When David Eagleman was eight years old, he fell off a roof and kept on falling. Or so it seemed at the time. His family was living outside Albuquerque, in the foothills of the Sandia Mountains. There were only a few other houses around, scattered among the bunchgrass and the cholla cactus, and a new construction site was the Eagleman boys’ idea of a perfect playground. David and his older brother, Joel, had ridden their dirt bikes to a half-finished adobe house about a quarter of a mile away. When they’d explored the rooms below, David scrambled up a wooden ladder to the roof. He stood there for a few minutes taking in the view—west across desert and subdivision to the city rising in the distance—then walked over the newly laid tar paper to a ledge above the living room. “It looked stiff,” he told me recently. “So I stepped onto the edge of it.”

“Time is this rubbery thing,” Eagleman said. The best example of that is the so-called oddball effect. Photograph by Dan Winters

In the years since, Eagleman has collected hundreds of stories like his, and they almost all share the same quality: in life-threatening situations, time seems to slow down. He remembers the feeling clearly, he says. His body stumbles forward as the tar paper tears free at his feet. His hands stretch toward the ledge, but it’s out of reach. The brick floor floats upward—some shiny nails are scattered across it—as his body rotates weightlessly above the ground. It’s a moment of absolute calm and eerie mental acuity. But the thing he remembers best is the thought that struck him in midair: this must be how Alice felt when she was tumbling down the rabbit hole.

Eagleman is thirty-nine now and an assistant professor of neuroscience at Baylor College of Medicine, in Houston. Physically, he seems no worse for the fall. He did a belly flop on the bricks, he says, and his nose took most of the impact. “He made a one-point landing,” as his father puts it. The cartilage was so badly smashed that an emergency-room surgeon had to remove it all, leaving Eagleman with a rubbery proboscis that he could bend in any direction. But it stiffened up eventually, and it’s hard to tell that it was ever injured. Eagleman has puckish, neatly carved features, with a lantern jaw and modish sideburns. In Baylor’s lab-coated corridors, he wears designer jeans and square-toed ankle boots, and walks with a bounce in his step that’s suspiciously close to a strut, like Pinocchio heading off to Pleasure Island.

If Eagleman’s body bears no marks of his childhood accident, his mind has been deeply imprinted by it. He is a man obsessed by time. As the head of a lab at Baylor, Eagleman has spent the past decade tracing the neural and psychological circuitry of the brain’s biological clocks. He has had the good fortune to arrive in his field at the same time as fMRI scanners, which allow neuroscientists to observe the brain at work, in the act of thinking. But his best results have often come through more inventive means: video games, optical illusions, physical challenges. Eagleman has a talent for testing the untestable, for taking seemingly sophomoric notions and using them to nail down the slippery stuff of consciousness. “There are an infinite number of boring things to do in science,” he told me. “But we live these short life spans. Why not do the thing that’s the coolest thing in the world to do?”

The Eagleman lab, on the ground floor of Baylor’s Ben Taub General Hospital, could be the lair of a precocious but highly distractible teen-ager. The doors are pinned with cartoons, the counters strewn with joysticks and other gizmos. The conference table is flanked by a large red rubber ball, for use as a chair or a Hippity Hop. When Eagleman first moved in, he had the walls painted baby blue, with a shiny finish designed to be erasable. By now, they’ve been covered from floor to ceiling with equations, graphs, time lines, to-do lists, aphorisms, and sketches of brain waves—a Pollocky palimpsest of red, green, purple, and black scribblings. “The old stuff is really hard to erase,” Eagleman told me. “It’s like memory that way.”

Although Eagleman and his students study timing in the brain, their own sense of time tends to be somewhat unreliable. Eagleman wears a Russian wristwatch to work every morning, though it’s been broken for months. “The other day, I was in the lab,” he told me, “and I said to Daisy, who sits in the corner, ‘Hey, what time is it?’ And she said, ‘I don’t know. My watch is broken.’ It turns out that we’re all wearing broken watches.” Scientists are often drawn to things that bedevil them, he said. “I know one lab that studies nicotine receptors and all the scientists are smokers, and another lab that studies impulse control and they’re all overweight.” But Eagleman’s ambivalence goes deeper. Clocks offer at best a convenient fiction, he says. They imply that time ticks steadily, predictably forward, when our experience shows that it often does the opposite: it stretches and compresses, skips a beat and doubles back.

The brain is a remarkably capable chronometer for most purposes. It can track seconds, minutes, days, and weeks, set off alarms in the morning, at bedtime, on birthdays and anniversaries. Timing is so essential to our survival that it may be the most finely tuned of our senses. In lab tests, people can distinguish between sounds as little as five milliseconds apart, and our involuntary timing is even quicker. If you’re hiking through a jungle and a tiger growls in the underbrush, your brain will instantly home in on the sound by comparing when it reached each of your ears, and triangulating between the three points. The difference can be as little as nine-millionths of a second.

