  Nothing---the absence of spacetime---can be either an endpoint of tunneling,
as in the bubble of nothing, or a starting point for tunneling, as in the
quantum creation of a universe. We argue that these two tunnelings can be
treated within a unified framework, and that, in both cases, nothing should be
thought of as the limit of anti-de Sitter space in which the curvature length
approaches zero. To study nothing, we study decays in models with
perturbatively stabilized extra dimensions, which admit not just bubbles of
nothing---topology-changing transitions in which the extra dimensions pinch off
and a hole forms in spacetime---but also a whole family of topology-preserving
transitions that nonetheless smoothly hollow out and approach the bubble of
nothing in one limit. The bubble solutions that are close to this limit,
bubbles of next-to- nothing, give us a controlled setting in which to
understand nothing. Armed with this understanding, we are able to embed
proposed mechanisms for the reverse process, tunneling from nothing to
something, within the relatively secure foundation of the Coleman-De Luccia
formalism and show that the Hawking-Turok instanton does not mediate the
quantum creation of a universe.
