I love flying because it offers an extended uninterrupted opportunity to read—and there’s even a drink cart! On a recent trip out of town, I packed two books.  One was a novel sent to me by its author with a request to review it; it proved unreadable. The Time Travelother was a chronicle of siblings who were executed for resisting the Nazis, a fine account that was nevertheless too depressing to be my sole read for the duration. Thus, while waiting for a connecting flight at an airport in Atlanta, I spotted a bookstore and walked out with a copy of Time Travel: A History, by James Gleick, which literally consumed me for the rest of my journey.

What if I had packed different books for my trip? What if I had the option to go back and make a different selection? Or, what if—prior to departure—I could have skipped ahead and evaluated how my choices turned out? Certainly, everyone at some point questions decisions made in life and yearns for that impossible opportunity for a do-over. Rarely, of course, would we muse over such mundane matters as to which book was plucked off a shelf. The choices revisited would more likely focus upon not having another drink before picking up the car keys, not sleeping with your wife’s best friend, or not taking the rent money to the casino. The flip side of looking back, naturally, is looking ahead to the consequences: the accident, the divorce, the eviction.  But as we are reminded more than once in Gleick’s marvelous existential excursion into the scientific, philosophical and literary manifestations of the concept of time travel, our lives and choices are part of an even greater chaos system than the planet’s weather, and an authentic butterfly effect governs it all. So yeah, even your pick of a paperback could somehow end up changing everything, not only for you, but for those in your orbit, perhaps all of humanity.

Of course, Gleick—who specializes in science and technology writing and is the author of Chaos: Making a New Science—is something of an expert on chaos theory, although Time Travel is far less concerned with science than it is with how notions of time run through every aspect of modern consciousness. That was not always the case, as Gleick notes in an especially fascinating focus on how that came to change, relatively recently in our history, when the advent of train transportation and by extension train schedules—together with the communications technology of the telegraph—brought standardization to clocks across a vast network. The long pre-industrial agricultural world governed almost entirely by sunrise and sunset and the phases of the moon was all at once turned upside down.  Clocks were not new, of course, but until regular train travel came to depend upon rigid scheduling, there was little pressure for the universal standardization of time. It was perhaps this now omnipresent sense of time, Gleick’s account seems to suggest, that sparked the whimsy to wonder what it could be like to travel forward or backward in it.

Time Travel opens with the guy who—it turns out—virtually invented the idea: H.G. Wells. While there were, Gleick recounts, prior random references to such a thing here and there, it was The Time Machine, Well’s 1895 novel—published at a time when the potential of science and technology seemed limitless—that conceived for a wide audience the very notion that travel through time might be plausible. The impact was immeasurable. For one thing, this slim work managed to generate a kind of massive cultural paradigm shift in the way those exposed to it perceived their past and future, riding Wells’ imaginary machine in their own minds-eyes: could it be possible to revisit the past or peer into the future?  (Never mind that Wells himself believed it impossible.)  For another, it spawned a whole cottage industry of science fiction tales in books—and later, in film—wrapped about this topic, the bulk of which are contained in this volume’s extensive bibliography. The reader suspects that Gleick has read or watched all of these, probably more than once, as throughout the narrative he skillfully employs these literary excursions to explore the philosophical and scientific ramifications of time travel.  What if you went back in time and blocked Booth from shooting Lincoln, or killed your mother, or met yourself? These all seem like “timeless” themes now (pun fully intended!), but it looks like no one asked questions like these before Wells came along. And Wells didn’t ask them either: his time machine only went forward to a far distant future of Morlocks and Eloi, and beyond that, but not into the past. It was left to his literary descendants to tap those veins.

Given Gleick’s resume, I expected more of a focus on science in this book than naval-gazing, but that’s not to say science is absent. We think of the elasticity of time in our perceptions of it—we indeed experience the same units of time quite differently during orgasm and root canal, for instance—but Einstein revealed with his theory of special relativity that in the nature of the universe time is indeed elastic. Wells may have invented the science fiction about time, but it was Einstein, just a decade later, who uncovered the science of it. In fact, Einstein shook the halls of science at its very foundation when his work showed that space and time are part of a single space-time continuum comprised of the coordinates of four dimensions. I’m hardly a physicist, but the way I usually explain this to the uninitiated is by asking the trick question: What would happen on the earth if the sun suddenly burned out?  The answer, of course, is: nothing … for about 8 minutes!

Time is, though, a tricky thing. On the macro level, there is something known as the “arrow of time” which proceeds in one direction, towards the future, along with the concomitant disorder known as entropy that is a component of the second law of thermodynamics, which accounts for the asymmetry between future and past.  On the quantum level, on the other hand, we cannot be certain of that: underlying laws of physics work the same going forward in time as in reverse, meaning they are time-symmetric. Most of the physics beyond this are also beyond me, but Gleick does a competent job of summarizing the thinking about this attached to the best minds out there, from Einstein to Eddington to Feynman to Hawking—to the current cutting-edge science probed by the likes of Sean Carroll—from theories of relativity to those of the multiverse, in language fully accessible to the layman.

So, does that mean time travel is possible, to the past or the future?  Gleick himself seems agnostic; others not so much.  Some scientists consider that hypothetical wormholes—predicted by the theory of general relativity—might create shortcuts through space-time and could thus be a potential mechanism for time travel to the future, but that remains very speculative. Few think that travel into the past, reversing the arrow of time, could ever be possible. Others believe, like Wells did, that time travel is an amusing idea, but simply impossible. And then there are those, like some champions of the multiverse, who suggest that time is passing differently but simultaneously in multiple parallel universes, which makes you wonder that if time travel was indeed a possibility, rather than moving backwards or forwards in time, might it be more interesting to instead go sideways? Perhaps the accident, the divorce, or the eviction have different outcomes elsewhere …

There’s another kind of time travel that has a more secure scientific foundation. We know that Einstein was correct about special relativity, and although we cannot directly measure the space-time continuum, phenomena predicted by it have been confirmed. Einstein noted that the speed of light in a vacuum is always the same, regardless of the speed an observer travels, because the speed of light is absolute and invariable.  This means that events might occur at the same time for one observer and at different times for another observer travelling at a different speed, because rates of time run differently based upon relative motion, something called time dilation.  This is where the notion comes in of a hypothetical explorer on a space ship travelling near the speed of light who spends 100,000 light years in space on a voyage to a distant star and hardly ages at all.

Our science is not nearly advanced enough to put that to a test, but recently astronaut Scott Kelly spent nearly a year in space at high-speed orbit while—as part of a focused NASA study—his twin brother Mark remained on earth, and it is believed that Scott returned to the planet 8.6 milliseconds younger than his twin.  On a more sobering note,  unexpected changes to Scott’s DNA have altered his gene expression so that it now appears to vary by seven percent from that of his twin, and some of these changes suggest potential harm to his biological processes, perhaps shortening his lifespan. Contrary to what was initially reported in the popular press, however, Scott and Mark are indeed still identical twins, and much further study lies ahead before definite conclusions can be reached. But it does make you think and make you wonder …

If you are, like me, a person who delights in thinking and wondering, then Time Travel by James Gleick should be near the top of your “to-be-read” list. Whether your interest is in science fact or science fiction, this fascinating book—which truly defies genre—will make an outstanding contribution to your intellectual development.  That is, of course, if you can find the time to read it …