Yet “brain time,” as Eagleman calls it, is intrinsically subjective. “Try this exercise,” he suggests in a recent essay. “Put this book down and go look in a mirror. Now move your eyes back and forth, so that you’re looking at your left eye, then at your right eye, then at your left eye again. When your eyes shift from one position to the other, they take time to move and land on the other location. But here’s the kicker: you never see your eyes move.” There’s no evidence of any gaps in your perception—no darkened stretches like bits of blank film—yet much of what you see has been edited out. Your brain has taken a complicated scene of eyes darting back and forth and recut it as a simple one: your eyes stare straight ahead. Where did the missing moments go?

The question raises a fundamental issue of consciousness: how much of what we perceive exists outside of us and how much is a product of our minds? Time is a dimension like any other, fixed and defined down to its tiniest increments: millennia to microseconds, aeons to quartz oscillations. Yet the data rarely matches our reality. The rapid eye movements in the mirror, known as saccades, aren’t the only things that get edited out. The jittery camera shake of everyday vision is similarly smoothed over, and our memories are often radically revised. What else are we missing? When Eagleman was a boy, his favorite joke had a turtle walking into a sheriff’s office. “I’ve just been attacked by three snails!” he shouts. “Tell me what happened,” the sheriff replies. The turtle shakes his head: “I don’t know, it all happened so fast.”

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A few years ago, Eagleman thought back on his fall from the roof and decided that it posed an interesting research question. Why does time slow down when we fear for our lives? Does the brain shift gears for a few suspended seconds and perceive the world at half speed, or is some other mechanism at work? The only way to know for sure was to re-create the situation in a controlled setting. Eagleman and one of his graduate students, Chess Stetson, who is now at Caltech, began by designing and programming a “perceptual chronometer.” About the size of a pack of cards, it had an L.E.D. display connected to a circuit board and powered by a nine-volt battery. The unit could be strapped to a subject’s wrist, where it would flash a number at a rate just beyond the threshold of perception. If time slowed down, Eagleman reasoned, the number would become visible. Now he just needed a good, life-threatening situation.

Late one afternoon in October, Eagleman and I pulled into a gravel parking lot northwest of Dallas. A dingy cinder-block ticket office stood to one side, with a sign above the door that said “Zero Gravity.” Inside, past a low chain-link fence, a collection of giant steel structures rose several stories into the sky. To the left was a rickety-looking platform with a rubber rope dangling from it; to the right, a monstrous orange windmill with seats attached to the tips of its blades. “We had to shut down the Scraper,” one of the park attendants told me, pointing at it. “It’s waitin’ for a part from Germany.”

Zero Gravity was billed as a thrill park, but it looked more like an abandoned construction site—or an arena for death matches in a post-apocalyptic film. When Eagleman first went there, five years ago, he knew it was the place for him. He had tried to test the chronometer on his grad students, on a field trip to Six Flags AstroWorld, in Houston, but even the largest roller coasters proved insufficiently terrifying. He needed something completely safe yet plausibly deadly. “I really chewed on this for a while,” he told me. “I couldn’t put people in a car accident.” Then he heard about the SCAD.

The ride stood in the middle of the lot at Zero Gravity, like a half-built oil derrick. A steel gondola hung between its four legs and could be lifted to the top by thick cables. SCAD was short for suspended catch air device—a phrase more confusing than its acronym. But the idea was simple: when the rider reached the top of the tower, he’d be hooked to a cable and lowered through a hole in the floor of the gondola. His back would be to the ground, his eyes looking straight up. When the cable was released, he would plummet a hundred and ten feet, in pure free fall, until a net caught him near the bottom. “I’ve been up this thing three times, and it’s gotten scarier every time,” Eagleman said. “The second you drop, every part of you locks up. Your abs are rock solid, and you can’t breathe. You’re falling backward, going fifty miles an hour when you hit the net.”

We scanned the lot for potential volunteers, but the park was deserted. There are only two SCADs in the country, both of which, until recently, had pristine safety records. Then, in July, a SCAD operator in the Wisconsin Dells triggered a drop before the net had been lifted fully into place. When the rider—a twelve-year-old girl named Teagan Marti—landed in the net, her momentum stretched it to the ground. The impact fractured her skull and broke her spine in ten places. Afterward, the SCAD operator was put on leave for reasons of mental health. “It was just human error,” the attendant in Dallas assured us. Nothing like that had happened here.

Just then, a young couple wandered into the park. They were both in their early twenties, moonfaced and a little fidgety. April had small round glasses and a long ponytail; T.J. had a baggy black T-shirt with a purple sword on it, and a modest mullet combed back on top. They’d met at the Walmart in Weatherford, an hour away, they told us. April had found this place online but already seemed to regret it: she clutched T.J.’s hand and peered at the SCAD, her shoulders hunched up around her ears. He followed her gaze. “I’ve jumped off cliffs into lakes before,” he said. “But that’s about it.”

