Linnaeus Physics Colloquium: Lorentz invariance violation with very high energy gamma rays

Image: NASA/JPL-Caltech

Title: Understanding the fine structure of the Universe by testing Lorentz invariance violation with very high energy gamma rays
Lecturer: Michael Daniel, Center for Astrophysics, Harvard & Smithsonian

Abstract

The two major cornerstones of Modern Physics — Quantum Theory on the small scales of particle physics and General Relativity on the large scale of gravity — happen to be mutually incompatible. This is a problem when trying to unite the four fundamental forces in a grand unified theory of everything. To produce a viable quantum theory of gravity therefore requires significant examination of the underlying principles. One of the founding principles of Modern Physics is the assumption of Lorentz invariance in Special Relativity, postulating that the laws of physics are the same for all observers no matter there frame of reference. But we know that other symmetries can be broken on local scales, such as in CP violation, and the same may be true for Lorentz invariance on sufficiently small scales that quantum gravity effects become dominant.

The energy scale at which these would manifest is far beyond anything the terrestrial laboratory can probe and so indirect methods must be investigated instead. Certain unified theories of quantum gravity predict Lorentz invariance violation (LIV) introducing an energy dependent dispersion to photon propagation, effectively an energy dependent variation of the speed of light. These effects would be small, but over a sufficiently large distance would add up to an observable signature of new physics at work. Therefore looking for energy dependent delays in highest energy photons travelling astronomical distances from pulsars, active galactic nuclei and gamma-ray bursts is a good place to start. LIV could also manifest as subtle changes in particle interactions, such as in pair production where two photons interact to become an electron-positron pair. This would effectively change the optical depth of the universe to gamma-ray propagation, changing the effective horizon and enabling us to see sources at greater distances than we would predict.

Here we shall explore a few of the ways we can use very high energy gamma-ray observations as a means to phenomenologically test for LIV and ultimately attempt to better understand, or at least constrain, our understanding of the fine structure of the universe.

About Michael Daniel

Michael Daniel received his PhD in particle astrophysics from the University of Durham in 2002. He has gone on to work on the prototype Very Energetic Radiation Imaging Telescope Array System (VERITAS) and then the High Energy Stereoscopic System (H.E.S.S.) ground based gamma-ray telescopes in a series of postdoctoral positions in the U.S.A., Ireland, and the U.K. until in 2016 he returned to VERITAS as its operations manager. Currently, in addition to managing the day-to-day operations of the telescopes, he co-chairs the VERITAS Astroparticle, Transient, Optical, and Multi-Messenger (ATOMM) Science Working Group in its mission to explore the universe ranging from the exotic physics of testing Lorentz invariance violation, to more traditional astronomical studies at the highest time and angular resolutions.