Re: Doomsday Example

Robin Hanson (
Wed, 26 Aug 1998 13:44:15 -0700

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 RWJF Health Policy Scholar, Sch. of Public Health 510-643-1884 140 Warren Hall, UC Berkeley, CA 94720-7360 FAX: 510-643-8614