Anthropic Bias Observation Selection Effects in Science and Philosophy Nick Bostrom

9 If there is a sense of “explanation” in which a multiverse theory would not explain why we observe a fine-tuned universe, then the prospect of a multiverse theory would not add to the need for explanation in that sense.

10 The meaning of “representative” is not equivalent here to “most numerous type of universe in the multiverse” but rather “the type of universe with the greatest expected fraction of all observers”.

11 One can easily imagine multiverse theories on which this would not necessarily be the case. A multiverse theory could for example include a physics that allowed for two distinct regions in the space of possible boundary conditions to be life-containing. One of these regions could be very broad so that most universes in that region would not be fine-tuned – they would still have contained life even if the values of their physical constants had been slightly different. The other region could be very narrow. Universes in this region would be fine-tuned: a slight perturbation of the boundary conditions would knock a universe out of the life-containing region. If the universes in the two life-containing regions in parameter space are equivalent in other respects, this cosmos would be an instance of a multiverse where representative observer-containing universes would not be fine-tuned. If a multiverse theory assigns a high probability to the multiverse being of this kind, then on the hypothesis that that theory is true, representative observer-containing universes would not be fine-tuned.

12 It may intuitively seem as if our observing a fine-tuned universe would be even more surprising if the only multiverse theory on the table implied that representative observer-containing universes were not fine-tuned, because it would then be even more improbable that we should live in a fine-tune universe. This intuition most likely derives from our not accepting the assumptions we made. For instance, the design hypothesis (which we ruled out by fiat) might be able to fit the four criteria and thus account for why we would find the fine-tuning surprising even in this case. Alternatively, we might think it implausible that we would be sufficiently convinced that the only available multiverse hypotheses would be ones in which representative universes would not be fine-tuned. So this represents a rather artificial case where our intuitions could easily go astray. I discuss it only in order to round out the argument and to more fully illustrate how the reasoning works. The point is not important in itself.

13 It’s not clear whether there is an alternative that would work here. There would be if, for instance, one assigned a high prior probability to a design hypothesis on which the designer was highly likely to create only one universe and to make it fine-tuned.

14 The figure 1 in 10^3,000 is Hart’s most optimistic estimate of how likely it is that the right molecules would just happen to bump into each other to form a short DNA string capable of self-replication. As Hart himself recognizes, it is possible that there exists some as yet unknown abiotic process bridging the gap between amino acids (which we know can form spontaneously in suitable environments) and DNA-based self-replicating organisms. Such a bridge could dramatically improve the odds of life evolving. Some suggestions have been given for what it could be: self-replicating clay structures, perhaps, or maybe something isomorphic to Stuart Kaufmann’s autocatalytic sets. But we are still very much in the dark about how life got started on Earth or what the odds are of it happening on a random Earth-like planet.

15 A similar definition was given by Barrow in 1983:
[The] observed values of physical variables are not arbitrary but take values V(x,t) restricted by the spatial requirement that x  L, where L is the set of sites able to sustain life; and by the temporal constraint that t is bound by time scales for biological and cosmological evolution of living organisms and life-supporting environments. ((Barrow 1983), p. 147)

16 There is also no contradiction involved in supposing that we might discover that we are not carbon-based.

17 (A) points to the teleological idea that the universe was designed with the goal of generating observers (spiced up with the added requirement that the “designed” universe be the only possible one). Yet, anthropic reasoning is counter-teleological in the sense described above; taking it into account diminishes the probability that a teleological explanation of the nature of the universe is correct. And it is hard to know what to make of the requirement that the universe be the only possible one. This is definitely not part of anything that follows from Carter’s original exposition.

(B) is identical to what John Wheeler had earlier branded the Participatory Anthropic Principle (PAP) ((Wheeler 1975; Wheeler 1977)). It echoes Berkelian idealism, but Barrow and Tipler want to invest it with physical significance by considering it in the context of quantum mechanics. Operating within the framework of quantum cosmology and the many-worlds interpretation of quantum physics, they state that, at least in its version (B), SAP imposes a boundary condition on the universal wave function. For example, all branches of the universal wave function have zero amplitude if they represent closed universes that suffer a big crunch before life has had a chance to evolve, from which they conclude that such short-lived universes do not exist. “SAP requires a universe branch which does not contain intelligent life to be non-existent; that is, branches without intelligent life cannot appear in the Universal wave function.” ((Barrow and Tipler 1986), p. 503). As far as I can see, this speculation is totally unrelated to anything Carter had in mind when he introduced the anthropic principle, and PAP is irrelevant to the issues we discuss in this book. (For a critical discussion of PAP, see e.g. ((Earman 1987))).

Barrow and Tipler think that statement (C) receives support from the many-worlds interpretation and the sum-over-histories approach to quantum gravity “because they must unavoidably recognize the existence of a whole class of real ‘other worlds’ from which ours is selected by an optimizing principle.” ((Barrow and Tipler 1986), p. 22). (Notice, by the way, that what Barrow and Tipler say about (B) and (C) indicates that the necessity to which these formulations refer should be understood as nomological: physical necessity.) Again, this seems to have little do to with observation selection effects. It is true that there is a connection between SAP and the existence of multiple worlds. From the standpoint of Leslie’s explication, this connection can be stated as follows: SAP is applicable (non-vacuously) only if there is a suitable world ensemble; only then can SAP be involved in doing explanatory work. But in no way does anthropic reasoning presuppose that our universe could not have existed in the absence of whatever other universes there might be.

18 For further critique of Tipler’s theory, see ((Sklar 1989)).

19 A non-zero cosmological constant has been considered desirable from several points of view in recent years, because it would be capable of solving the cosmological age problem and because it would arise naturally from quantum field processes (see e.g. (Klapdor and Grotz 1986), (Singh 1995), (Martel, Shapiro et al. 1998)). A universe with a cosmological density parameter and a cosmological constant of about the suggested magnitude would allow the formation of galaxies ((Weinberg 1987), (Efstathiou 1995)) and would last long enough for life to have a chance to develop.

Download 9.31 Mb.

Share with your friends: