It would also be a good candidate for the Great Filter - the hidden factor that has stopped the development of all the other intelligent civilisations that would otherwise be out there. That would mean this is good news - we'd rather the Great Filter lies in our past (because that means we've already survived it) than in our future.
I don't think it's a good candidate; certainly it pushes some amount of the Filter into our past, but even if only 1 in 700 solar systems is Sol-like, there are so many we'd still expect to see tons of aliens. For a single-step Filter or for solar systems to even be a majority of the filter, you need a step so unlikely that it can push the presence of interstellar life down from, like, trillions of star systems to ~1.
Thanks for the reply, and yes, that makes sense. What would you predict as the most likely other steps in the filter?
Looking at our own planet, I would guess that it's a combination of a) it being simply infeasible to send large animals to distant stars (we haven't yet colonised the moon or even made a serious attempt to colonise low earth orbit, and we might not do so before we run out of cheap energy) and b) technological singularities.
> What would you predict as the most likely other steps in the filter?
I don't know. Robin Hanson blogs a lot about it, but none of the steps seem terribly plausible although my favorite currently is big brains being feasible - in a number of ways, heads seem to be rare, big brains even rarer, and the costs of big brain almost too expensive to bear because neurons seem to be unable to get more efficient; one paper I liked on the topic was http://www.pnas.org/content/early/2012/06/19/1201895109.full... but you can find a lot of relevant material in http://www.gwern.net/Drug%20heuristics
LEO is a weird place to colonize. There aren't any resources there. The moon is also pretty tough, being no atmosphere and Carbon in ppb quantities. Mars is a good candidate, though.
I imagine that if there were another planet in the solar system capable of supporting native life with just 18th-century technology, we would already have sent people there, even if it were a multi-year trip. Which would provide big economic drivers for interplanetary transport.
Another apparently obvious filter is the development of advanced nanotechnology and AI. However...
Although I'd immediately agree that those are filters hindering the spread of any humanlike civilisation, it doesn't at all stop the spread of other, technological forms - the "vile offspring", to borrow a term.
Honestly, I have no idea what might work as a future filter, at this point. I don't think the expense could be it; that's going to drop, quite abruptly once uploading (or straight-up AI) works well enough.
There's not much suggesting that there is only one Great Filter. After all, we haven't met anyone else yet, so the likelihood of another filter being in our future is just as real as it is if there was one in our past.
Call it the "two neptune filter", to borrow from the article. :-)
