Here is one possible (plausible) answer to Fermi, I think: Some various assumptions: Space travel is limited by the speed of light and the faster you are goin

Here is one possible (plausible) answer to Fermi, I think: Some various assumptions: Space travel is limited by the speed of light and the faster you are going, the more energy it takes to go a little bit faster. Relatively is more or less correct and absolute that way. No Star Trek "hyperdrive" is possible. Nope, you can't extract energy from a quantum vaccuum or any other such tricks. Nope, you can't actually scoop up all that much fuel in inter-stellar space. Most viable solar systems don't have all that much practically accessible fuel for inter-stellar voyages. Transmutation of matter into arbitrary elements is possible, but not practical at scale. You can't, therefore, have (material) trade routes between any but perhaps the closest stars. Even then, the total mass of material you are trading in a unit of time is doomed to be disappointingly low - there is hardly any point to bothering. With assumptions along those lines, "colonization" per se would not exactly be possible -- if by "colonization" we mean building any kind of empire. At the very best, a species could accomplish a kind of "panspermia" -- sending out "seeds" to whatever stars are in reach, but once a seed is launched, its subsequent evolution is pretty much on its own. In a few cases, if one lucky civilization is close by a second, resource-rich solar system, maybe you get get a little bit of mining going. But such opportunities would be cosmologically rare and even more rarely economically viable. Worse, sending out seeds to a near-ish solar system and letting them do what they will would be dangerous to the life back in the solar system of origin. Right now, it doesn't appear that their is a hostile, alien intelligence anywhere in the galactic neighborhood. If we did a good job of launching "seeds", we'd increase the odds considerably. In short, there will never be a point at which it makes any kind of economic or existential sense for us to begin a colonization project - and the situation doesn't look like it would be all that different for other imaginable forms of life. So why not signal? Well, we already do a bit. Perhaps in a few years we'll have found some not too far off (but far enough!) solar systems that look like they might host life -- and we'll point some lasers at them and start sending "pings" to see if anyone is home. If other solar systems are already doing that, it would be very hard to detect from here on Earth. In other words, one answer to Fermi, but not quite the "Great Filter" that Bostrom posits, is simply that galactic colonization is a practical impossibility and, to the extent there is intelligent signaling that spans solar systems, it's a lot more directional and harder to detect than anything you could easily find with a few radio telescopes.