For the first time, physicists have built a working version of quantum mechanics without complex numbers — numbers that have been considered essential to the theory for nearly a century.

Complex numbers combine a regular “real” number with an “imaginary” one — a multiple of the square root of-1, represented by the symbol i — into a single value, like 3 + 4i. The square root of-1 doesn’t correspond to any quantity you could count or measure directly (you can’t have negative one apple, for instance), which is why mathematicians call it imaginary.