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Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: My current thoughts on the risks from SETI, published by Matthew Barnett on March 15, 2022 on The Effective Altruism Forum. SETI stands for the search for extraterrestrial intelligence. A few projects, such as Breakthrough Listen, have secured substantial funding to observe the sky and crawl through the data to look for extraterrestrial signals. A few effective altruists have proposed that passive SETI may pose an existential risk to humanity (for some examples, see here). The primary theory is that alien civilizations could continuously broadcast a highly optimized message intended to hijack or destroy any other civilizations unlucky enough to tune in. Many alien strategies can be imagined, such as sending the code for an AI that takes over the civilization that runs it, or sending the instructions on how to build an extremely powerful device that causes total destruction. Note that this theory is different from the idea that active SETI is harmful, ie. messaging aliens on purpose. I think active SETI is substantially less likely to be harmful, and yet it has received far more attention in the literature. Here, I collect my current thoughts about the topic, including arguments for and against the plausibility of the idea, and potential strategies to mitigate existential risk in light of the argument. In the spirit of writing fast, but maintaining epistemic rigor, I do not come to any conclusions in this post. Rather, I simply summarize what I see as the state-of-the-debate up to this point, in the expectation that people can build on the idea more productively in the future, or point out flaws in my current assumptions or inferences. Some starting assumptions Last year, Robin Hanson et al. published their paper If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare. I consider their paper to provide the best available model to-date on the topic of extraterrestrial intelligence and the Fermi Paradox (along with a very similar series of papers written by S. Jay Olson previously). You can find a summary of the model from Robin Hanson here, and a video-summary here. The primary result of their model is that we can explain the relatively early appearance of human civilization—compared to the total lifetime of the universe—by positing the existence of so-called grabby aliens that expand at a large fraction of the speed of light from their origin. Since grabby aliens quickly colonize the universe after evolving, their existence sets a deadline for the evolution of other civilizations. Our relative earliness may therefore be an observation-selection effect arising from the fact that civilizations like ours can't evolve after grabby aliens have already colonized the universe. Assuming this explanation of human earliness is correct, and we are not an atypical civilization among all the civilizations that will ever exist, we should expect much of the universe to already be colonized by grabby aliens by now. In fact, as indicated by figure 13 in the paper, such alien volumes should appear to us to be larger than the full moon. Given the fact that we do not currently see any grabby aliens in our sky, Robin Hanson concludes that they must expand quickly—at more than half the speed of light. He reaches this conclusion by applying a similar selection-effect argument as before: if grabby alien civilizations expanded slowly, then we would be more likely to see them in the night sky, but we do not see them. However, the assumption that grabby aliens, if they existed, would be readily visible to observers, is arguable, as Hanson et al. acknowledge, This analysis, like most in our paper, assumes we would have by now noticed differences between volumes controlled or not by GCs. Another possibility, however, is that GCs make their volumes look only subtly different, a differen...