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.

Given the unevenness of change and armed with this yardstick, we still face exhausting difficulties in measuring change. When we speak of the rate of change, we refer to the number of events crowded into an arbitrarily fixed interval of time. Thus we need to define the "events." We need to select our intervals with precision. We need to be careful about the conclusions we draw from the differences we observe. Moreover, in the measurement of change, we are today far more advanced with respect to physical processes than social processes. We know far better, for example, how to measure the rate at which blood flows through the body than the rate at which a rumor flows through society.