By Lutz Hüwel
Professor of Physics, Wesleyan University
We all know time can fly. And in a song by The Alan Parson Project time “keeps flowing like a river.” I like that metaphor: on occasion there is but a trickle and water-time hardly moves, but soon enough it rushes forward with great speed and force. Then again, the comparison has its limits – like all analogies do. Water flows, but time is not a substance. So what is flying when time flies? This is a difficult question. Maybe so difficult that all we can do is use analogies after all.
Still, many valiant attempts have been made to find an answer to the question of what time is. I marvel at Saint Augustine’s struggle with the concept of time in his Confessions. In the end though, he is forced to admit: “What, then, is time? If no one asks me, I know what it is. If I wish to explain it to those who ask me, I do not know….” In physics, the difficulty of coming to terms with what time is has been cleverly sidestepped by using an operational definition: time is what a clock says it is. Question: What is a clock? Answer: A device to tell time. While this word play is of course facetious, even more careful definitions of what constitutes a clock run into the risk of circular logic. Saint Augustine really has a point! Nevertheless, the seemingly limiting and vulnerable approach to restrict discussion of time to measurable aspects has yielded fascinating and deep insights into the nature of time.
Take Einstein. As a starting point for the theory of relativity, he substitutes “position of the small hand of my clock” for “time.” With that simple trick and the constancy of the speed of light — assumed to be perfect, measured to be true for all circumstances tested so far — he manages to predict properties of time, all verified by now, that a hundred years later remain strange and somewhat disturbing. Events judged to be simultaneous by one observer have a finite span of time between them for others. Temporal order is not absolute — the same event that occurred in the past for me can still lie in the future for you. Time can be stretched and compressed — two identical clocks can tick at their own, distinctly different beat. There is not just one absolute and universal flow of time as Newton had envisioned, but a multitude of time-rivers.
Another facet having to do with the notion of flowing time intrigues me. Is time continuous or does a smallest, indivisible unit exist? No change is truly experienced as continuous movement, but rather as a sequence of still images. But that does not imply time itself is discontinuous. The quest for ever more exactitude has given us clocks that chop a nanosecond into a million pieces; no limit seems to be in sight. It is quite astonishing how simple acts of observation and reasoning have contributed so much to what we know about time. It is likewise remarkable with what delicate precision we can measure time while a true understanding still eludes us.