  The quantum fluctuation of the stress tensor of a quantum field are
discussed, as are the resulting spacetime metric fluctuations. Passive quantum
gravity is an approximation in which gravity is not directly quantized, but
fluctuations of the spacetime geometry are driven by stress tensor
fluctuations. We discuss a decomposition of the stress tensor correlation
function into three parts, and consider the physical implications of each part.
The operational significance of metric fluctuations and the possible limits of
validity of semiclassical gravity are discussed.
