Since the 1970s, they’ve known how to find shortest paths essentially as fast as theoretically possible, assuming all weights are positive numbers. But on some graphs weights can be negative — ...

Negative numbers are irrelevant for such tasks, as is zero. Brahmagupta was also interested in such abstract problems, however.

Imaginary numbers—the square roots of negative numbers—are an inescapable part of quantum theory, a study shows By Marc-Olivier Renou, Antonio Acín & Miguel Navascués ...

This means they are made of both real numbers and the imaginary number i: the square root of negative 1. Remember, a square root of a number represents a number whose square is the original number.