Hal Finney Wrote:
>However I think the anthropic prediction in such cases is >that our history would have been just barely good enough to >allow life like us to form.  If meteor bombardment should >have wiped us out, we would predict that we would have >experienced a history of heavy meteor strikes, not quite >enough to wipe us out, but enough to be very troublesome.
.......
>.  Robin Hanson has a
>couple of papers on this, http://hanson.gmu.edu/greatfilter.html
>and a more technical one at http://hanson.gmu.edu/hardstep.pdf.
 
..Thanks, they were interesting. I don't think 'not getting suddenly wiped out' can be described in the same terms as the 'difficult steps' he analyses, but we have been hit by a lot of not-completely-deadly astronomical bodies. The effect of these could be very crudely fitted in to his analysis if we suggest that each asteroid impact wastes a certain amount of time, by wiping out promising species, etc. Then if we use as a random variable the total time wasted, and if the expected time wasted is much bigger than the time for one 'step' (about 1/2 a billion years), and the probability distribution of the time lost is sufficiently flat for small times, then we might expect to have 'wasted' one step-size recovering from meteors..
   However, I don't think all te conditions I've described are actually met: For one thing, some meteors might be helpful, for instance if they killed off powerful but incurably stupid competition. If the probability distribution for lost time had a large negative tail then we would expect to get as many helpful meteor strikes as possible until the unlikelyness cost of having a larger negative time-loss would equal the likelyness gain achieved by the 'extra time'.
   On a more general point, I think one thing that isn't stressed clearly enough in the articles you referenced is that if we take as given that intelligient life evolved on preisely one planet within our cosmic horizon, and look at the variation of some parameters corresponding to how difficult the evolutionary steps must have been (and must still be for the pond scum on other planets) we would expect them to be difficult enough so that intelligence only appears once, but for things to then be more difficult is less likely. So where he says the expected time for us to evolve is approximately greater than 30 billion years or something (the precise figure depending on the number of hospitable planets), I would say it's approximately equal.

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