How do we know that change is accelerating? There is, after all, no absolute way to measure change. In the awesome complexity of the universe, even within any given society, a virtually infinite number of streams of change occur simultaneously. All "things"—from the tiniest virus to the greatest galaxy—are, in reality, not things at all, but processes. There is no static point, no nirvana-like un-change, against which to measure change. Change is, therefore, necessarily relative.

It is also uneven. If all processes occurred at the same speed, or even if they accelerated or decelerated in unison, it would be impossible to observe change. The future, however, invades the present at differing speeds. Thus it becomes possible to compare the speed of different processes as they unfold. We know, for example, that compared with the biological evolution of the species, cultural and social evolution is extremely rapid. We know that some societies transform themselves technologically or economically more rapidly than others. We also know that different sectors within the same society exhibit different rates of change—the disparity that William Ogburn labeled "cultural lag." It is precisely the unevenness of change that makes it measurable.

We need, however, a yardstick that makes it possible to compare highly diverse processes, and this yardstick is time. Without time, change has no meaning. And without change, time would stop. Time can be conceived as the intervals during which events occur. Just as money permits us to place a value on both apples and oranges, time permits us to compare unlike processes. When we say that it takes three years to build a dam, we are really saying it takes three times as long as it takes the earth to circle the sun or 31,000,000 times as long as it takes to sharpen a pencil. Time is the currency of exchange that makes it possible to compare the rates at which very different processes play themselves out.