Abstract: Due to quantum fluctuations, spacetime is foamy on small scales.  The degree of foaminess is found to be consistent with holography, a principle prefigured in the physics of black hole entropy.  It has bearing on the ultimate accuracies of clocks and measurements and the physics of quantum computation.  Applied to cosmology, the holographic model of spacetime foam requires the existence of dark energy which, we argue, is composed of an enormous number of inert "particles" of extremely long wavelength.  We suggest that these "particles" obey infinite statistics in which all representations of the particle permutation group can occur, and that the nonlocality present in systems obeying infinite statistics may be related to the nonlocality present in holographic theories.  We propose to detect spacetime foam by looking for halos in the images of distant quasars, and argue that it does not modify the GZK cutoff in the ultra-high energy cosmic ray spectrum and its contributions to time-of-flight differences of high energy gamma rays from distant GRB are too small to be detectable.

Abstract: I shall describe a derivation of entropy of apparent horizons using coherent states in loop quantum gravity.