  We start with the field U. It has values U(6)=−2, U(4)=−3, U(3)=−4 U(2)=−6. If we assume that the mass M is at rest in space, then the potential field U will be constant through time. This field is illustrated in Figure 7 below.  It shows the space around the central mass at different times t=0, t=1 and t=2. The circles represent points in space with the same value of U. For example, all those points at radius r=6 have U=−2. The constancy of the field in time is represented by the vertical lines that connect points with the same value of U across time. For example, point at r=6 at each time instant have the same potential U=−2.