Russell Standish writes, regarding http://arxiv.org/abs/astro-ph/0607227 :

> Thanks for giving a digested explanation of the argument. This paper > was discussed briefly on A-Void a few weeks ago, but I must admit to > not following the argument too well, nor RTFA. > > My comment on the observer moment issue, is that in a Multiverse, the > measure of older observer moments is less that younger ones. After a > certain point in time, the measure probably decreases exponentially or > faster, so there will be a mean observer moment age. I had a similar thought. I am curious to know your reasoning or justification for why this should be true. I have not read the papers referenced by this one, but the authors allude to previous work: "Given some a priori distribution of the values of the fundamental constants across the ensemble, the probability for a 'typical' obserer to measure a certain value of one or more of these constants is usually taken to be proportional to the number of observers (or the number of observers per baryon)." It is this last parenthetical comment I found interesting. Apparently there has been a difference in previous work about whether the measure should be proportional to observers vs observers per baryon. Consider two cases: one observer in a universe of a given size, or one observer in a universe twice that big. These would be considered the same by a number-of-observers measure, but the first would have twice the measure if it was observers per baryon. I argued some time past, based on some hand-wavey arguments, that the latter measure is better - we attribute a portion of a universe's measure to an observer, proportional to the fraction of the universe that the observer takes up. This came from the UDASSA concept I was describing in detail last year. It amounts to the observers-per-baryon measure. It's interesting that physicists have considered a similar idea. In terms of time, like Russell I would say that ancient observer-moments should get less measure than early ones, for the same basic reason - it takes more information to specify the location of the "physical" system that instantiates the OM within the universe. My reasoning though would imply that measure should be inversely proportional to age, rather than Russell's suggestion of an exponential decay. So I am curious where he got that. I could describe my reasoning in more detail if there is interest. > So contra all these old OMs dominating the calculation, and giving > rise to an expected value of Lambda close to zero, we should expect > only a finite contribution, leading to an expected finite value of > Lambda. > > We don't know what the mean age for an observer moment should be, but > presumably one could argue anthropically that is around 10^{10} > years. What does this give for an expected value of Lambda? I don't know if I know enough physics to figure that out. I'll take another look at the paper. I see that I misstated the reason why the CC limits observation. It's not that the universe becomes uninhabitable. Rather, computation and observation is assumed to be proportional to internal energy divided by external universe temperature. It turns out that the optimal strategy is to accumulate and store up as much energy as you can, as the universe expands and cools. Then, when the universe is all cooled down, you go ahead and do all your observations and calculations. In a universe with a high CC, you can't accumulate as much energy, because it expands more quickly and hence mass-energy thins out faster. It cools down sooner and you don't have as much stored up at that time, so you can't do as much. So what we would have to say is that that strategy is no longer optimal because such distant observer-moments will have low measure, and we care more about OMs which have high measure. (I admit that few people take the idea seriously that seemingly undetectable OM measure changes should matter, but I can assume that this is a super-advanced civilization and everyone is smart, so of course they will agree with me!) Instead, the optimal strategy maximizes the total measure of OM- computations, and that requires doing more computations early. OTOH, it is more efficient to wait until the universe is cooler, we can do more computing with the same amount of energy. Maximizing the product of these two effects would require a detailed model for how quickly measure decays with time. (We'd also have to consider whether measure should change with temperature, which it might in my model, I have to think about it more.) > Of course their argument does sound plausible for a single universe - > is this observational evidence in favour of a Multiverse? I think what you're saying is that if this is the only universe, and if civilizations adopt the strategies advocated in this paper, then most OMs will be far in the future, hence by the ASSA we are unlikely to be experiencing present-day OMs. This was the basic concept of a paper we discussed back in 2002: > Dyson, L., Kleban, M. & Susskind, L. Disturbing implications of a > cosmological constant. Preprint <http://xxx.lanl.gov/abs/hep-th/0208013>, > (2002). They used a slightly different physics model but came up with the same idea, that most OMs should be in the distant future, contradicting what we call the ASSA, which I think they just considered an implication of the anthropic principle. You might be right that these papers could be read as an argument against a single-universe model, if in fact we could come up with a good justification within a multiverse model for decreasing OM measure in the future. We'd probably have to have a pretty strong argument in that regard, though. Hal Finney --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Everything List" group. To post to this group, send email to everything-list@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/everything-list -~----------~----~----~----~------~----~------~--~---