those neg rates are going to be tough on the fed as well. when it gets to the point that the natural rate should turn negative the fed may find they must tighten instead. if they can't tighten their only ammo against inflation will be to stop printing which will crash the system. if they do tighten the debt payments explode in excess of 1.2 trillion PER YEAR again crashing the economy. the fed is walking a razors edge.