From: Robin Hanson (hanson@econ.berkeley.edu)
Date: Wed Aug 26 1998 - 14:44:15 MDT
Nick Bostrom writes:
>> >Suppose the only alternatives were (1): one in ten Earth-like planets
>> >evolve intelligent life; or (2) one in a thousand does. Suppose the
>> >prior probability is fifty-fifty.
>> >
>> >Now you observe that the Earth has evolved life. Based on your
>> >clarification above, I now take it that you think this observation
>> >should increase your confidence in (1). Is this right?
>>
>> Yes.
>>
>> >But then what happened to the selection-effect you spoke of in the
>> >"Early life"-paper? "Since no one on Earth would be wondering about
>> >the origin of life if Earth did not contain creatures nearly as
>> >intelligent as ourselves, the fact that four billion years elapsed
>> >before high intelligence appeared on Earth seems compatible with any
>> >expected time longer than a few billion years"
>>
>> I don't see this as inconsistent with the other position.
>
>What do you mean when you say that quick evolution is compatible with
>any expected time longer than a few billion years? Nobody thought
>quick evolution were *logically* incompatible with a very long
>expected time. So presumably you were pointing out that they were not
>even probabilistically incompatible. But now, it seems, you say that
>quick evolution gives probabilistic grounds for ruling out a very
>long expected time.???
Consider an exponential model of the time to evolve life on a planet,
where the probability of life evolving t years after the planet forms
is proportional to exp(-t/h) within the window [0,T]. The average
number of planets with life is then proportinal to 1 - exp(-T/h).
The observation that t ~ T/2 exponentially suppresses the posterior,
relative to the prior, for h << T. Values of h >> T are only linearly
suppressed if one expects one is more likely to appear in a universe
that has more life. In my paper I was focusing on the exponential,
not the linear, suppression, but I admit both effects are there.
Robin Hanson
hanson@econ.berkeley.edu http://hanson.berkeley.edu/
RWJF Health Policy Scholar, Sch. of Public Health 510-643-1884
140 Warren Hall, UC Berkeley, CA 94720-7360 FAX: 510-643-8614
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