Fluctuations of Spacetime Itself
Gravity differs fundamentally from the other basic forces, which act
in spacetime. For the other three basic forces, spacetime serves as a
passive "stage" for physical events.
But gravity distorts spacetime itself and is in turn
generated by distortion of spacetime.
Therefore, quantum fluctuations of
the gravitational field don't occur on a passive spacetime stage. Rather, it is the spacetime stage
itself that
fluctuates at the quantum level of the gravitational field. This accounts for the strange
possibilities described in the left panel.