Both string theorists and loop quantum gravity theories have proposed that space-time may be something other than the perfectly smooth, perfectly local space of classical general relativity, which can potentially lead to path dependent phase alteration of light from a common source due to the cumulative effect of a discrete or non-uniform space-time over the course of the journey that affects photons taking different paths differently.(adsbygoogle = window.adsbygoogle || []).push({});

These effects should be easiest to see in high frequency photons that have traveled long distances, such as light from distant quasars.

A new experimental search but boundaries on a parameter alpha (with potential values from 0 to 1) which is a function of how much space-time differs from classical general relativity which has a value of 1. The lower bound for alpha from experiment is now about 0.76 for the highest frequency gamma rays, and 0.46 in measurements from infrared light.

Two proposals that have an alpha that is independent of wavelength, a "random walk" hypothesis that would imply alpha = 0.5, and a "holographic" model which would imply alpha = 2/3 are ruled out by the results, but the parameter space still leaves plenty of room for space-time with a character different from quantum gravity so long as the alpha predicted is in the range from 0.77-.99 or so.

A discussion of the experiment is found at: http://www.science20.com/quantum_gr...antum_spacetime_foam_from_quasar_light-155928

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# Experimental Limits on Spin-Foam Effects From Quasars

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