And it turns out the second solution, with negative energy, was when the theory is telling you, hey, there are particles and there are anti-particles, and the negative energy when properly interpreted will describe anti-particles.

结果,后来证明,第二个根,也就是能量为负的那个根,正是理论的关键,世界上既有粒子,也有反粒子,负能量就是用来解释反粒子的

耶鲁公开课 - 基础物理课程节选