Leaving aside the wonderful progress of Greek and Arab science, it can be said that century physics started its successful march forward during the sixteenth and seventeenth centuries by addressing simple problems. In the case of mechanics, this means solving two-body problems. By this term, physicists designate phenomena in which only two components interact. For example, the movement of the Earth around the Sun, the free fall of heavy bodies, the oscillations of a pendulum, or, in a somewhat extended sense, the deviation of a light beam when it interacts with water. Some three centuries later, investigation of the hydrogen atom assuming it was composed of a proton and an electron was another attempt guided by the same principle of simplicity.