happen if you went back in time and killed your mother before you were born. Heady stuff. How are we to interpret and understand all of this? The truth of the matter is that it simply isn't possible to "understand" these and other esoteric phenomena of relativity in any meaningful sense of the word—this, after all, is "Old Testament Science" at its best.

Enter Clifford Pickover, research staff member at the IBM Thomas J. Watson Research center, columnist, and author of an impressive if odd collection of books with titles such as The Alien IQ Test, Black Holes: A Traveler's Guide, Chaos in Wonderland, Keys to Infinity, The Loom of God, and Strange Brains and Genius.

Now, in the vein of his guided tour of black holes, Pickover offers Time: A Traveler's Guide. "This book," he tells us in the preface, "is mostly about the science of time travel and touches only briefly on mysticism. However, the line between science and mysticism sometimes grows thin." He adds that one of his goals is to help the reader "understand that time travel is possible." What the book is really about is Einstein's theory of relativity, particularly those aspects that touch on time.

Sir Arthur Eddington, the great Quaker physicist who invested considerable effort in bringing relativity to both scientific and popular audiences, was once asked if it were true that only three people in the world understood relativity. After some reflection, Eddington is said to have answered, "I understand it, and certainly Einstein understands it, but I am trying to think of who the third person might be." (Eddington led the expedition that took the photographs mentioned in the opening of this essay.)

Pickover's book is only the latest in a long series of attempts to explain relativity to nonscientists. One of the first was Einstein's own Relativity, The Special and the General Theory: A Popular Exposition (1921); the most famous is Stephen Hawking's A Brief History of Time: From the Big Bang to Black Holes (1988), which has sold millions of copies. What sets Pickover's apart from the rest is his offbeat approach to the topic and his speculative exposition near the end, as some of the more shaky theoretical elements of the theory become incarnated as a working time machine.

But it would be a mistake to overlook the significance of what Pickover is doing: he is taking a body of rather difficult scientific material and making it both interesting and accessible. Readers who find conventional science exposition dry and lifeless will stay with Pickover to see if his characters actually do get to travel through time; readers eager to understand Einstein's very difficult theory will benefit from the excellent exposition.

Time: A Traveler's Guide opens with an alien named Mr. Veil playing the bassoon. We gather from the dialogue that he is an assistant of sorts to you, the reader. You are a scientist who delights in explaining relativity to your friends. In chapter 2 we are introduced to another assistant, a young woman named Constantia Gladkowska, to whom you are attracted. The majority of the book is an ongoing conversation between you, Mr. Veil, and Constantia as you explain the "physics of time travel" to them and prepare to travel back in time to watch Chopin perform. Along the way the reader learns about Mr. Veil's strange characteristics and Constantia's surprising origins (and her increasingly less insistent rejections of your amorous advances), not to mention a variety of details about life in the year 2063.

Each chapter has two parts: a conversation, generally bizarre but always interesting, between the trio mentioned above; and a conclusion, generally entitled "The Science Behind the Science Fiction," in which the relevant science is presented in a conventional manner. The conversational setting works well in allowing for some of the more curious bits of the science to get some extra attention in response to questions and objections; it also provides a lively setting for the exposition. The conclusion is complementary to the conversation, repeating and reinforcing what is often very difficult material.

Pickover's exposition of relativity begins in the traditional way, pointing out that simultaneity is an ambiguous concept because of the finite speed of light. Two separated events at locations A and B that appear to occur simultaneously to an observer midway between them will not appear to be simultaneous to observers at other locations. An observer close to B, for example, will see that event first and event A later. An observer at A will experience the two events in the opposite order. This is due to the fact that light does not travel infinitely fast but takes some time to get from the event to the observer.

Light also travels at exactly the same speed in a vacuum, regardless of the motion of either the light source or the frame of reference in which the speed is being measured. The fact that light is the single solitary exception to the common experience that things appear to increase their speed of approach when we move toward them is one of the most counterintuitive ideas in all of science. The absolute constancy of the speed of light is one of the starting points of relativity and can be motivated with some physical arguments or simply presented as a postulate.

Pickover opts for the latter, which unfortunately makes relativity seem even more like "Old Testament Science" than is necessary. Readers without a scientific background could easily draw the quite natural, although erroneous, conclusion that the odd results of relativity depend on accepting the starting postulates on faith. The reality is that the foundations on which relativity is built are quite secure empirically. The reader should understand that relativity is, for the most part, uncontroversial. (Ideally, readers would "recall" from high school that special relativity is well-established science, but the fact that so few in the United States even take physics in high school is one of the reasons why we need books like Pickover's.)

When the implications of the finite speed of light are taken into consideration, the result is the rather startling conclusion that time slows down for an observer in motion. This leads to the famous twin paradox, in which one twin travels away in a rocket ship at great speed and returns a few years later to discover that time has passed much faster on earth and his twin is now much older than he is. The paradox arises because the motion of the twins is supposedly relative, which means that it should be possible to construe this case as one in which the twin on the earth traveled away and the twin in the rocket ship stayed put.

