Stress Tensor Fluctuations and Passive Quantum

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 semi-classical gravity are discussed.