equation are grossly over-estimated: there are fewer habitable zones, less planets, less life, less intelligence and less willingness to communicate than Drake and his followers believe. In short, we are either superior or unique. Pluralists have been more speculative. Maybe they came, did not see anything interesting and left without trace. Maybe they have not reached us yet, or they have been and we are the descendants. Maybe we missed intelligent life: it evolved, colonized the galaxy, and self-destructed long before humanity existed. Now we are among the very first in the second time around. Perhaps the Frank Tipler probes are really there, but are programmed not to respond until we encounter them. Maybe they came, dumped their rubbish and contaminated the Earth and out of the chemical trash emerged the seeds of prebiotic life. Thomas Gold (1960) describes this as the ‘garbage hypothesis’. It is, of course, an untestable hypothesis. Maybe they are transmitting but their signals are too weak, failing to penetrate the dense gas clouds between the stars. Perhaps they have no detectable physical structures and exist as pure states of consciousness having long ago discarded their material bodies, or having rejected their planetary homes because they regard them as dirty, unhealthy places, have no wish to visit another one. Perhaps their technology evolved long ago beyond a stage where they would want anything from us. Perhaps our environment is inhospitable to the species which dominates our part of the galaxy. Perhaps, contrary to Hart and Tipler, they have no desire to colonize, no desire to migrate. Expansion is by no means inevitable. Ming Dynasty China was a leading sea power from 1405 to 1435 which then turned inwards and made it a capital offence to manufacture large sea-going junks. Advanced ETs might not suffer from any half-crazy desire to strip-mine the galaxy, running from planet to planet polluting and raiding its resources.
Strictly speaking, the basic premise in Fermi’s Paradox need not be accepted. Strange as it may seem, we could argue that it has not been decisively shown that they are not here. They could very well be here – not in the sense that they are having secret meetings with the world’s leaders or participating in experiments in Area 51 Research Center – but they could be sending messages which we have not yet received; they might be teleporting various ‘sims’ of themselves across the galaxy, but as we lack the appropriate receivers we are unaware of them. They, or their robot probes, might be mining asteroids which we have not yet looked at, or maybe they are currently colonizing the galaxy with robots or have evolved into post-organic beings but have continued further in the direction taken by our technology, and have succeeded in even greater levels of miniaturization. In which case they are too small for us to observe them.
One explanation of the silence rests on a weak interpretation of the SETI programme: it concedes that the combined fractions of f 1 and f c in the Drake equation (planets with intelligent and communicative life) are much lower than previously estimated. Drake estimated a rate of one planet in every hundred. But according to the weak interpretation it can be conceded that, even with the
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precursors of life, such as sugar and amino acids, the next steps towards life and then intelligence are more difficult than imagined. Perhaps some rare, as yet unknown, catalyst is required if life is to emerge. Perhaps some, as yet unknown, software is required if intelligence is to develop. It might also be conceded that intelligent life is more prone to natural disasters than previously thought. For example, the Cretaceous Tertiary event, plus evidence of four other mass extinctions found in the sedimentary record, destroyed most species of land animal over 440 kilos in weight and prohibited any potential for intelligence in the first 3 billion years of Earth life. The same thing, to even greater proportions, might occur throughout the galaxy.
Events in social history could reduce the fraction of L – the lifetime of an advanced civilization. Expanding populations, driven primarily by demographic pressure, tend to wreck their environments, as the Europeans did in their colonies. If this were repeated on an interstellar scale, it could be argued that the termination of an advanced civilization coincides with its capacity for interstellar communication and travel. The problem with this argument is that one cannot claim that civilizations will destruct before they colonize unless one can demonstrate that every single race will do so before it reaches the capacity for interstellar flight (Birch, 1990: 1). For it would only require one exception for the galaxy to be populated with ETs. There is no scientific law which prohibits colonization. It may, of course, turn out that no one wants to, but this is not supportive of a thesis which claims that no one can.
According to Hart and Tipler, given the age of the universe, there has been ample time for advanced extraterrestrials to have established colonies near or even on the Earth. The absence of these colonies is invoked to establish the claim that we are unique. Many of SETI’s exponents reject this appeal to colonization. Fred Hoyle and Chandra Wickramasinghe (1978: 160) rebut the appeal to colonization with a counter-appeal to the predator–prey analogy in biology. No predator, they argue, seeks prey that cannot be obtained within its own lifetime. Consequently, colonization which takes longer than the colonizer’s lifetime is biologically useless. Hence Hart and Tipler’s appeal to colonization fails, because galactic colonization would require too long a commitment. Now it might be suggested that the extraterrestrials have exceptionally long life-spans, although it is improbable that they would survive for the millions of years that would be required for galactic colonization. However, the weakness of Hoyle and Wickramasinghe’s objection is in the fact that human colonization projects are frequently carried out with reference to the benefits of future generations. Conquests, which take many generations, are conducted with references to historical destiny, one thousand year Reichs, with today’s sacrifices balanced against the benefits to be gained by future generations.
Perhaps they have no need to colonize. For example, our Sun and nearby planets can provide enough energy and materials for the inhabitants of the Earth for several millions of years. Non-aggressive, non-expansionist civilizations consisting of pacifists might have no desire to communicate with us. Carl Sagan
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