Gedankenexperiment, German for “thought experiment,” was Albert Einstein's famous name for the imaginings that led to his greatest breakthroughs in physics. He traced his realization of light's finite speed—the core idea of special relativity—to his teenage daydreams about riding beams of light. General relativity, his monumental theory of gravitation, has its origins in his musings about riding up and down in an elevator. In both cases, Einstein crafted new theories about the natural world by using his mind's eye to push beyond the limitations of laboratory measurements.