[Mycroft Holmes]

I suspect that due to your 4: life is on average as aggressive a humans that the average lifespan of a technological civilization is less than a thousand years. YMMV.

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[conejo99]

We’ve been looking for radio signals for some time. We haven’t found any we believe to be from an intelligent civilization. I’d think someone might be able to estimate the minimum distance a civilization could be expected to be from Earth and still not be found.
Additionally, if other technical civilizations exist then there must be an average lifespan (or radio transmition spans) for those civilization. Any technical civilizations would probably start out using the types of radio signals we used and presumably are looking for. I wonder if over time the valuable and scarce radio spectrum might be used in a way that would look different to an outside observer. Perhaps a more efficient use of the spectrum might look like noise to a observer that doesn’t have decryption codes.