The vacuum is central to physicists' best theories of subatomic particles, gravitation, and cosmology. Nothingness provides the reference point with which to compare new particle creation and annihilation. Cosmologists use empty universes to study the causal structure of spacetime. Paradoxically, our best physical theories of particles, gravity, and spacetime are theories of nothingness. Stranger still, the physicists' vacuum is a hive of activity. Quantum fluctuations fill empty space wit...