Quantum gravity

Quantum gravity can manifest itself as an effective energy-dependence to the velocity of light in vacuum caused by propagation through a gravitational medium containing quantum fluctuations on distance scales near the Planck length ( $\simeq 10^{-33}$cm) with time-scales on the order of the Planck mass ( $\simeq 10^{19}$GeV) (e.g., [Amelino-Camelia et al. 1998]). Recent work within the context of string theory indicates that quantum gravity may manifest itself at a much lower energy scale than the Planck mass, perhaps as low as 10¹⁶GeV ([Witten 1996]). TeV observations of variable emission from astrophysical objects provide a means of searching for the effects of quantum gravity. For example, the Whipple Collaboration has recently used data from a rapid flare of the AGN Mrk 421 to constrain the energy at which quantum gravity couples to electromagnetic radiation to be $>4 \times 10^{16}$GeV for some models ([Biller et al. 1999]). VERITAS will significantly improve short time-scale variability measurements and also will detect more distant objects. If variability on a time-scale of 1 minute is seen above 5TeV from a source at z=0.1, VERITAS would be sensitive to quantum gravity effects within a factor of 5 of the Planck mass. In addition to AGN flares, measurements of pulsed emission from Galactic sources and of gamma-ray bursts may provide avenues for investigating the effects of quantum gravity.