Isaac Newton had a funny notion of time. He thought that it flowed smoothly— "equably," as he put it—at a steady rate from past to future. We know, of course, that this is nonsense. Time has different tempi. In the run-up to New Year's Eve, for instance, it goes at a gallop. Then, in January and February, it creeps along at a miserable pace.

Moreover, time moves faster for some of us than others. Old people are being rushed forward into the future at a cruelly rapid clip. For little children, however, time seems to go quite slowly. A single summer can expand into an eternity. One psychologist has estimated that, subjectively, by the age of eight you have lived two-thirds of your life.

One way researchers have tried to measure the subjective flow of time is by asking people in different age groups to estimate when a certain amount of time has passed. People in their early twenties tend to be quite accurate in judging when three minutes has elapsed, typically being off by less than three seconds. Those in their sixties, by contrast, miss the mark on average by forty seconds. Somewhat counterintuitively, their sense of time speeds up because their inner clocks are ticking off seconds at a slower pace. Three minutes and forty seconds slip by in what seems to them like only three minutes.

The river of time may have its rapids and its calmer stretches, but one thing would seem to be certain: It carries all of us, willy-nilly, in its flow. Irresistibly, irreversibly, we are being hastened toward the turn of the millennium, and thence to our deaths. That is not only what Newton thought, it's what we know in our hearts.

But it's all a monstrous illusion—so says contemporary science. The revolution in our understanding of time began with Albert Einstein, who discovered that there is no such thing as now. Whether or not two distinct events are happening "at the same time" all depends on where an observer is located, as well as on that observer's speed and direction of motion, Einstein proved. Suppose, for example, that Jones is walking uptown on Fifth Avenue and Smith is walking downtown. Because of their different trajectories, there will be a discrepancy of several days in what they would judge to be happening "now" in the Andromeda galaxy at the moment they pass each other. For Smith, the space fleet launched to destroy life on Earth is already on its way; for Jones, the Andromedan council of tyrants has not even decided whether or not to send the fleet.

Once simultaneity goes by the board, the very division of moments into past and future becomes meaningless. Events judged to be in the past by one observer may still lie in the future of another observer, so the future must be as definite as the past. "If there's one thing we can be sure of in physics," said the great cosmologist Fred Hoyle, "it is that all times exist with equal reality." In place of the fleeting present, we are left with a vast frozen timescape. Here, you are being born. There, you are celebrating the arrival of the year 2000. And over there, you've been dead for a while. Nothing is "flowing" from one event to another.

Einstein furnished a rigorous justification for a view of time that goes back to McTaggart, to Spinoza, to Augustine, even to Parmenides. Time is an illusion, according to this "staticist" view. The only rational way to see the universe is as God sees it: sub specie aeternitatis.

Yet despite Einstein et al., we cannot help feeling ourselves to be slaves to one part of this timescape (the past) and hostages to another part (the future). Nor can we help feeling ourselves to be quite literally running out of time.

Sir Arthur Eddington, one of the first physicists to grasp Einstein's relativity theory, declared that our sense of time's passage is so powerful that it must correspond to something in the objective world. If science could get no purchase on all this, we might be justified in saying: So much the worse for science! But things are not quite that bad. Scientists have recently learned, for instance, that our conscious now—also known as the "specious present"—spans about three seconds, because that is the interval over which our brains knit up arriving sense data into a unified experience. Evidence also suggests that a cluster of neurons in the midbrain acts as a sort of "time organ," and that a brain chemical called dopamine influences the way that organ measures subjective temporal spans. In a traumatic event like a car accident, dopamines may flood the brain, slowing down your sense of time so that seconds can feel like minutes.

It is also pretty clear that the nature of memory has something to do with the feeling that we are moving in time. The past and the future might be equally real, but—for reasons having to do, oddly enough, with thermodynamics—we cannot "remember" events in the future, only in the past. This fact gives a direction to time's arrow. It does not, unfortunately, explain why that arrow seems to fly.

Still, you do not have to be a scientist to know the speed with which we are all edging into the future: precisely one second per second. That is the rate at which we must go on killing time—until time kills us.

Jim Holt