I find much confused logic in Nick Bostrom's argument, and huge assumptions:
"I will return to this scenario shortly, but first I shall say a few words about another theoretical possibility: that the extraterrestrials are out there, in abundance but hidden from our view. I think this is unlikely, because if extraterrestrials do exist in any numbers, it’s reasonable to think at least one species would have already expanded throughout the galaxy, or beyond. Yet we have met no one."
Sorry mate, but it's not reasonable to think that at all. The galaxy is colossal. We have exactly zilch insight into how even a two-star civilization might work, and the kind of problems it would have - and he's saying it's reasonable to assume the existence of a civilization with dozens of thousands, or hundreds of thousands, of millions of stars?
It's possible, maybe, but it's not reasonable, and it doesn't disprove the idea that we simply haven't looked deeply enough for ET yet. As Jill Tarter of the SETI Institute once remarked [1], it's like asking if there are any fish in the ocean, filling a cup with ocean water, and upon discovering that it has no fish in it, you conclude that all the world's oceans don't, either.
In short, we haven't done the experiment yet - we may as well refrain from making conclusions. Our galaxy could house a million times the diversity of species in Star Trek, Star Wars, Battlestar Galactica and Mass Effect combined, but it may take a while to find evidence of any of that.
"I will return to this scenario shortly, but first I shall say a few words about another theoretical possibility: that the extraterrestrials are out there, in abundance but hidden from our view. I think this is unlikely, because if extraterrestrials do exist in any numbers, it’s reasonable to think at least one species would have already expanded throughout the galaxy, or beyond. Yet we have met no one."
Sorry mate, but it's not reasonable to think that at all. The galaxy is colossal. We have exactly zilch insight into how even a two-star civilization might work, and the kind of problems it would have - and he's saying it's reasonable to assume the existence of a civilization with dozens of thousands, or hundreds of thousands, of millions of stars?
It's possible, maybe, but it's not reasonable, and it doesn't disprove the idea that we simply haven't looked deeply enough for ET yet. As Jill Tarter of the SETI Institute once remarked [1], it's like asking if there are any fish in the ocean, filling a cup with ocean water, and upon discovering that it has no fish in it, you conclude that all the world's oceans don't, either.
In short, we haven't done the experiment yet - we may as well refrain from making conclusions. Our galaxy could house a million times the diversity of species in Star Trek, Star Wars, Battlestar Galactica and Mass Effect combined, but it may take a while to find evidence of any of that.
[1]: http://kepler.nasa.gov/education/sagan/JillTarterEssay/