Fluctuations of Energy Density and Validity of Semiclassical Gravity

From calculations of the variance, or the fluctuations, and the mean energy densityof a massless scalar field in the Minkowski vacuum as a function of an intrinsicscale defined by the world function between two nearby points (as used in pointseparation regularization), we show that, contrary to prior claims, the ratio ofvariance to its mean squared being of the order unity does not imply a failureof semiclassical gravity. It is more a consequence of the quantum nature of thestate of the matter field than any inadequacy of the theory of spacetime withquantum matter as source.