Acosmic peek at spacetime foam

Far from being a passive backdrop, spacetime is generally thought to be dynamic, undergoing quantum fluctuations that make it foamy on the scale of the minuscule Planck length, about 10^–35 m. There is no consensus on the size or character of the fluctuations: Two current models are the so-called holographic model, which is consistent with black-hole physics, and the uniform random-walk model. For any model, the fluctuations give rise to exceedingly tiny uncertainties in distance measurements. Three physicists from the University of North Carolina at Chapel Hill have found a way to use extragalactic point sources—like very distant active galactic nuclei or quasars—to amplify those infinitesimal effects. The key idea is that a propagating wavefront acquires uncertainties in both phase and direction as it moves through and scatters off the spacetime fluctuations. If the light’s wavelength is short enough (radio won’t work) and the travel path long enough, those uncertainties will show up in interferometric observations as a decreased fringe visibility. Using Hubble Space Telescope observations of an appropriately bright and distant source, the UNC researchers have decisively ruled out the random-walk model. According to them, proposed large optical instruments such as the VLT and Keck interferometers will be on the verge of probing the holographic model. (W. A. Christiansen, Y. J. Ng, H. van Dam, Phys. Rev. Lett. 96 , 051301, 2006 http://dx.doi.org/10.1103/PhysRevLett.96.051301 .)