Title: Limits from CGRO/EGRET Data on the Use of Antimatter as a Power Source by Extraterrestrial Civilizations

Abstract: I argue that the existence of cold antimatter in bulk is not permitted by the Standard Model, so that if a gamma-ray signature from antiproton annihilation were to be detected, it must represent either new physics or the action of intelligence. Time variability of the signal would strongly support the second alternative. The entire sky was scanned at the relevant energies (30- 928 MeV) by the EGRET experiment on board the Compton Gamma Ray Observatory during 1991-1995. A search of this database for the antiproton annihilation signature yielded only upper limits on the flux (an intriguing spectrum detected from QSO 2206+650 = 3EG J2206+6602 is probably not related to SETI). The all-sky, longterm 99 upper limit is 2.3 x 10^{-8} photon/(cm2 s); it is a factor 10 worse in the Galactic plane due to the higher diffuse gamma-ray background emission. I give brief, but quantitative, illustrations of what this limit means for nearby intelligent activities.