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FiftyTwo comments on Open Thread, September 1-15, 2012 - Less Wrong
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Another great filter related question I posted a while ago but didn't get much response to:
Could the great filter just be a case of anthropic bias?
So the universe could be teeming with intelligent life, and theres no good reason there can't be any near us, but if there were we would not have existed. Hence we don't see any.
This is an interesting idea but I think it doesn't work. Say for example that another species starts 200 million light years away and is spreading a colonization wave at .5 which is a pretty extreme value. Then one should have at least 400 million years to notice that. And it is going to be pretty hard to do a fast colonization wave without some astronomically detectable signs. Reducing the colonization speed makes it less likely to be detected but increases the time span.
It seems no less plausible that what spreads outward at a sizable fraction of lightspeed is a wave of "terraforming" agents, altering all planets in the neighborhood into more suitable colony planets. Meanwhile colonization spreads at a rate roughly bounded by the ratio of reproduction rate to death rate, which might well be significantly slower than that.
That scenario would be enough to ensure that if an intelligent species evolves, it is necessarily far from any spreading interstellar empire (since otherwise the terraforming agents would have destroyed it), without having to posit such a fast colonization wave.
That said, though, why assume that a colonization wave is astronomically detectable? Being detectable at this range with our instruments is surely an indication of wasting rather an enormous amount of energy that could instead be put to use by a sufficiently advanced technology, no?
Waste heat is one thing there's not much one can do about. Even a Dyson sphere will have it. In the case of Dyson spheres there have been active attempts to find them. See here although some other work suggests that Dyson spheres are just not that likely(pdf). Most largescale engineering projects will leave a recognizable signature. In this example, systematic searches have only been done out a few hundred light years, but stellar engineering is in the more blunt forms noticeable even at an intergalactic level.
Moreover, many ship designs lead to detectable results. For example, large fusion torch drives have a known sort of signature that we've looked for and haven't found.