James Gleick’s Time Travel riffs on the topics that might pop up in a serious bull session about time: eternity; memory; the theory of relativity; causation; personal identity (what makes me the same person today as yesterday); free will versus determinism; the subjective time of poets, novelists, and (some) philosophers versus the operational time of physicists (and other philosophers); time as a cycle, as an arrow, as an eternal recurrence; time travel and its paradoxes. It is in part a treasury of quotations. The opening chapters alone cite (among others) St. Augustine, Newton, Richard Feynman, Hobbes, Schopenhauer, Susan Sontag, Saint Paul, Tennyson, Poe, Borges, and one of my favorite comedians, Stephen Wright: “Right now I’m having amnesia and déjà vu at the same time. I think I’ve forgotten this before.”

The hook is the publication, in 1895, of H. G. Wells’s novel The Time Machine, the first story about traveling in time. Fiction had previously shown heroes, like Rip Van Winkle and Hank Morgan (the Connecticut Yankee), who one day found themselves in a different era. But they hadn’t gone adventuring. Gleick asks: Why then? What changed?

He’s full of suggestive anecdotes. There is no record, he says, of anyone celebrating the centennial of anything until Americans celebrated the centennial of the Declaration of Independence in 1876. The phrase “turn of the century” didn’t exist before the twentieth. The idea of sending messages to the future, to inform them about ourselves, seems to have originated in 1938 with the time capsule, a “tragicomic time machine.” The future itself was a modern invention, when it became thinkable that, for better or worse, what was to come might differ radically from what is or has been. In the words of Paul Valéry (which Gleick, to my surprise, doesn’t quote), “The problem of our times is that the future is not what it used to be.” Wells himself was a thoroughly and self-consciously modern man who believed in Progress and in the importance of creative people determined to bring the future about.

Gleick concerns himself not just with highfalutin thoughts but with what they turn into after trickling down into general culture. Victorian England, for example, had a fling with “the fourth dimension,” which could serve as a hiding place for any mysterious possibility one wanted to believe in, including telepathy and clairvoyance. Wells’s Time Traveller (not otherwise named) finds it a good-enough-for-sci-fi explanation of time travel: in addition to having dimensions of height, width, and depth, objects cannot exist unless they persist in a fourth dimension of duration; one can maneuver not only in the three spatial dimensions but in that one as well.

Gleick’s typical chapter is more like an improvised tune than a parsable argument.

I doubt Wells knew it, but mathematicians had long since demystified the idea of dealing with any number of dimensions. Geometrical terms like “space” and “dimension” were retained as useful analogies, but their application was no longer restricted to idealizations of the physical space of everyday experience; rather, the number of dimensions associated with a model of any physical system is simply the number of things needed to describe it. A model that characterizes a boiler by pressure, temperature, the setting of a throttle, and the length of time it’s been running has four dimensions. The state of the boiler corresponds to a list of those four values, and that list is called a “point” in the model’s four-dimensional “space.” There is no good way to associate each of those four-number lists with a point in our three-dimensional physical space, but so what? (To say that is not to disparage the intellectual reorientation required.) Gleick says that spaces with more than three dimensions are abstractions that lack a physical meaning, but what they lack is a representation in physical space.

Gleick’s typical chapter is more like an improvised tune than a parsable argument, and even ends with what a jazz musician would call a tag: “We are all futurists now”; “This is the way the world ends”; “Only the Time Traveller can call himself free.” Here, for example, is a sample from the flow of a chapter titled “Philosophers and Pulps”: Laudatory reviews of The Time Machine, leading to an outlier criticizing the story’s logic, leading to a paper in a philosophical journal about the logic of time travel . . . eventually passing through a Felix the Cat short subject (Father Time sends him into the past by turning the hands of his clock backward) . . . winding up with Hugo Gernsback, “selfmade inventor . . . entrepreneur . . . and . . . bullshit artist,” and prophet of a marvel-filled future (which would include “electrically propelled roller skates”), who founded Amazing Stories, the first magazine devoted to what he called scientifiction. Then a quick coda, a sci-fi plot: a time traveler becoming his own father. “Page Einstein indeed.”

This sophisticated noodling retails amazing amounts of information about novels, stories, movies, popular culture, and—at a well-informed layman’s level—science and philosophy. It can be highly entertaining. The stories often have witty plots, and Gleick dispenses many diverting asides from his magpie hoard. Who knew that the first Borges story to appear in English was published in, of all places, Ellery Queen’s Mystery Magazine? Or that Ursula K. Le Guin went to high school with Philip K. Dick? (Though, contra his suggestion, the fact that The Time Machine appeared shortly before Einstein’s first papers on relativity is pure coincidence. A hook is just a hook; not everything’s a fish.)

Now for two complaints.

