A new paper has taken a fresh look at the Fermi Paradox, arguing that if we continue to find no evidence of advanced alien life as our technology progresses we will soon be left with two options: The Zoo Hypothesis, or nothing.
If you haven't heard of the Fermi Paradox, it goes something like this: given the vastness of the universe, the sheer amount of time it has gone on for, and the septillion stars out there, how come we can see no signs of alien civilizations, and why haven't they got in touch? We have found many exoplanets in the brief time we've been looking. Surely there must be someone else out there who, like us, desperately wants to find others?
Since it was posed in 1950 by Enrico Fermi, there have been a range of answers, from the benign to the absolutely terrifying. One is that there simply hasn't been enough time yet. Alien civilizations may prioritize, as we do, searching for techno signatures, which we simply haven't been broadcasting for long enough. On the other end of the spectrum, it could be that the tendency throughout the universe is for civilizations to destroy themselves before they reach sufficient advancement to make contact.
One explanation, proposed by John Allen Ball in 1973, is the Zoo Hypothesis. In this idea, aliens may be aware of us but are hiding themselves from us.
"Among currently popular ideas about extraterrestrial intelligence, the idea that 'they' are trying to talk to us has many adherents," Ball wrote in his paper. "This idea seems to me to be unlikely to be correct and the zoo hypothesis is in fact the antithesis of this idea."
"I believe that the only way that we can understand the apparent non-interaction between 'them' and us is to hypothesize that they are deliberately avoiding interaction and that they have set aside the area in which we live as a zoo."
Just as we set aside areas of land as nature reserves and leave uncontacted tribes uncontacted, advanced civilizations may choose to allow us to evolve on our own and watch our progress, as we watch animals in a zoo. As civilizations become sufficiently mature – technologically or politically – they would make contact. Like on Earth, these previously uncontacted civilizations would "eventually be engulfed and destroyed, tamed, or perhaps assimilated".
This would imply that there was a ruling civilization/collection of civilizations, who all agreed to respect the zoos, much like the "prime directive" in Star Trek.
Absence of evidence is not evidence of absence, but since the Fermi Paradox and the Zoo Hypothesis were proposed, the lack of evidence of advanced civilizations (e.g. in the form of probes) has only grown more puzzling. For instance, we have discovered over 5,000 exoplanets. If it turns out we find signs of simple alien life on these planets, could it imply that simple life is abundant but advanced technological civilizations are rare?
Perhaps, as suggested in the new paper, planets with enough oxygen to allow for combustion are rare. Earth is the only planet we know of that has fire, and it was billions of years before it did. Without fire, maybe alien life is unable to create advanced technologies and machinery, even if they are intelligent. The team suggests other potential bottlenecks, such as intelligence relying on developing certain rare characteristics like humans' brain size and dexterous hands, as well as language, which could be a rare leap, given that it has only happened on Earth the once.
"In the absence of such a late-stage evolutionary filter, the Fermi paradox would return, arguably leaving the zoo hypothesis as the last remaining plausible explanation," the team write in their paper.
The zoo hypothesis has been criticized for being untestable. As our methods of detection progress, you could always say "perhaps the aliens just have more advanced ways of evading our detection". However, this paper argues that even if alien civilizations were actively hiding they would have a harder time doing it, particularly hiding their waste heat production, as our own detection capabilities increase.
"Even if they can hide evidence of their technology (space probes, communications traffic and so forth), hiding the large number of inhabited planets in the background implied by such a scenario would probably prove challenging," they add.
The team argues that we need to continue our search for extraterrestrial life outside the Solar System, as well as inside and around nearby stars, given the constraints finding life would place on how prevalent early life is through the universe. An answer could be closer than we thought.
"The longer we do not detect any signs of advanced intelligent life around us, the less likely the zoo hypothesis becomes as an explanation, forcing us to conclude that technological intelligent life is rare in the Universe," the team writes. "It is in this sense that we propose that the solution to the Fermi paradox is the zoo hypothesis or nothing. Fortunately, by undertaking comprehensive searches for biosignatures and technosignatures, both within the Solar System and beyond, we may have it in our power to distinguish between these possibilities within the next few decades."
The study is published in Nature Astronomy.