Until this century we envisioned time as an absolute, a fixed period or set of periods within which the species lived out its history. Time provided the stage upon which the human drama unfolded. Throughout the twentieth century, however, our concept of time has changed. Theoretic and experimental work in physics has led to such notions as parallel universes, relativity, and the emergent idea that time and space are perhaps equivalent aspects of existence. These new ideas forced the species to reconsider the nature of time as not so much a fixed quantity, but a variable one.