If you're young, this won't make much sense to you, but you had better read it anyway so you'll be prepared.

It's about an experience that everyone has as he or she gets older: as the years go by, time passes faster and faster. You don't really have to be old to notice it, but the effect becomes more dramatic as the years go by, so that you eventually find yourself saying, "I can't believe it. . . it's almost the middle of 2008 and I'm just getting used to 2007!"

Almost everyone experiences this phenomenon; the question is, why? One straightforward suggestion is that our sense of the passage of time depends on how much time we've experienced in our lives. For instance, one year for my 11-year-old son, Max, is about 9 percent of his total life; that same year for me is less than 2 percent. If the feeling of a year is based on how much of our total experience that year represents, then obviously, because it is relatively more event-filled for Max than it is for me, it will seem to pass by more slowly for him than for me.

But this is only a suggestion; there have to be some data to back it up, and there are.

The most interesting piece of work on this issue was published by Robert Lemlich of the University of Cincinnati in the mid-1970s (which of course seems like just yesterday to me). Lemlich argued that since it's all subjective anyway, the apparent length of a year should be related not to chronological years, but to subjective years. In other words, your estimate of the length of a year should be compared not with the actual number of years you've lived, but with your subjective sense of the length of your life.

Being an engineer, he took a no-nonsense mathematical approach to the question, and came up with a set of equations. For instance, let's assume you are a 40-year-old who will live to the age of 80. Lemlich calculated that time would be passing by twice as fast now as it did when you were 10, which feels about right-I remember how long summer vacation seemed to last, and how the return to school in the fall felt like I was turning a whole new page in my life.

But the extension of this calculation has some disturbing implicatiorjS; it might not sit quite as comfortably. If you're 40, the numbers tell you that you're halfway through your life-but because time seems to be passing ever more rapidly, Lemlich's math suggests that you will feel something quite different. In fact, he calculates that at the age of 40 you have already lived-subjectively-71 percent of your life. It gets worse too: by the time you're 60, even though you have 20 years remaining (25 percent of your actual life), those 20 years will feel like only 13 percent of your life.

These numbers are shocking enough (and you may well disagree with them, but before you do, ask your grandparents how they feel), but they take on an even more bizarre twist when you work backward and ask the question, when do we feel we're at the halfway point of life? This same 40-year-old would have felt as if she were at the halfway point when she was . . . just 20. I don't mean by this that at 20 she felt half as old as 40; I mean that when she was 20 she had already experienced half of her total subjective life.

These numbers are amazing and slightly unbelievable, but Lemlich backed them up with Esome experiritents. He asked some people of different ages how much faster time seemed to be passing in the present, compared to when they were one-quarter or one-half their present age. His theory predicted their answers almost exactly: these subjects felt time was passing twice as fast as when they were one-quarter their present age, and 1.4 times faster than when they were half their present age.

So, people do seem to experience time ing more quickly as they age. But is there a more scientific explanation? It may not be simply that each year represents a smaller percentage of the total. Might it be that, as we get older; our biological clock runs slower and slower it does, then external events would appear to be passing faster and faster.

We have several biological clocks in our brains, including one that tracks the 29 days or so between menstrual periods and the set of daily, "circadian" clocks in various organs that measure 24-hour periods. At the other end of the time spectrum is a clock that allows us to estimate the passage of small packets of time, on the order of seconds. (If you're questioning the usefulness of a clock that measures intervals of a second or less, question no further. Without such a timer you wouldn't be able to play "Jeopardy": rapid and accurate movement of your finger to the buzzer requires extremely accurate timing and coordination on short time scales.) Many experiments have revealed that sensitivity to small intervals of time is more complicated than the clock analogy suggests.

Imagine you're asked to listen to a musical note, then after it ends there is a pause; then the note is played again, but this time you have to say "Stop!" when you think it has lasted as long as it did the first time. It's a simple enough task, but there is a minimum set of equipment you need in your brain to be able to do it accurately. One is a biological clock. A second piece of equipment is something that can use that clock to judge the length of the note. And a third is something that can re-create that length while the second note is playing-that's the only way you can know when to say "Stspp." Any clock in the brain that allows you to estimate the passage of time needs these three components. Experiments like the one just described have given scientists a rough idea of what parts of the brain are involved in timekeeping, and have provided evidence that the clocks can slow down.

Tiny intervals of time-fractions of a second-are tracked by centers buried deep in the brain, below our awareness. One of these areas is affected early in Parkinson's disease; as a result, Parkinson's patients are inaccurate at judging these short time intervals. The neurotransmitter molecule dopamine (the chemical that allows neurons in this part of the brain to "talk" to each other) is crucial to accurate timing; it is depleted in Parkinson's disease and can also be boosted or depleted by a variety of drugs. Marijuana lowers dopamine and slows the clock, so that time speeds up. (If you think a slowed brain clock would logically make time seem to pass more slowly, re-think it this way: if your slowed clock now estimates a second as being only half a second, then twice as much as usual seems to be happening in that interval, and therefore time will seem to be speeding up-what seems like one minute will actually be two.) Conversely, cocaine raises dopamine levels and speeds the clock.

If you're trying to estimate anything longer than a second or so, you recruit different areas of the brain, those concerned with paying attention and remembering. Namely, you use the cortex, the elaborately folded surface of the brain. The right parietal cortex (just behind and below your right ear) and the right frontal cortex (tucked in behind your right eye) help you judge time intervals if few seconds. The parietal cortex is important for paying attention: this helps you keep track of the number of ticks of the clock that go by while a tone plays. And the frontal cortex is necessary for short-term memory: this helps you compare the length of a preceding tone with the one that is being played. Patients with damage to the right underside of their frontal cortex grossly underestimate the passage of a few seconds, but those with damage on the left side tend not to make those errors.

Although we know the right side of the brain is crucial to timekeeping, there is a medical case that illustrates just how important the left hemisphere is as well. The patient was a 66-year-old man who was admitted to a hospital m in Dusseldorf, Germany, with a tumor on the left side of his frontal cortex. He complained that everything seemed to be happening with "accelerated" motion. He had had to stop his car by the side of the road because traffic was rushing at him at an incomprehensible speed. The already breakneck speed of TV was intensified so much he was unable to watch. He felt that life was passing by too quickly for him to cope, so he abandoned his activities and hobbies and curtailed his social life.

When asked to estimate 60 seconds, this man's guesses averaged 286 seconds. In other words, more than four and a half minutes was the amount of time that seemed like a minute to him. No wonder the world seemed chaotic. Imagine four minutes' worth of traffic passing by in what seems like a minute! This suggested that his internal clock might have been damaged by the tumor and slowed dramatically.

This unfortunate patient suffered from an extreme version of what seems to happen to all of us, gradually, as we get older. Fergus Craik and Janine Hay at the Rotman Research Institute in Toronto used an experiment with a computerized task to test what happens to our sense of time with age. When asked, "How long have you been working on this trial?" the guesses of older people averaged around 30 seconds; in fact, 60 seconds of real time had elapsed. At about 120 seconds, they thought that just a minute had passed. Their timers seemed to be running at about half the speed of the younger people in the xmt.

Now we have two threads to tie together: the years pass by ever more quiackly; and our clocks are slowing down. Does the lattercompletely explain the former? I'm tempted to say yes, but I'm afraid the question is still a little too complicated for that. Neither process is steady and inexorable; both are thrown off track by life events, chemical changes, or the combination. All I know is that, clocks or not, I'm determined to enjoy the 19 percent of my subjective life that I have left.

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