Constraints on Lorentz Invariance Violation with Fermi-LAT Observations
  of Gamma-Ray Bursts
  Some Quantum Gravity (QG) theories allow for a violation of Lorentz
invariance (LIV), manifesting as a dependence of the velocity of light in
vacuum on its energy. If such a dependence exists, then photons of different
energies emitted together by a distant source will arrive at the Earth at
different times. High-energy (GeV) transient emissions from distant
astrophysical sources such as Gamma-ray Bursts (GRBs) and Active Galaxy Nuclei
can be used to search for and constrain LIV. The Fermi collaboration has
previously analyzed two GRBs in order to put constraints on the dispersion
parameter in vacuum, and on the energy scale at which QG effects causing LIV
may arise. We used three different methods on four bright GRBs observed by the
Fermi-LAT to get more stringent and robust constraints. No delays have been
detected and strong limits on the QG energy scale are derived: for linear
dispersion we set tight constraints placing the QG energy scale above the
Planck mass; a quadratic leading LIV effect is also constrained.
