&gt; <i>We really don&#x27;t know if there&#x27;s some branch of physics that allows FTL travel</i><p>I agree, and I didn&#x27;t mean to come down on either side of the opinion spectrum on FTL. Personally, I consider it a strong possibility that several methods of FTL travel might exist in practice.<p>&gt; <i>I think that assumption relies on the assumption that ETs have similar lifespans as us, which is probably a faulty assumption.</i><p>No, you misunderstood me there. My basic assumption is that ET lifespans (if they are individual beings at all) are for all intents and purposes infinite, as you would expect from any technologically advanced civilization. Instead, my conclusion is mostly shaped by the young age of our own civilization, the meagerness of RF signals emanating from our system, and the vastness of the distances involved.<p>&gt; <i>There&#x27;s hundreds of stars within a few dozen lightyears or so, so an advanced civilization should be able to build a powerful robotic ship to tour them all and gather data.</i><p>I agree that a swarm of robotic probes meets the criteria for blanket early detection of civilizations. I assert that you would need to cover more than a few dozen light years though, but that&#x27;s not really relevant. It comes down to the willingness of deploying these probes and the speed at which probe data can be relayed back to someone willing to act on it.<p>&gt; <i>Honestly, it shouldn&#x27;t be that hard to detect other civilizations from a reasonable distance; we&#x27;re already detecting various exoplanets</i><p>Planets: yes. Civilizations: it depends. I&#x27;m personally doubtful we could detect our own civilization today if we had a clone of humanity orbiting Alpha Centauri, and that&#x27;s still well within our RF light cone.