Gleick’s breezy tone usually serves him well but sometimes conveys not knowledge but a pose of knowingness. For example, he cites a famous fragment from Newton’s Principia:

Absolute, true, and mathematical time, in and of itself and of its own nature, without reference to anything external, flows uniformly . . .

This, he says, is “handed down to us as if engraved on tablets of stone”—which is, to begin with, a cliché beneath such a good writer. More important, the implication of dogmatism is misleading. The words quoted occur at the beginning of a discussion in which Newton lays out his underlying ideas about time, space, and motion, and then presents arguments for them (though the most elaborate and detailed concern not time but motion). The patronizing rhetoric continues: For Newton, “Absolute time is God’s time. He had no evidence for it.” Scholars disagree about what Newton commits himself to, but insofar as he goes beyond setting out the axioms of a model—which would be justified by its spectacular success—he’s engaged in a metaphysical dispute, whose weapons are not data but arguments. (“God’s time” alludes to the so-called General Scholium, which Newton added to the second edition of Principia in response to his critics.) Sniffs of condescension are elsewhere directed at Aristotle and the great mathematician Laplace.

Sniffs of condescension are directed at Aristotle and the great mathematician Laplace.

Complaint number two: Gleick’s account of “space-time,” one of the central scientific ideas in the book, grabs the wrong end of the stick. The concept, though not the term, comes from Hermann Minkowski. With it he reformulated the theory of special relativity, proclaiming that “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.” Gleick’s exposition seizes on a technical aspect of Minkowski’s model that does not differentiate it from Newtonian physics: namely, representing a body moving through space not as a succession of snapshots (here, then there, then there . . .) but from a god’s eye view, as a history laid out all at once. The maneuver is familiar. A newspaper lays out the history of a stock as a graph showing its price through time. A stock moves in one dimension (price up or down), so its graph is a track through the two dimensions of the page—using one dimension for price and one for date. We can similarly think of the entire history of a body moving in physical space as a track (which Minkowski calls its worldline) through a four-dimensional space aptly called space-time. Each point in a model of space-time is a list of four numbers that represents an event—three specifying where the event happens and the fourth specifying when. (Every observer makes his own model of the public universe of events, using his own clocks and yardsticks; whether all observers can agree on the clock and yardstick is the deep question that divides Einstein from Newton.)

Minkowski models the history of the cosmos as a collection of world-lines, analogous to a single newspaper image on which the tracks of all the stocks in the market have been printed. This aspect of the model is important to Gleick because it presents a world in which the distinctions between present, past, and future can be regarded as an illusion. All moments of history are simultaneously present in the set of worldlines; “now” is just a way of selecting a point from each of them.

But Newton’s universe can also be modeled as a collection of world-lines, so the crucial difference lies elsewhere. Imagine two events: button pushed; missile lands. Different observers can use their yardsticks to measure the spatial separation of those events (the distance between the button and the target) and their clocks to measure temporal separation (the time between push and boom). In Newton’s world, measurements of time and distance are independent of one another and all observers agree on the results. Neither is true in the Einstein/Minkowski world. There is, however, a more fundamental measure of separation, not between the spatial aspects or the temporal aspects of events but between events themselves. Called the space-time interval, its measurement requires both yardsticks and clocks, which cannot be disentangled. All observers—strictly speaking, all inertial observers—do agree when they measure intervals. Space-time is characterized by this special geometry.

To describe the state of time in the early twentieth century, Gleick contrasts the views of Einstein and the philosopher Henri Bergson, whom a journalist visiting from the twenty-first century would no doubt call a major public intellectual and a rock star. (The Stanford Encyclopedia of Philosophy suggests that one of his lectures was responsible for the first-ever traffic jam on Broadway.) For Bergson, time doesn’t exist independently of us and our perceptions of it, and cannot be understood exclusively through science. Time can be grasped only by an act of intuition. For Einstein, time is what a clock says—a theoretical clock defined in a sophisticated and physically fundamental way. One may legitimately speak of a subjective, psychological experience of time but, as Einstein said in a famous public debate with Bergson, “The time of the philosophers does not exist.”

Gleick wraps up his set with choruses that reassert the importance of our experience. He cites the great modernist writers, whose work is saturated with Bergsonian ideas. (Proust and Faulkner read Bergson, and Eliot attended his lectures.) Ultimately, Gleick says, we “need” time travel (odd word) “to elude death.” The ersatz immortality offered by our hyper-connected world, a kind of permanent present, is shallow. And the chilly view that time is somehow illusory, that the world lines have been laid down, hardly speaks to the fact that the drumbeat of life is loss. I know that the future will differ from the present in a very important way: it will not contain me. And the past was enriched by people that I loved (and still do). “Our entry into the past and the future,” Gleick says, “fitful and fleeting though it may be, makes us human.”