A cornerstone of Einsteinĝ€™s special relativity is Lorentz invarianceĝ€"the postulate that all observers measure exactly the same speed of light in vacuum, independent of photon-energy. While special relativity assumes that there is no fundamental length-scale associated with such invariance, there is a fundamental scale (the Planck scale, l(Planck)1.62×10-33cm or E(Planck) ≤ M(Planck)c 21.22×1019GeV), at which quantum effects are expected to strongly affect the nature of spaceĝ€"time. There is great interest in the (not yet validated) idea that Lorentz invariance might break near the Planck scale. A key test of such violation of Lorentz invariance is a possible variation of photon speed with energy. Even a tiny variation in photon speed, when accumulated over cosmological light-travel times, may be revealed by observing sharp features in-ray burst (GRB) light-curves. Here we report the detection of emission up to 31GeV from the distant and short GRB090510. We find no evidence for the violation of Lorentz invariance, and place a lower limit of 1.2E(Planck) on the scale of a linear energy dependence (or an inverse wavelength dependence), subject to reasonable assumptions about the emission (equivalently we have an upper limit of l(Planck)/1.2 on the length scale of the effect). Our results disfavour quantum-gravity theories in which the quantum nature of spaceĝ€"time on a very small scale linearly alters the speed of light.
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Acknowledgements The Fermi LAT Collaboration acknowledges support from a number of agencies and institutes for both the development and the operation of the LAT as well as scientific data analysis. These include NASA and DOE in the United States, CEA/Irfu and IN2P3/CNRS in France, ASI and INFN in Italy, MEXT, KEK, and JAXA in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council and the National Space Board in Sweden. Additional support from INAF in Italy for science analysis during the operations phase is also acknowledged. J. Granot gratefully acknowledges a Royal Society Wolfson Research Merit Award. The Fermi GBM Collaboration acknowledges the support of NASA in the United States and DRL in Germany. J. Conrad is a Royal Swedish Academy of Sciences Research Fellow, funded by a grant from the K. A. Wallenberg Foundation. E.T. is a NASA Postdoctoral Program Fellow and a Canon Foundation in Europe Fellow. A. J.v.d.H. is a NASA Postdoctoral Program Fellow. We thank J. Ellis for comments.