MG14 - Talk detail

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Abstract

One aspect of the quantum nature of spacetime is its ``foaminess" at very small scales. Astronomical observations of cosmologically distant sources may be important to testing models for quantum gravity, because the Planck-scale spatial uncertainties posited by those models will produce phase fluctuations in the wavefront of radiation emitted by a source , which may accumulate over large path lengths. In the so-called alpha-models, the path-length fluctuations accumulate according to $\delta \ell \simeq \ell^{1 - \alpha} \ell_P^{\alpha}$ where the source is at distance $\ell$, and $\ell_P$ is the Planck length. We reassess proposals to use astronomical observations of distant quasars to test models of spacetime foam. We show explicitly how wavefront distortions cause the image intensity to decay to the point where distant objects to become undetectable if the path-length fluctuations become comparable to the wavelength of the radiation. We use X-ray observations from the Chandra X-ray Observatory to set the constraint {\bf $\alpha \gtrsim 0.58$, which rules out the random walk model (with $\alpha = 1/2$). Much firmer constraints can be set utilizing detections of quasars at GeV energies with the Fermi observatory, and at TeV energies with ground-based Cherenkov telescopes: $\alpha \gtrsim 0.67$ and $\alpha \gtrsim 0.72$, respectively. These limits seem to rule out $\alpha = 2/3$, the model of some physical interest.

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