When Eagleman showed them the perceptual chronometer, they looked a little dubious. Eagleman’s excitement about his research is usually infectious. He’s a good talker, with a gift for distillation and off-the-cuff analogy, and he tends to gather steam as he goes, leaping from idea to idea until his voice is hoarse and his mind is catapulting off to distant dimensions. (“What if we were to land on a planet with aliens who live at a different time scale from us?” he asked me at one point. “Would we seem like statues to them the way trees do to us?”) In this setting, though, it was a little hard to take him seriously. The more sober and scientific he tried to sound, the more April and T.J. seemed to take him for some unhinged Trekkie babbling on about his time machine.

Still, they agreed to give it a try. The attendant fitted them with harnesses, latched them into the gondola, and sent them lurching into the Texas sky. I could see April’s ponytail whipping around above her head like a wind sock. “What is it, Tuesday?” Eagleman said. “How does someone, on a Tuesday, wake up and decide, ‘This is the day that I’m going to scare myself to death’?” Then he pulled a stopwatch from his pocket and waited for the bodies to drop.

Eagleman traces his research back to psychophysicists in Germany in the late eighteen-hundreds, but his true forefather may be the American physiologist Hudson Hoagland. In the early nineteen-thirties, Hoagland proposed one of the first models for how the brain keeps time, based partly on his wife’s behavior when she had the flu. She complained that he’d been away from her bedside too long, he later recalled, when he’d been gone only a short while. So Hoagland proposed an experiment: she would count off sixty seconds while he timed her with his watch. It’s not hard to imagine her annoyance at this suggestion, or his smugness afterward: when her minute was up, his clock showed thirty-seven seconds. Hoagland went on to repeat the experiment again and again, presumably over his wife’s delirious objections (her fever rose above a hundred and three). The result was one of the classic graphs of time-perception literature: the higher his wife’s temperature, Hoagland found, the shorter her time estimate. Like a racing engine, her mental clock went faster the hotter it got.

Psychologists spent the next few decades trying to identify this mechanism. They worked with mice, rats, fish, turtles, cats, and pigeons, then moved on to monkeys, children, and brain-damaged adults. They shocked their subjects with electrodes, strapped them into heated helmets, dunked them in water baths, and irritated them with insistent clicks, hoping to speed up or slow down their internal clocks. Hoagland believed that timing was a “unitary chemical process” tied to metabolism. But later studies suggested a hodgepodge of systems, each devoted to a different time scale—the cerebral equivalent of a sundial, an hourglass, and an atomic clock. “Mother Nature’s a tinkerer instead of an engineer,” Eagleman says. “She doesn’t just invent something and check it off the list. Everything is layers on layers built on top of each other, and that provides tremendous robustness.” Parkinson’s disease can impair our ability to time intervals of a few seconds, for instance, but leave split-second timing intact.

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Just how many clocks we contain still isn’t clear. The most recent neuroscience papers make the brain sound like a Victorian attic, full of odd, vaguely labelled objects ticking away in every corner. The circadian clock, which tracks the cycle of day and night, lurks in the suprachiasmatic nucleus, in the hypothalamus. The cerebellum, which governs muscle movements, may control timing on the order of a few seconds or minutes. The basal ganglia and various parts of the cortex have all been nominated as timekeepers, though there’s some disagreement on the details. The standard model, proposed by the late Columbia psychologist John Gibbon in the nineteen-seventies, holds that the brain has “pacemaker” neurons that release steady pulses of neurotransmitters. More recently, at Duke, the neuroscientist Warren Meck has suggested that timing is governed by groups of neurons that oscillate at different frequencies. At U.C.L.A., Dean Buonomano believes that areas throughout the brain function as clocks, their tissue ticking with neural networks that change in predictable patterns. “Imagine a skyscraper at night,” he told me. “Some people on the top floor work till midnight, while some on the lower floors may go to bed early. If you studied the patterns long enough, you could tell the time just by looking at which lights are on.”

Time isn’t like the other senses, Eagleman says. Sight, smell, touch, taste, and hearing are relatively easy to isolate in the brain. They have discrete functions that rarely overlap: it’s hard to describe the taste of a sound, the color of a smell, or the scent of a feeling. (Unless, of course, you have synesthesia—another of Eagleman’s obsessions.) But a sense of time is threaded through everything we perceive. It’s there in the length of a song, the persistence of a scent, the flash of a light bulb. “There’s always an impulse toward phrenology in neuroscience—toward saying, ‘Here is the spot where it’s happening,’ ” Eagleman told me. “But the interesting thing about time is that there is no spot. It’s a distributed property. It’s metasensory; it rides on top of all the others.”

The real mystery is how all this is coördinated. When you watch a ballgame or bite into a hot dog, your senses are in perfect synch: they see and hear, touch and taste the same thing at the same moment. Yet they operate at fundamentally different speeds, with different inputs. Sound travels more slowly than light, and aromas and tastes more slowly still. Even if the signals reached your brain at the same time, they would get processed at different rates. The reason that a hundred-metre dash starts with a pistol shot rather than a burst of light, Eagleman pointed out, is that the body reacts much more quickly to sound. Our ears and auditory cortex can process a signal forty milliseconds faster than our eyes and visual cortex—more than making up for the speed of light. It’s another vestige, perhaps, of our days in the jungle, when we’d hear the tiger long before we’d see it.