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this post was submitted on 30 Dec 2023
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Today I Learned
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We have only a single advanced civilization to use as a comparison point for the strength or our telescopes, and that's ourselves. From my understanding of it, the most powerful broadcast we've made out is 15-20MW for an over the horizon radar system, and that only ran for 40 years or so. I don't have an exact answer, but my understanding is that even for our largest radio telescope, 20 megawatts at a distance of 100 lightyears would be below the noise floor.
Nuclear tests are slightly more visible than that, but only occur periodically, so you'd have to have a telescope facing the right way by coincidence. Basically, if there's an Earth-like civilization 200 lightyears away, I think we would be entirely blind to it, and that's over a tiny distance in the scheme of things.
The farthest known exoplanet is 27,710 lightyears away, and was discovered by the transit method - but this was made possible because the planet is very big (bigger and heavier than Jupiter), and orbits quite close to its star (with a 43 hour orbit). To be detectable at that range, a signal has to be stronger than some stars are bright.