The distinction between Newton's and Einstein's idea about gravitation has somtimes been illustrated by picturing alittle boy playing merble in a city lot. The ground is very
uneven , ridged with bumps and hollows. An observer in an office ten stories above the street would not be able to see these irregularities in the ground. Noticing that the
marbles appear to avoid some sections of the ground and move toward other sections, he migth assume that a "force" was operating which replled the marbles from certain spots
and attracted them to others. But another observe on the ground would instantly percieve that the path of the marbles was simply governed by the curvature of the field. In
this analogy Newton is the upstairs observer who imagines a "force" is at work, and Einstein is the observer on the ground, who has no reson to make such an assumption.