The edges of any landscape—horizons, the lip of a valley, the bend of a river around a canyon wall—quicken an observer’s expectations. That attraction to borders, to the earth’s twilit places, is part of the shape of human curiosity. And the edges that cause excitement are like these where I now walk, sensing the birds toying with gravity; or like those in quantum mechanics, where what is critical straddles a border between being a wave and being a particle, between being what it is and becoming something else, occupying an edge of time that defeats our geometries. In biology these
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