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Despite the naive storylines about interstellar travel in science fiction, biological creatures were not selected by Darwinian evolution to survive travel between stars. Such a trip would necessarily span many generations, since even at the speed of light, it would take tens of thousands of years to travel between stars in our galaxy’s disk and 10 times longer across its halo. If we ever encounter traces of aliens, therefore, it will likely be in the form of technology, not biology. Technological debris could have accumulated in interstellar space over the past billions of years, just as plastic bottles have accumulated on the surface of the ocean. The chance of detecting alien technological relics can be simply calculated from their number per unit volume near us rather than from the Drake equation, which applies strictly to communication signals from living civilizations.

On a recent podcast about my book Extraterrestrial, I was asked whether extraterrestrial intelligence should be expected to follow the rational underpinning of morality, as neatly formulated by the German philosopher Immanuel Kant. This would be of concern to us during an encounter. Based on human history, I expressed doubt that morality would garner a global commitment from all intelligent beings in the Milky Way.

Instead, a code of conduct that allows systems of alien technology to dominate the galaxy would also make them more likely to be the way we would first encounter extraterrestrials. Practically, this rule will act as a sort of Darwinian evolution by natural selection, favoring systems that can persevere over long times and distances; and multiply quickly and spread at the highest speed with self-repair mechanisms that mitigate damage along their journey. Such systems could have reached the habitable zones around all stars within the Milky Way, including our sun, by now. Most stars formed billions of years before ours did, and technological equipment sent from habitable planets near them could have predated us by enough time to dominate the galaxy before we came to exist as a technological species.

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