From: Robin Hanson (hanson@econ.berkeley.edu)
Date: Fri Aug 14 1998 - 17:08:29 MDT
Hal F. writes:
>Many times I've convinced myself that I have a refutation for the DA,
>only to decide on further consideration that it is more difficult than
>it seems.
I've just spent the last 24 hours reconsidering the Doomsday
argument, including Leslie's book, Nick's paper, and the two
articles Nick cites that I could find on the web (at:
http://www.hedweb.com/nickb/140797/doomsday.html
http://xxx.lanl.gov/abs/gr-qc/9407002
http://www.cs.monash.edu.au/~jono/TechReports/analysis2.ps )
The basic argument is that, all else equal, we should not expect
to be temporally among the first, say, 5% of creatures "like us."
So given exponential growth with a sudden end, that end is near.
I'm currently pretty negative on the doomsday argument.
1) Even when it is valid, the argument suggests "change," and
not necessarily "doom". Creatures like us can no longer exist
because their descendants are different, but not destroyed.
2) Our concept of "like us" can be biased by where we are
in time. No fair picking the last big intelligence change, and
then defining "like us" to be anything at least as smart as we
are. Better to exclude smarter creatures as well as dumber.
3) It's actually hard to formalize the argument so that the
implications are big.
For example, Oliver & Korb show that accepting one's birth rank
as a random draw from some total population, with a uniform prior
over that total population, an observation of a low rank changes
the expected value by less than a factor of ten. Kopf, Krtous,
& Page also show that expected values don't change much for a
power law prior on total population, unless the power is near
minus two.
As another example, Leslie admits in his "shooting room" example
that if the probability of "doom" is constant with time independent
of population size, the doomsday argument fails. (The expected
population is infinite here, so stuff depends on how you slice it.)
Finally, Nick shows how the argument gets weakened as one allows
for alien creatures "like us" who won't get hurt by our local doom.
I suspect that the more these models get elaborated with our
detailed knowledge relevant for forecasting "doom" or change,
the less important the doomsday type selection effect would be.
4) Nick explains well how the doomsday argument assumes that
one was guarenteed to show up as a creature "like us" at sometime
in our universe, regardless of how many such creatures this
universe produces. If one instead assumes that the probability of
finding oneself in a universe is proportional to population of that
universe, the doomsday argument evaporates. This later assumption
seems much more reasonable to me, and to Kopf, Krtous, & Page.
I buy it even if the different universes are only "possible"
rather than coexisting in some way.
In summary, I don't buy a basic assumption of the doomsday argument,
and even if I did the implication seems only a moderately higher
chance of change, not doom.
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: 510-643-2627
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