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The central question of why the earth happens to harbor intelligent life has occupied many scientific- and theology-minded thinkers. On one hand, physicist Stephen Hawking laments that "The human race is just a chemical scum on a moderate-sized planet orbiting round a very average star in the outer suburb of one among a hundred billion galaxies." [Deutsch1997, pg. 177-178]. On the other hand, scientists such as Simon Conway Morris say that "there is ... seeded into the initiation of the universe itself the inevitability of intelligence" [ConwayMorris2005]. Christian de Duve declares that life is a "cosmic imperative" [DeDuve1995, pg. 300]. Similarly, distinguished physicist Andre Linde writes, "I cannot imagine a consistent theory of everything that ignores consciousness. ... In the absence of observers, our universe is dead." [Folger2002; see also Linde2004, pg. 449].
Astronomer-physicist Paul Davies is even more explicit [Davies2007, pg. 231]:
Let me now turn to the philosophical question for why I believe the mind occupies a significant place in the universe. It concerns the fact that minds (human minds, at least) are much more than mere observers. We do more than just watch the show that nature stages. Human beings have come to understand the world, at least in part, through the processes of reasoning and science. In particular, we have developed mathematics, and by so doing have unraveled some -- maybe soon, all -- of the hidden cosmic code, the subtle tune to which nature dances. Nothing in the entire multiverse/anthropic argument (and certainly nothing in the unique no-free-parameters theory) requires that level of involvement, that degree of connection. In order to explain a bio-friendly universe, the selection processes that features in the weak anthropic principle merely requires observers to observe. It is not necessary for observers to understand. Yet humans do. Why?
I am convinced that human understanding of nature through science, rational reasoning, and mathematics points to a much deeper connection between life, mind, and cosmos than emerges from the crude lottery of multiverse cosmology combined with the weak anthropic principle. ... Somehow, the universe has engineered its own self-awareness.
What then is the answer to the topic question? Davies, at the conclusion of his book Cosmic Jackpot: Why Our Universe Is Just Right for Life, lists seven "positions." It is worth reviewing these positions, which are paraphrased here from Davies' book [Davies2007, pg. 261-265], as a summary of the various options of scientific thinking on this key question:
Advantages: This is an easy position to hold, but as Davies points out, it is "easy to the point of being a cop-out."
Disadvantages: If one assumes this view, then the success of the scientific enterprise to date is a complete enigma. The fact that life exists against fantastically small odds must be attributed to an extraordinary accident. The fact that life has further developed conscious minds is another strange accident of nature. Finally, one has to assert that human brains, which evolved to survive on savannas, somehow have also evolved the capacity to understand the profound patterns of modern physics and cosmology, when there is no good reason for them to do so.
Advantages: The advantage of this is that science in general, and physics in particular, now becomes a straightforward process of deriving consequences of known physical laws.
Disadvantages: Unfortunately, several very big questions would still remain: why has this theory been selected as the underpinnings of our universe? And how was life breathed into the theory to instantiate it into a physical system? Most of all, the bio-friendliness of the universe remains utterly unexplained -- it must be tossed aside as a curious fact, but of no cosmic significance, the same as with the absurd universe.
Advantages: The multiverse provides a natural and easy explanation of why the universe is fine-tuned for human existence.
Disadvantages: The chief disadvantage of such a notion is that it represents an inconceivably flagrant violation of Occam's razor. Indeed, at what point is the complexity of identifying our location in the enormous multiverse (or our location in the "landscape" of potential mathematical structures for a universe) greater than the complexity of what we are trying to explain? More extreme versions of the multiverse, such as Tegmark's ensemble of all conceivable logical structures realized in physical universes, are even more problematic: a theory that explains everything explains nothing very well.
Advantages: Attributing the design and execution of the universe to a transcendent being has obvious appeal to many traditional Judeo-Christian people. Such a notion at least provides some insight into why the universe is life-friendly.
Disadvantages: One overriding difficulty with this approach is that invoking an "intelligent designer" whenever convenient becomes a "thinking stopper" -- it buries numerous questions of our existence in the inscrutable mind of a mysterious Designer: Why the world was designed the way it was? How did the design and creative process transpire? What physical principles and/or laws were employed? Why those particular laws? What prompted the Designer to proceed with this creation? Have other worlds similarly been designed or created? Where? Surely there is a more fruitful avenue for finding a harmony between science and religion.
Advantages: It takes life seriously, placing it neither as a completely unexplained and meaningless add-on to the universe, nor as a mere passive agent as with the multiverse. In other words, it inserts a real purpose into the fundamental underpinnings of the universe.
Disadvantages: The insertion of purpose into the laws of physics ("teleology") represents a severe break with traditional science in general and physics in particular, which has always avoided any suggestions that the unfolding of the universe is heading towards any future goal. For example, how exactly does the universe "know" about life or where it is heading -- what mechanism is there to achieve this? Suggestions of backwards-in-time causality, which have been discussed here as a possible mechanism, are definitely very weird, even if they can be constructed without clear violations of physical laws. Why is life and consciousness the target of the universe? How was this "life principle" established and set into motion?
Advantages: It is self-contained and avoids infinite regress of explanations.
Disadvantages: This is a very speculative notion, with little if any solid basis in known physical laws.
Advantages: This "explains" our universe, in much the same sense as the "intelligent design" universe.
Disadvantages: But it also shares many of the same disadvantages listed above for the "intelligent design" universe. Besides, if one concludes that the universe is a sham, why bother to figure out how it works?