The final class of solutions to the Fermi paradox is that "they" — extraterrestrial civilizations advanced enough for us to communicate with them — do not exist.
Within this class of solutions, one can discern different approaches to Fermi's question. Ultimately, though, these solutions depend upon making one or more of the terms in the Drake equation tiny. If a single term is close to zero, or else if several of the terms are small, the effect is the same: when all the terms are multiplied together, the result is N = 1. The only technologically advanced civilization in the Galaxy, and perhaps the whole Universe, is our own.
Recently, Peter Ward and Don Brownlee, scientists at the University of Washington, wrote a stimulating and thought-provoking book called Rare Earth.176 They presented a coherent argument about why complex life may be an unusual phenomenon. (Strangely, they make no mention of the Fermi paradox.) In this chapter I will discuss several of the ideas made in Rare Earth. Since each of these ideas has been proposed individually as a resolution to the Fermi paradox, I discuss them individually. However, they could equally have been grouped as a single "Rare Earth" solution to the paradox.
Advanced ETCs may not exist because of a lack of suitable environments: Earth-like planets may be rare. But perhaps they do not exist because life itself is a rare phenomenon; perhaps the emergence of life from non-living material is an almost miraculous fluke, or perhaps the evolution of complex life-forms is unlikely to occur. I will discuss several solutions established on these ideas, but it is worth bearing in mind that the discussions will contain a major limitation: I will assume throughout that naturally occurring life is carbon-based and requires water as a solvent. Some scientists have argued that other chemicals, notably silicon, could be used instead of carbon; some have even argued that other solvents, perhaps methane, could be used in place of water. Personally — and this may be a failure of imagination on my part — I find it difficult to conceive of a biochemistry that does not feature water or carbon. Water in particular I am sure is necessary for life. Find water, and you have a chance of finding life. If you believe life can take quite different forms — perhaps as persistent patterns in plasma clouds, or as information-carrying whorls in viscous fluids, or whatever — then the solutions I present here will seem narrow-minded.177 We may later discover that many of the proposed solutions in this chapter stemmed from a lack of scientific imagination. But we are in the difficult position of trying to generalize from a single instance — as far as we know, Earth is the only planet with life. It is dangerous to draw conclusions from a sample size of one, but in this case what else can we do? Inevitably we will be influenced — perhaps biased is a better word — by those factors that seem necessary for our continued existence. We are bound by the Weak Anthropic Principle (WAP), which states that what we can observe must be restricted by the conditions necessary for our presence as observers. Since it is impossible to avoid the WAP in a discussion of the Fermi paradox, it makes sense to begin this part of the book with a solution based upon an-thropic reasoning. Anthropic solutions are rather abstract; later solutions will be based on more concrete proposals.
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