Pose a question to a Magic 8 Ball, and it’ll answer yes, no or something annoyingly indecisive. We think of it as a kid’s toy, but theoretical computer scientists employ a similar tool. They often imagine they can consult hypothetical devices called oracles that can instantly, and correctly, answer specific questions. These fanciful thought experiments have inspired new algorithms and helped researchers map the landscape of computation. The researchers who invoke oracles work in a subfield of co...