This equation explained not only the intrinsic angular momentum of the electrons called "spin", it also led to the prediction of the antiparticle of the electron (the positron), and fine structure could only be fully explained with special relativity.
Not only the intrinsic angular momentum of the electrons called "spin", it also led to the prediction of the antiparticle of the electron (the positron). This was what this equation explained. Fine structure could only be fully explained with special relativity. This was what this equation explained.