“…Since then, they have become cornerstones for our understanding of a wide range of physical processes [8,9,10,11,12]. This fact notwithstanding, the unification of both concepts poses a theoretical challenge still nowadays (classical references are [13,14,15,16,17,18]; recent contributions include [19,20,21,22,23,24,25,26,27,28,29,30,31]; potential applications in high-energy physics and astrophysics are considered in [32,33,34,35,36,37]). The relatively slow progress in this field can be attributed to the severe difficulties that arise when one tries to describe N -body systems in a relativistically consistent manner [38,39].…”