• In the 1920s the young English physicist Paul Dirac began trying to understand and describe the space-time evolution of the electron, the first elementary particle discovered by J.J. Thomson in 1897. Dirac was puzzled by an unprecedented property of space-time, discovered by Lorentz in his studies of electromagnetic forces, whereby if space was real, time had to be imaginary, and vice versa. In other words, space and time had to be a ‘complex’ mixture of two quantities, one real and the other imaginary.