Couple of points: -I am not convinced that von Neumann probes are as easy as presented. The assumption "it is done in nature we will do it ourselves". That point was laid aside pretty fast. And if the probe is based on nature, why not assume problems in replication(due to radiation damaging probes, or other effects)? Errors in the code and how to ensure continued existence after errors upon errors? I could go on, but I think there are problems that are cast aside too simply. Heh if you want to have fun imagine a interstellar ecology of probes by colonizers and colonist-hunters by civilizations with opposing views regarding utilization of the universe. Btw: the lecture had values of 99% an 50% of light speed, from what I remember from various discussions on the subject, survivable speed is about 25%-30%, after that potential collisions with particles become too strong to withstand by known and theoretical materials. -We have resources to do lot of things-engineer rivers, build Orion ships, colonize Antarctica, have a Moon Base.We don't do it. With increasing development of our civilization the concepts of colonization and territorial expansion seem to go down in favour of centralization of population and lower population growth with focus on nature conservation. So perhaps ideas of stellar engineering, Dyson spheres, are reminder of different time and different attitudes? -Nature conservation could be a good and sensible answer for Great Silence. We try more and more to conserve nature rather than colonize it. Perhaps civilizations advanced enough colonize in very sustainable way and don't want to disturb other civilizations(if they encounter them), since their own unique development would seem more valuable to observing civilization(and let's face it any technological entity advanced even 1000 years ahead of us, would probably destroy our development). If lowered population growth and conservation of energy go hand in hand than you can have slow expanding civilizations that probably won't cover the whole galaxy in their territorial influence. I notice that some students mentioned this probability at the end :) -As to the Paradox and lack of signals towards us-we could but we aren't sending signs of our presence all around space(directed radio signals, laser beams). Why assume others would? But perhaps it is simpler for advanced civilizations like ours to reach a level where will be able to actually detect life bearing planets and sings of civilization before being producing enough energy to constantly send out signals into space.

one word: Reapers! All kidding aside, I always loved the Reaper concept from Bioware's Mass Effect; it does a good job at exploring a cause of Fermi and ETI massive and hostile civilisation

Has there been enough time for a universe colonisation civilization to have claimed everything? How many years does it take for the universe to be "fenced". Then again the meteors that have wiped out earlier life (Ordovician, Devonian, Permian, Triassic & Cretaceous) could be the aliens pre-empting us. Using earth as an example its not possible politically. our civilisation hasnt "matured" enough to accept the project "Dyson sun" . Sort of like Romans and their non utilized steam engines. Another idea that sparked was that our galaxy system itself could be a von Neumann probe with a designated target.

Arenamontanus wrote:

Well, a lot of this will be expanded on in the full paper. Basically, there are fairly plausible arguments for why self-replicating machines are doable with near-term technology (see Merkle and Freitas book on the topic), and we have practical demonstrations of arbitrarily high fidelity copying in our own technology (sequences of checksums/hashes, for example).

Exactly. The same nanotechnology that can assemble a structure with molecular perfection could also analyze a structure for imperfections. Simple verification code placed in blueprints would help ensure perfect production fidelity, especially if you're dealing with open-air nanoswarm construction such as with habitats. There, the chances of flaws cropping up due to dust, exposure and other effects makes error-correction mandatory.

Arenamontanus wrote:

Maybe. But would you argue that *all* alien civilizations at *all* times and for *all* subgroups would want to stay put? (or send out nature conservation probes) Our argument makes the strong convergence assumption even tougher to swallow too. Basically, our argument boils down to 1) SETI people have been thinking too small, 2) physics seem to allow spamming the universe with relatively small resources, 3) this means the great silence is at least a million times more deafening than previously thought. 4) the real explanation for that will hence be (in some sense) a million times more extreme than previously thought. *Personally*, since I think life is not too hard and intelligence not too unusual, that civilizations do not converge strongly yet are not rapidly killed off by self-made xrisks (since they are so different), and that our paper is realistic... I end up thinking that the aliens might already be here via Bracewell probes. Most likely in the form of a few extremely hard to spot structures out in the Kuiper belt that are enforcing their claim to the solar system. Whether that means running a full interdict, just preventing attempts at spamming the universe, or some form of welcome to whatever game-theoretic alliance that makes sense once humans make proper contact, I don't know.

I've always worked under the assumption that while intelligence and life may be common, technology (or at least our specific technological path) may not be. We as a species followed a fairly unique process of evolution. We were tool using social animals when we were primitive, and we became smarter from there. All of our tech was an extension of that. What if other intelligent life does not evolve in this manner? What if a solitary non-tooled predator evolves to sapience? What if a herd animal did? Is that creature automatically destined to produce technology? Is it automatically destined to reach for the stars? Or could intelligence, like life as a whole, be so divergent that the multitude of possibilities for an intelligent species could run the gamut, and the rarity here isn't our intelligence, or our life, but our specific combination and the path we have followed up to this point? Perhaps we should be less worried that there's no other intelligent life out there, and more worried that the intelligent life out there isn't going to be the rubber-headed aliens we want them to be. Okay, maybe that was harsh, but to some degree it's true. Our society has a premade assumption that life brings civilization, civilization brings technology, and technology makes space communication that could be detected. I don't think this is necessarily the case. Perhaps they skipped the invention of basic civilization and went to tech. Maybe they never produced tech but have a flourishing civilization. Whatever the case, we cannot say for sure if intelligence and advanced technology are inexorably tied, since we are the only example of both that we have to observe. If we look out into the stars expecting to see an analog, we might as well be staring out into space looking for mirrors.

Decivre wrote:

If we look out into the stars expecting to see an analog, we might as well be staring out into space looking for mirrors.

Stanisław Lem had a couple of good books on the subject. "His Master's Voice" for example. I also recommend "Blindsight" by Peter Watts.