Pickover shows, quite correctly, that this paradox only arises because we have failed to take into account that the spaceship had to accelerate to get going and that this acceleration was not something that the earthbound twin experienced. Hence the two twins are not in similar reference frames and there is no real paradox. The demonstration of this extraordinary conclusion requires nothing beyond high school math and geometry, and is fully worked out in Pickover's treatment of special relativity. (Special relativity is a subset of the larger theory of general relativity that deals with motion in reference frames that do not have forces acting on them.) The traveling twin does not illustrate the truly paradoxical example of time travel, because the twin cannot go back into the past; he can only move into the future at a slower pace than his nontraveling counterpart.

Speculation about travel into the past requires the much more sophisticated theoretical machinery of general relativity, machinery whose exposition eventually overwhelms all but the advanced specialist. Thus Pickover's book—which is not primarily intended for specialist readers, though surely many will enjoy it as a diversion—slowly but surely migrates from straightforward and thorough exposition of securely established science in the early chapters to a sensational and superficial romp through playgrounds of speculative conjecture at the end. The transition occurs slowly, making it difficult for the reader to tell exactly where the textbook ends and the tabloid begins—where Nova turns into Star Trek. The early chapters provide the relevant mathematical derivations making it possible to see how the curious conclusions follow from the more straightforward premises. But as the level of mathematical sophistication required to explicate the theory increases beyond what can be reasonably expected of the reader, the equations give way to graphical representations and then to abstract pictures of ever-decreasing scientific pedigree.

To ward off criticism that his speculations have passed beyond science into science fiction, Pickover reminds the skeptical reader near the end that history has proved embarrassing to previous skeptics. After all, the great Lord Kelvin assured us in 1895 that "heavier than air flying machines are impossible"; the chairman of IBM stated in 1943 that "there is a world market for maybe five computers"; and the founder of Digital Equipment told us in 1977 that "there is no reason for any individual to have a computer in their home." (As I write these words, Compaq, the world's largest producer of "useless" home computers is completing its successful takeover of Digital Equipment, now considered a technological dinosaur.) As we all know, technological advance soon put the lie to Lord Kelvin's claim that we would never have machines that could move through the air. Pickover suggests that this may happen with machines that move through time.

The reader may be forgiven if the difficulties in achieving flight seem to be of an entirely different scale from those that must be solved to make time travel possible. It is important to note, however, that the puckish Pickover is not alone in proposing the possibility of time machines. The idea has appeared before in more conventional works from leading theoretical physicists who are actively working and publishing in the area of general relativity.

Perhaps the leading thinker in this area is Kip Thorne, who holds the prestigious Feynman Chair of Theoretical Physics at the California Institute of Technology. Thorne's massive book Black Holes and Time Warps: Einstein's Outrageous Legacy concludes with a chapter on time machines in which he recounts the fascinating story of how the first paper on this topic was published in the prestigious physics journal Physical Review Letters.3 This seminal publishing event is also recounted in Michio Kaku's Hyperspace, another semipopular book by a mainstream theorist that takes time travel to be a serious possibility.4 (Kaku's book, which has a chapter entitled "To Build a Time Machine," was written while he was working at the Princeton Institute for Advanced Study, Einstein's home for most of his professional career.)

Thorne relates the fascinating story of how he was driven to consider what would be the most plausible design for a "ship" that could maneuver through spacetime by Carl Sagan's request for some technical help for his novel Contact. Working backward from the "science fiction" conclusion that Sagan needed, Thorne discovered, to his surprise, that there were indeed certain types of "time machines" whose existence was not precluded by the laws of physics. Such machines require the existence of a certain "exotic" form of matter, which most physicists think probably does not exist. But time machines, unlike the apparently much simpler "perpetual motion machines" that continue to captivate crank "inventors," cannot be proven to be impossible based on any known laws of physics.

Popularizations of science—from such demanding but accessible works as Thorne's, to idiosyncratic treatments like Pickover's—are in a long and noble tradition that goes back to Galileo and his controversial decision to publish some of his work in Italian, the language of the common people, rather than Latin. Since the time of Galileo, science has changed and continues to change our world. To be literate requires that we understand this. And readers of this journal should be only too aware that evangelicals have generally failed to "keep up" with science; one looks almost entirely in vain through the offerings from our major Christian publishers for books written for the purpose of improving the science literacy of their evangelical readership.

Pickover's book may not appeal to every reader's taste: those who like their science straight up may find his intergalactic soap opera a nuisance, while those who prefer their science fiction with a heavy emphasis on the fiction may be equally disappointed. But readers of more catholic tastes will relish the mixture of some excellent science exposition with one astonishing yet possible implication of that science.

Karl Giberson is professor of physics at Eastern Nazarene College. He is the author of Worlds Apart: The Unholy War Between Science and Religion and the winner of a 1996 Templeton Award for an Outstanding Course in Science and Religion.

1. Paul Johnson, Modern Times: The World from the Twenties to the Nineties, rev. ed. (HarperPerennial, 1991).

2. Chris Toumey, Conjuring Science: Scientific Symbols and Cultural Meanings in American Life (Rutgers Univ. Press, 1996), p. 7.

3. Kip Thorne, Black Holes & Time Warps: Einstein's Outrageous Legacy (Norton, 1994); M. S. Morriss, K. S. Thorne, and U. Yurtsever, "Wormholes, Time Machines, and the Weak Energy Condition," Physical Review Letters, Vol. 61 (1988), p. 1446.

4. Michio Kaku, Hyperspace: A Scientific Odyssey Through Parallel Universes, Time Warps, and the 10th Dimension (Oxford Univ. Press, 1994).

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