Imaginary Numbers

Rafael Bombelli wrote in 1572 that he had 'found a new type of number' when solving cubic equations — numbers involving the square root of negative quantities. For two centuries, mathematicians used these 'imaginary' numbers (the dismissive name stuck) while being deeply uneasy about them. Then Gauss showed in 1799 that they are geometrically real — rotations in a plane — and all unease evaporated. The complex numbers are now considered the natural domain of physics, not a convenience but a fundamental feature of reality.