From: Robin Hanson (hanson@econ.berkeley.edu)
Date: Sun Aug 30 1998 - 21:53:07 MDT
Nick Bostrom and I have had a long conversation here about
the Doomsday Argument, and I think I need to stop now. So
let me now summarize my position.
Our standard approach to analyzing the chances for doom soon
is to imagine various sorts of descructive processes, and then
choose a probability distrubution over the frequency of such
events. In doing so one trivally and usually implicitly conditions
on the fact that we have made it so far without doom. And one
typically does not infer that doom is likely soon.
There are two doomsday arguments I can see. The first one notes
that for exponential growth processes which end suddenly, most
population members appear near the end. Thus estimating that
doom is not soon implies that we are not typical/random population
members.
Since people often like to think they are special, the fact that
an inference suggests we are special does suggest we may have
fooled ourselves somehow. But while this may motivate skeptics
to seek weak points in standard assumptions, it does not by itself
seem to me much of a reason to much modify standard analyses.
We *are* special on some dimensions, and this could be one of them.
The second doomsday argument invites us to imagine that we had
amnesia regarding which person in history we are, and that in this
situation our probability distribution over doom was that suggested
by standard analyses. The argument then correctly notes that
learning that we appear early in history must make us expect that
doom occurs earlier in history. It then concludes that we should
expect doom earlier than standard analyses would suggest.
The weak point of this argument is the assumption that we should
invoke our standard prior probabilities over doom in the amnesia
context. The usual place to invoke priors over doom is in the
context where we don't know whether humanity lasted long enough
for us to be born. In this context, learning that one exists, but
has amnesia, gives one a lot of information, and in particular suggests
that doom occurs later than suggested by the priors over doom.
Learning that one is alive with amnesia makes one more optimistic,
and then learning that one was born early makes one more pesimisti.
If you started with standard priors, you end up with standard
expectations about how soon doom may be. Let me illustrate with
some examples.
Imagine that you found yourself hung over in a hotel room, and
couldn't remember who you were, other than that you are a musician
on tour. You wonder: am I a one-hit-wonder, or do/will I have a
lasting music career? If on average only 25% of musicians have a
lasting career, but musicians with lasting careers spend three
times as much time in hotel rooms on tour, then you should estimate
a 50% chance you have a lasting career. This is because only half
of the total musician hotel room-days are filled with one-hit-wonders.
Amensia implies optimism.
Similarly, if you can't recall how old you are, you should expect
to be older than the average person. Why? Because older people
have more hotel-days in their lifetime. Similarly, it seems to
me that if you can't recall who you are in humanity, you should
become more optimistic about humanity's chances.
The amnesia situation does not seem to me to be a good place for
imposing priors, unless one takes into account the fact that
amnesia suggests optimism. The fact that amnesiacs who learn they
are early in history should become more pessimistic does not seem
a reason for me to be pessimistic. After all, I don't have amnesia.
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