Nick Bostrom's new book
I don't know why Nick hasn't told us about his new book. I just found out about it on his web site: Anthropic Bias: Observation Selection Effects in Science and Philosophy Nick Bostrom, Routledge, New York, July 2002 There are some free sample chapters at http://www.anthropic-principle.com/book. I had to buy a copy of the book because apparently only 6 libraries in the world has it, and none of them are near me. If you want to buy one too you can use one of these links to save $7 off the $70 list price. Each one can be used by only one person so don't click on it if you're not going to buy the book. http://www.amazon.com/exec/obidos/redirect?tag=love102-20path=stl/4PY1Y12NQ23J http://www.amazon.com/exec/obidos/redirect?tag=love102-20path=stl/H1OOZT25VXRJ
Doomsday-like argument in cosmology
- Forwarded message from Wei Dai [EMAIL PROTECTED] - Date: Thu, 15 Aug 2002 13:28:43 -0700 From: Wei Dai [EMAIL PROTECTED] To: [EMAIL PROTECTED] Subject: Re: Nature Article On Thu, Aug 15, 2002 at 12:45:17AM -0400, [EMAIL PROTECTED] wrote: Dyson, L., Kleban, M. Susskind, L. Disturbing implications of a cosmological constant. Preprint http://xxx.lanl.gov/abs/hep-th/0208013, (2002). This is a variant on the Doomsday argument. The core argument of the paper is this: If we live in a world with a true cosmological constant, then the observers whose observable universe is macroscopically indistinguishable from ours are a tiny fraction of all observers. Therefore the only reasonable conclusion is that we do not live in a world with a true cosmological constant. Compare this with the Doomsday argument (see http://www.anthropic-principle.com/primer1.html): If we live in a world without a doomsday in the near future, then the observers whose birth ranks are similar to ours are a tiny fraction of all observers. Therefore the only reasonble conclusion is that we do not live in a world without a doomsday in the near future. So you should accept the conclusion of this paper only if you think the Doomsday type of argument is sound. - End forwarded message -
Re: Doomsday-like argument in cosmology
I think that the difference is that invoking the SIA does not affect the conclusion of the paper. Saibal Wei Dai wrote: On Thu, Aug 15, 2002 at 12:45:17AM -0400, [EMAIL PROTECTED] wrote: Dyson, L., Kleban, M. Susskind, L. Disturbing implications of a cosmological constant. Preprint http://xxx.lanl.gov/abs/hep-th/0208013, (2002). This is a variant on the Doomsday argument. The core argument of the paper is this: If we live in a world with a true cosmological constant, then the observers whose observable universe is macroscopically indistinguishable from ours are a tiny fraction of all observers. Therefore the only reasonable conclusion is that we do not live in a world with a true cosmological constant. Compare this with the Doomsday argument (see http://www.anthropic-principle.com/primer1.html): If we live in a world without a doomsday in the near future, then the observers whose birth ranks are similar to ours are a tiny fraction of all observers. Therefore the only reasonble conclusion is that we do not live in a world without a doomsday in the near future. So you should accept the conclusion of this paper only if you think the Doomsday type of argument is sound. - End forwarded message -
Re: Doomsday-like argument in cosmology
On Thu, Aug 15, 2002 at 11:28:28PM +0200, Saibal Mitra wrote: I think that the difference is that invoking the SIA does not affect the conclusion of the paper. Why do you say that? I think SIA affects the conclusion of the paper the same way it affects the Doomsday argument. It's kind of funny that the authors of this paper is playing the role of the presumptuous philosopher (in the thought experiment I just discussed in a previous post), except they're physicists, and they're making the opposite argument (in favor of the hypothesis that implies fewer observers rather than the one that implies more observers).
Re: Doomsday-like argument in cosmology
I haven't read the paper in detail, so I could be wrong. Consider the two alternatives: 1) true cosmological constant 2) no true cosmological constant We also assume SIA. Is it the case that there are much fewer observers in case of 2) than in case of 1) ? I haven't seen such a statement in the paper (but again, I could have missed it). So, I would say that given our observations of the universe a probability shift takes place, such that 2) is favored (assuming that 1) and 2) have a priory probabilities of the same order). Saibal - Oorspronkelijk bericht - Van: Wei Dai [EMAIL PROTECTED] Aan: Saibal Mitra [EMAIL PROTECTED] CC: [EMAIL PROTECTED] Verzonden: donderdag 15 augustus 2002 23:46 Onderwerp: Re: Doomsday-like argument in cosmology On Thu, Aug 15, 2002 at 11:28:28PM +0200, Saibal Mitra wrote: I think that the difference is that invoking the SIA does not affect the conclusion of the paper. Why do you say that? I think SIA affects the conclusion of the paper the same way it affects the Doomsday argument. It's kind of funny that the authors of this paper is playing the role of the presumptuous philosopher (in the thought experiment I just discussed in a previous post), except they're physicists, and they're making the opposite argument (in favor of the hypothesis that implies fewer observers rather than the one that implies more observers).
Re: Doomsday-like argument in cosmology
On Fri, Aug 16, 2002 at 12:26:10AM +0200, Saibal Mitra wrote: I haven't read the paper in detail, so I could be wrong. Consider the two alternatives: 1) true cosmological constant 2) no true cosmological constant We also assume SIA. Is it the case that there are much fewer observers in case of 2) than in case of 1) ? I haven't seen such a statement in the paper (but again, I could have missed it). You're right, we need to look at the alternative hypothesis. But there's not just one alternative, there are several. 1) True cosmological constant, therefore heat death and endless Poincare recurrences. 2a) The universe ends soon. 2b) The universe runs for a while longer, then gets reset to a low entropy state and starts over. This happens in an endless cycle. 2c) The universe never ends, and life become ever more complex and intelligent. 2d) No true cosmological constant, but we get heat death and endless Poincare recurrences for some other reason. 2e) The universe never ends, but the total number of observers is a relatively small finite number. I think these exhaust all of the possibilities. A huge problem with SIA is that 1, 2b, 2c, and 2d all imply an infinite number of observers, which makes SIA impossible to use. But for sake of argument let's say these universes do eventually end, and they all have the same (very large) number of observers. Applying just DA (Doomsday argument) favors 2a, 2b and 2e. Applying both SIA and DA favors 2b. So I guess you're right, whether or not you apply the SIA does not affect the the paper's conclusion that a shift away from 1 is warranted. This makes me realize that SIA doesn't perfectly counteract the Doomsday argument. DA makes you shift to 2a, 2b, and 2e. SIA then makes you shift to 2b, whereas what we really want is to shift back to the original distribution so we don't have to rule out 2c.
