Stringy Space-Time Foam and High-Energy Cosmic Photons

TL;DR

This review explores how string-inspired space-time foam models predict energy-dependent photon delays from cosmic sources, potentially observable as Lorentz violation effects, while evading many astrophysical constraints.

Contribution

It highlights a stringy space-time foam model that predicts specific photon time delays and avoids common constraints on Lorentz violation in quantum gravity theories.

Findings

Photon time delays fit stringy foam predictions

Model evades astrophysical constraints on Lorentz violation

Foam is transparent to charged matter and non-birefringent

Abstract

In this review, I discuss briefly stringent tests of Lorentz-violating quantum space-time foam models inspired from String/Brane theories, provided by studies of high energy Photons from intense celestial sources, such as Active Galactic Nuclei or Gamma Ray Bursts. The theoretical models predict modifications to the radiation dispersion relations, which are quadratically suppressed by the string mass scale, and time delays in the arrival times of photons (assumed to be emitted more or less simultaneously from the source), which are proportional to the photon energy, so that the more energetic photons arrive later. Although the astrophysics at the source of these energetic photons is still not understood, and such non simultaneous arrivals, that have been observed recently, might well be due to non simultaneous emission as a result of conventional physics effects, nevertheless, rather surprisingly, the observed time delays can also fit excellently the stringy space-time foam scenarios, provided the space-time defect foam is inhomogeneous. The key features of the model, that allow it to evade a plethora of astrophysical constraints on Lorentz violation, in sharp contrast to other field-theoretic Lorentz-violating models of quantum gravity, are: (i) transparency of the foam to electrons and in general charged matter, (ii) absence of birefringence effects and (iii) a breakdown of the local effective lagrangian formalism.