If x2 = 4,then what is x? Did you just contemplate "2"? Is that correct? Well, yes and no. The fact that there is a parallel but equally valid reply that x is negative 2 has been a difficult and intriguing conundrum to everyone who comes across it. But this concept can be applied to far more than simple algebra. It can be applied to one of the most confusing areas of science—antimatter, and an odd sort of "opposite" to the stuff that makes up our ordinary world.
Source: phys.org