# Quantum Gravity

**VHE Gamma Rays as Probes of Space-Time**

Due to their extremely short wavelengths and long propagation distances, very high energy gamma-rays from distant objects such as active galaxies (AGN) and gamma ray bursts (GRBs) are sensitive to the microscopic structure of space-time. Small-scale perturbations of the smooth space-time continuum should manifest themselves in a tiny energy dependence of the speed of light. Such a violation of the Lorentz invariance, on which the theory of special relativity is based, is present in some quantum gravity (QG) models. Burst-like events of gamma-ray production, e.g. in active galaxies, allow this energy-dependent dispersion of gamma-rays to be probed and can be used to place limits on certain classes of quantum gravity scenarios, or may possibly lead to the discovery of effects associated with Planck-scale physics.

CTA could detect characteristic time-scales and QG effects in AGN light curves (if indeed any exist) on a routine basis without the requirement for exceptional source flux states and in short observation times. CTA could resolve time scales as small as few seconds in AGN light curves and QG effects down to 10 s.

Very good sensitivity at energies >1 TeV is especially important for probing QG effects at higher orders. Fermi recently presented results based on observations of a GRB which may rule out linear-in-energy variations of the speed of light up to 1.2× the Planck scale. To test quadratic or higher order dependencies, the sensitivity provided by CTA will be needed.

**Further Reading**

Hossenfelder, Experimental Search for Quantum Gravity, available via http://arxiv.org/abs/1010.3420

Stecker, Gamma-ray and Cosmic-ray Tests of Lorentz Invariance Violation and Quantum Gravity Models and Their Implications, available via http://arxiv.org/abs/0912.0500

Bolmont et al. (H.E.S.S. Collaboration), Search for Lorentz Invariance Violation effects with PKS 2155-304 flaring period in 2006 by H.E.S.S, available via http://arxiv.org/abs/0904.3184

The effects of quantum gravity: in this
illustration, the purple photon carries a million times more energy than
the yellow photon. Some theories of quantum gravity predict time-delays
for higher energy photons, which interact more strongly with 'foamy'
space-time. (Image credit: NASA/Sonoma State University/Aurore
Simonnet) |