Living in a physical world Ⅵ. Gravity and life in the air

摘要

In our perceptual world, no physical agency imposes itself with greater immediacy than does gravity. We depend on it to walk or run; it injures us if we trip. It makes each of us about half a centimeter shorter at the end of each day than when we first arise. Our flesh sags as we age; more slowly, the glass of a window thickens at the bottom and thins at the top. We dream of escaping its constant crush, although our recent experiences in orbiting spacecraft reveal an addiction with a difficult withdrawal. Physicists may regard the gravitational attraction between two objects as the universe's definitional weak force, but to us large, terrestrial creatures it feels anything but weak. Since the consequences of gravity depend on one's size, scaling will loom at least as large in this and then the next as in any of the preceding essays. Even more important than the ways gravity's effects, scale will be another message-the surprisingly wide range of biological situations in which it plays some role. One knows that no massless world exists; I would argue that a weightless world is almost as hard to imagine.