One possible solution to this puzzle is the zoo hypothesis, which proposes that extraterrestrial civilizations exist but are deliberately hiding from us, perhaps to avoid interfering with our development or to observe us as part of a cosmic experiment.
The zoo hypothesis was first suggested by John Ball in 1973, who argued that any sufficiently advanced civilization would have the ability to conceal its presence from less advanced ones, and that this might be a common ethical or practical choice among such civilizations.
He wrote: “They are content to leave us alone, perhaps until we reach some level of maturity or technology at which point they will reveal themselves to us”.
There are several possible reasons why an alien civilization might choose to adopt the zoo hypothesis. One is the prime directive, a concept popularized by the science fiction series Star Trek, which states that no interference should be made with the natural evolution of other cultures or societies.
Another is the precautionary principle, which suggests that any contact with an unknown civilization might have unpredictable and potentially harmful consequences, both for them and for us.
A third is the scientific curiosity, which implies that observing a civilization without influencing it might provide valuable insights into its history, culture, psychology, and biology.
However, there are also several problems and challenges with the zoo hypothesis. One is the coordination problem, which asks how multiple alien civilizations could agree on a common policy of hiding from us, and how they could enforce it among themselves.
Another is the motivation problem, which questions why an alien civilization would care about us at all, and why they would not have any interest in communicating or interacting with us.
A third is the leakage problem, which points out that hiding from us might not be easy or foolproof, and that there might be signs of their existence or activities that we could detect with our current or future technology.
One way to test the zoo hypothesis is to look for such signs of leakage, such as artificial radio signals, megastructures, spacecrafts, probes, or artifacts.
This is the main goal of SETI (Search for Extraterrestrial Intelligence), which has been scanning the sky for decades without success.
However, this does not necessarily mean that the zoo hypothesis is false, as it might be possible that alien civilizations are using more advanced or stealthy methods of communication or engineering that we cannot detect or recognize.
An additional way to test the zoo hypothesis is to try to break out of it, by sending messages or signals that might attract their attention or provoke their response.
This is the main idea behind METI (Messaging Extraterrestrial Intelligence), which has been sending intentional transmissions to nearby stars since 1974. But this approach is controversial and risky, as it might expose us to hostile or harmful aliens, or violate their rules or preferences.
The zoo hypothesis is one of the most plausible solutions to the Fermi paradox, but it is also one of the most difficult and uncertain to verify. It implies that we are not alone in the Universe, but we are isolated and ignorant of it.
The only way to find out if it is true or not is to keep exploring and searching for evidence of extraterrestrial intelligence, either by looking for signs of their presence or by trying to contact them directly.