Anthropic Bias Observation Selection Effects in Science and Philosophy Nick Bostrom

20 I.e. that space is singly connected. There is a recent spate of interest in the possibility that our universe might be multiply connected, in which case it could be both finite and hyperbolic. A multiply connected space could lead to a telltale pattern consisting of a superposition of multiple images of the night sky seen at varying distances from Earth (roughly, one image for each lap around the universe which the light has traveled). Such a pattern has not been found, although the search continues. For an introduction to multiply connected topologies in cosmology, see ((Lachièze-Rey and Luminet 1995)).

21 A widespread misconception is that the open universe in the standard Big Bang model becomes spatially infinite only in the temporal limit. The observable universe is finite, but only a small part of the whole is observable (by us). One fallacious intuition that might be responsible for this misconception is that the universe came into existence at some spatial point in the Big Bang. A better way of picturing things is to imagine space as an infinite rubber sheet, and gravitationally bound groupings (such as stars and galaxies) as buttons glued on. As we move forward in time, the sheet is stretched in all directions so that the separation between the buttons increases. Going backwards in time, we imagine the buttons coming closer together until, at “time zero”, the density of the (still spatially infinite) universe becomes infinite everywhere. See e.g. ((Martin 1995)).

22 See e.g. ((Hawking and Israel 1979)): “[I]t is possible for a black hole to emit a television set or Charles Darwin” (p. 19). (To avoid making a controversial claim about personal identity, Hawking and Israel ought perhaps to have weakened this to “… an exact replica of Charles Darwin”.) See also ((Garriga and Vilenkin 2001)).

23 In fact, there is a probability of unity that infinitely many such observers will appear. But one observer will suffice for our purposes.

24 I restrict the assertion to human observations in order to avoid questions as to whether there may be other kinds of possible observations that perhaps could have infinite complexity or be of some alien or divine nature that does not supervene on stuff that is emitted from black holes – such stuff is physical and of finite size and energy.

25 Some cosmologists are recently becoming aware of the problematic that this section describes (e.g. (Vilenkin 1998), (Linde and Mezhlumian 1996)). See also (Leslie 1992).

26 This does not rule out that there could be other principles of assigning probabilities that would also provide plausible guidance in Dungeon, provided their advice coincides with that of SSA. For example, a relatively innocuous version of the Principle of Indifference, formulated as “Assign the same credence to any two hypotheses if you don’t have any reason to prefer one to the other”, would also do the trick in Dungeon. But subsequent thought experiments impose additional constraints, and for reasons that will become clear, it doesn’t seem that any straightforward principle of indifference would suffice to express the needed methodological rule.

27 Setting aside, as is customary in contexts like this, any risk aversion or aversion against gambling, or computational limitations that the person might have.

28 We suppose the incubator to be a mindless automaton that doesn’t count as an observer.

29 David Lewis ((Lewis 1986), (Lewis 1994)). A similar principle had earlier been formulated by Hugh Mellor ((Mellor 1971)).

30 An additional problem with the principle of indifference is that it balances precariously between vacuity and inconsistency. Starting from the generic formulation suggested earlier, “Assign equal credence to any two hypotheses if you don’t have any reason to prefer one to the other”, one can make it go either way depending on how a strong an interpretation one gives of “reason”. If reasons can include any subjective inclination, the principle loses most if not all of its content. But if having a reason requires one to have objectively significant statistical data, then the principle can be shown to be inconsistent.

31 Boltzmann attributes the idea to his assistant, Dr. Schuetz. Thank heaven for postdocs.

32 The only observational consequence such theories would have on that view is that we don’t make observations that are logically incompatible with the laws of nature which that theory postulates. But that is much too weak to be of any use. Any finite string of sensory stimulation we could have seems to be logically compatible with the laws of nature, both in the classical mechanics framework used in Boltzmann’s time and in a contemporary quantum mechanical setting.

33 One natural way of explicating this is to think of it as asking for what fraction of all Earth-like planets actually develop intelligent life, provided they are left untouched by alien civilization.

34 Define the three time intervals: , “the expected average time … which would be intrinsically most likely for the evolution of a system of ‘intelligent observers’, in the form of a scientific civilization such as our own” ((Carter 1983), p. 353); t_e, which is the time taken by biological evolution on this planet  0.4  10¹⁰ years; and , the lifetime of the main sequence of the sun  10¹⁰ years.

The argument in outline runs as follows: Since at the present stage of understanding in biochemistry and evolutionary biology we have no way of making even an approximate calculation of how likely the evolution of intelligent life is on a planet like ours, we should use a very broad prior probability distribution for this. We can partition the range of possible values of roughly into three regions: , , or . Of these three possibilities we can, according to Carter, “rule out” the second one a priori, with fairly high probability, since it represents a very narrow segment of the total hypothesis space, and since a priori there is no reason to suppose that the expected time to evolve intelligent life should be correlated with the duration of the main sequence of stars like the sun. But we can also rule out (with great probability) the first alternative, since if the expected time to evolve intelligent life were much smaller than , then we would expect life to have evolved much earlier than it in fact did. This leaves us with , meaning that life was very unlikely to evolve as fast as it did, within the lifetime of the main sequence of the sun.

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