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#206 Finely Tuned Universe

March 28, 2011

I was listening to Dr. Craig talking about how some atheists are very impressed by the Infinite Universes Hypothesis (IUH) -- namely, the idea that there is an infinite number of universes, in which all possible combinations of the cosmological constants have taken or are taking place, and they conclude that therefore our universe's amount of fine tuning is unimpressive.

Forgive me, but I don't see how the postulate of a genuinely infinite number universes helps them at all, if the variables within those possible universes are also infinite. In that case, they have an infinite chance that NO repetition of the ideal conditions for life will EVER take place.

It seems to me that they need a LIMITED number of variations of the constants, plus a VERY LARGE number of repetitions of the variables, rather like rolling a six-sided die many times increases the chance that a four will be rolled at some point.  On the other hand, if the die itself had infinite sides, then having an infinite number of rolls, it seems to me, would NOT increase the chance of rolling any fours; in fact, there would be an infinite number of chances it would NEVER happen.

If the actual number of possibilities is itself infinite, then how does having an infinite number of instances help to increase the chances of "hitting the right numbers" to produce an ordered, life-sustaining universe?  Am I missing something, or are they taking for granted a limited number of cosmological constants?  In that case, doesn't the question come up, "What causes the particular range of limited cosmological constants?"  And if that's so, how does the IUH help them evade the Singularity problem?

I realize that this question may be a bit too esoteric for posting, but if Dr. Craig turns out to be interested in answering it, I sure would like to know what the answer is.



Dr. craig’s response


Fine-tuning of the universe

Stephen, yours is an unusually insightful question about the cogency of appealing to a World Ensemble or multiverse in order to rescue the hypothesis of chance as the best explanation of the observed fine-tuning of the universe.

Defenders of the Many Worlds Hypothesis will typically appeal to some physical theory which serves to generate or describe the ensemble of worlds that exist. For example, M-theory, which unifies various string theories of physics, permits a cosmic landscape of 10500 different states of the quantum vacuum characterized by different fundamental constants. One can then conjoin M-theory with inflationary cosmology to produce different bubble universes in the wider sea of expanding false vacuum to try out the various combinations permitted by the cosmic landscape. If we assume an infinity of worlds, each combination will be repeated infinitely many times.

As you say, this move to restrict the nature of the multiverse raises all over again the question of whether the laws which govern it must be special in some way. If so, then nothing has been achieved by appeal to the World Ensemble.

Fine-tuning of the universe – the multiverse doesn’t explain why interacting agents exist

But an even more fundamental fallacy plagues such appeals to the Many Worlds Hypothesis. Just this week I’ve been reading a very insightful piece by Robin Collins on this subject.[1]Collins points out that simply postulating an ensemble of worlds large enough and varied enough for some phenomenon to appear in some worlds is not enough to explain that phenomenon. Otherwise you can explain almost anything (like my getting four aces every time I deal) by saying that somewhere in the World Ensemble there are worlds exhibiting the phenomenon in question. Science and rational behavior itself would become impossible if virtually everything can be written off as a random occurrence in a World Ensemble.

Defenders of the World Ensemble explanation of the fine-tuning of the universe therefore appeal to a self-selection effect on the part of observers. Observers can observe only those worlds which are characterized by life-permitting parameters. Obviously, observers don’t exist in worlds which are incompatible with the existence of observers! So all the worlds in the ensemble which have observers in them must be observed to be life-permitting. The assumption here is that observable worlds must be characterized by fine-tuning. That permits the partisan of chance to say observers must observe their world to be fine-tuned, and so there’s nothing to be explained.

What Collins points out is that that assumption is false. Here he appeals to the famous Boltzmann Brains. It is physically possible to have an observable world in which a single brain forms via a quantum fluctuation in the vacuum. Boltzmann Brains can fluctuate into existence even in worlds which are not finely tuned for embodied, interacting agents (for example, a world in which the cosmological constant is too large for galaxies and stars to form). So observable worlds need not be fine-tuned as ours is. In that case, postulating a World Ensemble or multiverse and appealing to a self-selection effect by observers will not explain why, if we are a random world in such an ensemble, we observe a fine-tuned universe.

Here is how Collins puts it:

The reliance on the observer selection principle brings up an enormous problem with the multiverse explanations of the fine-tuning: the fine-tuning data is not that we live in a “observer-structured universe,” by which I mean a universe structured in such a way that a large number of observers will arise; rather, the data is that we exist in a universe that is precisely set so that the predominant kind of observers that are likely to occur are embodied conscious agents that can significantly affect each other’s welfare, either for good or ill. . . . The B[oltzmann] B[rain] problem, therefore, shows that the constants are not fine-tuned for observers, but rather for interacting agents that arise through a standard evolutionary process. Yet, because of its reliance on the observer-selection principle, the multiverse hypothesis could at best explain fine-tuning for the existence of observers, not fine-tuning for interacting agents.

In order to save the appeal to the World Ensemble, the partisan of chance must provide some reason for thinking that the majority, or at least a significant proportion, of worlds with observers are worlds which are fine-tuned for the existence of embodied, interacting agents. Otherwise the explanation doesn’t even get off the ground. But there’s no way to do that.

Collins’ analysis thus suggests that the appeal to a World Ensemble or multiverse is quite wrong-headed. The problem is not that if we are random members of a World Ensemble we should each take ourselves to be a Boltzmann Brain with the illusion of a wider universe; it is, more fundamentally, that what needs explaining is, not that the universe is observer-permitting, but that it is permitting of embodied, interactive agents. As Collins explains,

In the literature, it is taken as given that the multiverse explains the fine-tuning, with the BB problem being seen as an untoward consequence of multiverse explanations that needs to be somehow fixed. My argument reframes the debate by arguing that the existence of BBs shows that the universe is not fine-tuned for observers but rather for interacting agents; thus the multiverse fails to get to square one of explaining the fine-tuning since it relies on the observer-selection principle, not an “interacting-agent” selection principle [my emphasis].

Since what cries out for explanation is not the fine-tuning of the universe just for observers but rather fine-tuning for embodied, interactive agents, the World Ensemble Hypothesis is a non-starter.

  • [1]

    “Modern Cosmology in Philosophical and Theological Perspective: Three methodological approaches” (preprint).

- William Lane Craig