Episode four of Five Numbers, the BBC radio series presented by Simon Singh.

The imaginary number takes mathematics to another dimension. It was discovered in sixteenth century Italy at a time when being a mathematician was akin to being a modern day rock star, when there was 'nuff respect' to be had from solving a particularly 'wicked' equation. And the wicked equation of the day went like this: "If the square root of 1 is both 1 and -1, then what is the square root of -1?"