Someone told me it's all happening at the zoo.
I do believe it, I do believe it's true.
In 1973, John Ball proposed the zoo scenario as a means of resolving the Fermi paradox.58 (In fact, Ball called it the "zoo hypothesis"; variants of the idea, some of which are described below, also call themselves "hypotheses," and they appear as such in the literature. I prefer to call them scenarios, because in science an hypothesis usually implies a speculation framed in such a way that it can be tested. As we shall see, Ball's speculation cannot be tested. This is not to say the zoo scenario is untrue or is somehow more unlikely than other explanations. Indeed, we shall meet ideas that appear far more wild and improbable than Ball's speculation; but they merit the term "hypothesis" because they present testable predictions.)
Ball argued that ETCs are ubiquitous; many technological civilizations will stagnate or face destruction (from within or without), but some will develop their level of technology over time. Arguing in analogy with terrestrial civilizations, he reasoned that we need only consider the most technologically advanced civilizations. The advanced ETCs will, in some sense, be in control of the Universe; the less advanced will be destroyed, tamed or assimilated. The important question becomes: how will highly developed ETCs choose to exert their power? Arguing in analogy with how mankind exerts its power over the natural world, wherein we set aside wilderness areas, wildlife sanctuaries and zoos so that other species can develop naturally, Ball speculated that Earth is in a wilderness area set aside for us by ETCs. The reason there seems to be no interaction between them and us is that they do not want to be found, and they have the technological ability to ensure that we do not find them. The zoo scenario suggested that advanced ETCs are simply observing us. (Variants on the idea were less appealing; the laboratory scenario would have us as the subjects of laboratory experiments.)
This general idea has a long history in science fiction, predating Ball's publication. For example, Star Trek had the "Prime Directive," which stated that the Federation should not interfere with the natural development of a planet. (The Directive was more honored in the breach than the observance, of course, since the writers had to generate plots.) And before that, an established trope of the Astounding of the 1950s (under the strong but quixotic editorship of John Campbell, Astounding was the leading SF magazine of the day59) was of Earth under quarantine — either because ETCs were protecting us or, more commonly, because mankind was a threat to them. One could also argue that Tsiolkovsky's solution to the paradox — that ETCs have set Earth aside in order to let mankind evolve to a state of perfection — contains the seeds of the zoo scenario.
Believers in flying saucers tend to favor the zoo scenario as if it legitimizes their belief. Yet the zoo scenario specifically predicts that we should not see flying saucers or any other manifestation of superior technology. If flying saucers are spacecraft then the zoo scenario is wrong. (James Dear-dorff proposed a variant of Ball's idea, known as the leaky embargo scenario, which is compatible with observations of flying saucers. The idea is that advanced and benevolent ETCs have put in place an embargo on official contact with mankind. But the embargo is not total: aliens contact those citizens whose stories are unlikely to be credible to scientists and the government. The aliens want to slowly prepare us for the shock that might come later when they reveal themselves. Deardorff's proposal is so unscientific — though again not necessarily untrue — that it probably does not merit even the term "scenario."60)
The zoo scenario has been attacked on several grounds. A major drawback is that it leads us nowhere; it is not a testable hypothesis. A good hypothesis generates ideas for observations that might confirm or falsify it, and in doing so generates new hypotheses. It is difficult to think of any observation that could test the validity of the speculation. Its one prediction is that we will not find ETCs, but the failure to find them hardly confirms the initial statement. There is something unsatisfying about an approach in which, no matter how hard we look, no matter how thoroughly we search, the absence of ETCs is explained simply by saying they do not want us to see them. (I can explain the lack of observational evidence for fairies at the bottom of my garden by saying they become invisible whenever people look their way. Irrespective of whether fairies exist, this is a poor sort of explanation from a scientific standpoint.)
Another criticism is that it is anthropocentric. Why should an ETC have any interest at all in a backward species like us? (Assuming, of course, that it is us they are interested in and not dolphins or monkeys or bees.) However, since we have absolutely no conception of what alien minds might find diverting, we cannot rule out the possibility that Earth — for whatever reason — has been set aside as the Galactic equivalent of a national park. A more serious weakness is that the zoo scenario fails to explain why Earth was not colonized long before complex life-forms appeared. Perhaps the scenario describes the reaction of ETCs to the discovery of intelligent life on Earth, but would the reaction be the same if all they found were primitive single-celled organisms?
A further criticism is that it takes only one ETC to break the embargo, and just one immature civilization to poke its fingers through the bars of the cage, for us to see them here on Earth. Furthermore, it fails to explain why we do not observe any evidence of them out there in the Galaxy. Ball proposes that advanced, intelligent life is ubiquitous. So where are their engineering projects? Where are their communications? It is one thing for them to keep Earth free from development, but quite another for them to stop all activity on our account.
Finally, it suffers in a way common to all solutions to the Fermi paradox that depend upon the motivations of alien intelligences. It supposes that all ETCs at all times behave in the same way with regard to us.
An expanded version of the idea, known as the interdict scenario, attempts to generalize Ball's idea and address some of the weaknesses.
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