The Great Filter – Are We Almost Past It? (1998)

1) The more important observation is that everything we see is a wilderness of apparently natural origins and processes, not that we have apparently not been visited.

2) No hypothesis involving choice that leads to failure to carry out self-replicating colonization can explain (1). All it takes is one small group in one sufficiently advanced species. This rules out wolves, berserkers, relinquishment, self-destruction, and so on and so forth.

Clearly something is missing or incorrect in how the pieces of the various sciences fit together to explain what we see. Some of the best reading on the subject is speculation on what it is that we don't know or the relative odds on where the present consensus is incorrect.

"No alien civilizations have substantially colonized our solar system or systems nearby. Thus among the billion trillion stars in our past universe, none has reached the level of technology and growth that we may soon reach."

While the article is in a way all about exploring that disconnect, it's a bit strange to find it written down so explicitly in the conclusion. We can ascribe a very high level of confidence to hypothesis that neither our own nor any of the directly neighboring star systems have been colonized. But to postulate that therefore the universe has not yet produced a civilization as technologically sophisticated as our own is still a non-sequitur, at least in my opinion. The article also makes a lot of assumptions (it has to!), some plausible, some seemingly arbitrary, which might in the end not be important factors at all. For example, any consideration about radio communication must be on very shaky grounds, in particular when charting the absence of it.

The reason why we're not seeing alien neighbors or predecessors (which would be "neighbors" in time), might well be a combination of factors. While we can exclude certain scenarios, such as a cloud of Von Neuman Probes eating the galaxy, it still leaves room for a lot of little factors that can change the galactic stage for us in a fundamental way - especially if they work in conjunction.

For example, the average duration of the technologically recognizable phase a civilization goes through may be smaller, and there may well be less of them than expected. In addition, willful avoidance or isolation may well play a role; the zoo hypothesis is an extreme example of this, but in a sparse cosmos the measures you have to undertake in order to isolate an emerging civilizations become less heroic.

It's also worth mentioning that we ourselves are not past the Filter by any means. In fact, as time progresses, it seems to become less likely we'll ever move off the Earth as an exclusive habitat. Part of this is due to psychology, we could have made it happen by now but didn't. The best bet at becoming an interstellar civilization right now seems to be to wait until we're pure computer systems, unburdened by aging and the passage of time, but that development, too, might end up being something we never elect to actually pursue.

Luckily, this whole thing is a mystery we can solve over time, just by gathering data about what's out there.

Consider the energy and material engineering necessary to achieve long-distance space transport of any sizable amount of people, equipment, and supplies. Regardless of whether the transport is subluminal or FTL, the energy requirements are tremendous and the engineering requirements for a durable vessel astounding.

My guess is that by the time our materials science and our energy production are up to the task, our interests may well have changed - because (and here I speculate wildly) science at engineering at that level will reveal so much about the fundamental nature of the universe and give us so much interesting practical technology that biochemical life clinging desperately to lucky rocks in the lucky zone won't be near so interesting anymore.

No one comes near because we have nothing to offer those able to come near and get home again to report what they found.

As to simply detecting signals, we have to consider how lucky we are to have so much ore so close to the surface: Metallurgy and eventually radiated communications were a natural for us.

Other civilizations may well start with fantastically engineered wood and eventually plastics, and go all-fibre, all line-of-sight, and have very, very different communications systems, systems undetectable at any remove.

Barring several fairly large changes to our energy and resource systems, our civilization is mathematically and physically guaranteed to collapse within the next 200 years at most.

That's not a hypothetical filter. It's a real one that we can see, and we are not past it.