Once past your fortieth or fiftieth birthday, a year seems to last but a fraction of the years you knew when you were fifteen or twenty. Life seems to speed up as you grow older--a form of acceleration that causes the years to shrink. That mysterious acceleration hides a puzzle, which American psychologist and philosopher William James mentioned in 1890 in his Principles of Psychology: How can the years speed up when the hours and days do not, and seem to be the same as they always have been?
For the acceleration of time, metaphors are easier to provide than explanations. "Time," wrote Gerrit Krol in Een Fries Huilt Niet (Frisians Do Not Cry), "is a small chain that you twirl round your finger." But why does that chain keep twirling more and more quickly? The French philosopher Paul Janet suggested in 1877 that the apparent length of a period in somebody's life is related to the length of his life. A child age ten would experience one year as a tenth of his life, a man of fifty as a fiftieth. James considered this "law" a description, rather than an explanation, of the subjective acceleration, and he was right. He attributed the apparent contraction of the years to:
... the monotony of the memory's content, and the consequent simplification of the backward-glancing view. In youth we may have an absolutely new experience, subjective or objective, every hour of the day. Apprehension is vivid; retentiveness strong; and our recollections of that time, like those of a time spent in rapid and interesting travel, are of something intricate, multitudinous, and long drawn-out. But as each passing year converts some of this experience into automatic routine, which we hardly note at all, the days and the weeks smooth themselves out in recollection to contentless units, and the years grow hollow and collapse.
This explanation places memory at the center of our experience of time. Psychological time ticks away on an internal clock, to the accompaniment of our recollections; duration and tempo are manufactured in the memory. The experience that life speeds up is part of a whole family of time illusions. Some of these occupy a scale of seconds or minutes; others take days, years, or even long periods of a human life. But no matter what their length, measured by the clock or the calendar, they all have this in common: They link the experience of time to what takes place in our consciousness.
As early as 1885, many of the psychological factors influencing subjective time were described by French philosopher and psychologist Jean-Marie Guyau. In his all-too-short life, ravaged by tuberculosis, he developed an elegant theory of man's idea of time. The basic analogy in Guyau's theory of time is space--not the geometric kind, but that used in perspective; space as it manifests itself to the observer. The experience of time is a case of "internal optics"; the memory orders our experiences in time much as a painter orders space with the use of perspective. Memories lend depth to our consciousness. As soon as the order in our memory is broken, as happens during the imperceptible transitions between dream images, our sense of time has also gone.
Guyau summed up a handful of factors that influence the internal optics of psychological time. Duration and tempo depend on the intensity of our sensations and ideas--their alternation, their number, the tempo with which they succeed one another, the attention we pay them, the effort it takes to store them in the memory, and the emotions and associations they call forth in us. In our personal perceptions, intensity is also a factor in our estimates of duration: Thinking back about an event that has made a great impression on us, we tend to underestimate the time interval separating us from that event. Such illusions have their counterparts in psychiatry. Traumatic events are repeated in flashbacks, memories that penetrate the psychological present and …