Photons, by the measure of specific impulse, are the ultimate rocket exhaust because they are emitted at the fastest velocity possible. Therefore, we should expect to astronomically see such exhaust created by advanced spacefaring alien civilizations who have built these ultimate slower-than-light rockets: giant lasers pointing backwards. What would their light look like from earth?
One possibility is short duration gamma ray bursts: due to proper motion and great distance, the exhaust trail only intersects an observer on earth for a brief moment. It is mind-boggling to imagine someone has built a very narrow-beam gamma ray laser. But this is probably false, all beams must spread, especially over intergalactic distances.
If we don't see such exhaust signatures, then it could be proof that photon rockets are impossible to build, or that there are no spacefaring civilizations in the universe.
If photons are too unwieldy, then the next best thing would be to accelerate small particles nearly to the speed of light. Are cosmic rays alien rocket exhaust?
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