The philosopher of science Karl Popper coined the term risky prediction to describe the process by which scientists verify untested theories. Good theories, Popper proposed, generate risky predictions. They presage an unanticipated fact or event that runs a real risk of not occurring or being proven incorrect. When this unanticipated fact proves true or the event does occur, the theory gains credibility and robustness. Newton’s understanding of gravitation was most spectacularly validated when it accurately presaged the return of Halley’s comet in 1758.
