So a sufficiently advanced civilization could construct a self-replicating probe that heads to each solar system, replicates and recharges itself, then heads on to the next stars until the whole galaxy is populated.
From the perspective of the civilization sending the probes, this is relatively cheap; just the price of the materials necessary to build a single ship capable of carrying itself outside the solar system, even if it takes hundreds of years to get anywhere.
Additionally, if all you are talking about building is a device that would, encountering our solar system, take up residence in orbit, take pictures, and report "back" - how do you know there aren't already several of them present? It would be rather hard to tell!
Colonization on Earth always had economic incentive. Populations migrated for economic opportunities, even if the selection of who did and did not migrate was heavily political. Colonization, as opposed to mere migration, was pretty much always, more or less by definition, for resource extraction. The colonized pay taxes and/or their raw materials and unique finished goods are exported back to the empire, and/or their local economy is converted to the coin of the empire's realm and taxes are extracted. It doesn't seem like any such program of empire building could work at inter-stellar scales unless there is some surprising physics that we've yet to discover that allows super-luminal communication and transport.
Fermi is "wrong" if intelligent, space-faring life only "works out" given the rules of physics at roughly the scale of our higher organisms and if, for organisms at that scale, colonization is not a viable option. There's no need for any "Great[er] Wall" than that.
Why do you assume that? There are plenty of self-replicating machines on earth that have been extremely successful at colonizing their environment, but we would not typically view them as "rival life forms".
> What functionality would you build into such a probe and by what means would you prevent a "boomerang" effect wherein it comes back and causes problems in the home system?
The simplest method would be to have them accept any order signed by a standard public key. I'm not sure why people tend to assume that a self-replicating machine cannot be controlled. There are plenty of self-replicating programs that are controlled very effectively.
> Additionally, if all you are talking about building is a device that would, encountering our solar system, take up residence in orbit, take pictures, and report "back" - how do you know there aren't already several of them present?
Because if have been many intelligent species in our galaxy over the past billion years, it seems unlikely that all would be careful to remain undetected. Certainly our species has little interest in hiding its activities.
> Colonization, as opposed to mere migration, was pretty much always, more or less by definition, for resource extraction.
Plenty of species on Earth colonize new environments without bothering to send resources back to their origins. And there have been plenty of human colonizations not linked to a controlling empire, especially when we were just hunter-gathers.
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.