## The Doomsday Argument

Graham Giller sent me an e-mail the other day.

Greg, are you familiar with the Doomsday Argument? Since you’ve done work on long term evolution of systems, I was wondering what you thought of it. My feeling is that it fails because of the assumption that there exists a final human with absolute certainty. That is Pr(Exists

Ms.t.M>Nfor allN)>0, whereNis the ordinal number of the last human, which seems to be unjustifiable.Any thoughts?

A number of lines of evidence make it quite clear that we live in a universe with a non-zero cosmological constant. The rate of expansion of non-gravitationally bound regions of the universe is accelerating, and if the cosmological constant is truly fixed, this acceleration will proceed indefinitely. Kraus and Starkman have done an extensive back-of-the-envelope treatment of the survival of life (or more specifically computation) in an accelerating universe. They conclude that computation cannot proceed indefinitely — the cosmological constant implies a minimum temperature for space, which will eventually make it impossible to store and retrieve information.

I think *N* is finite. Any guesses as to its value?

If you think that N is finite, the Doomsday Argument logic is plausible. I think N is probably finite, but I don’t know that it is almost certainly. Using the Bayesian logic of the Doomsday Argument I have to give a non-zero weight to the probability of no extinction. This means that the principal DA sum (E{N} = 2n, where n is estimated from the approximate cumulative human population to this point) is not valid.

Graham GillerMay 26, 2009 at 4:25 am