Beyond “Fermi’s Paradox” VIII: What is the Zoo Hypothesis?

Welcome back to our Fermi Paradox series, where we take a look at possible resolutions to Enrico Fermi’s famous question, “Where Is Everybody?” Today, we examine the possibility that we haven’t heard from aliens because a super-advanced civilization is deliberately avoiding us.

In 1950, Italian-American physicist Enrico Fermi sat down to lunch with some of his colleagues at the Los Alamos National Laboratory, where he had worked five years prior as part of the Manhattan Project. According to various accounts, the conversation turned to aliens and the recent spate of UFOs. Into this, Fermi issued a statement that would go down in the annals of history: “Where is everybody?

This became the basis of the Fermi Paradox, which refers to the disparity between high probability estimates for the existence of extraterrestrial intelligence (ETI) and the apparent lack of evidence. Since Fermi’s time, there have been several proposed resolutions to his question, which includes the Zoo Hypothesis, which states that aliens are keeping their distance to allow humans to evolve without interference.

To recap, the Fermi Paradox addresses the disparity that exists between what we believe is the statistical likelihood of life in the Universe and the lack of evidence for it. Whereas many proposed resolutions have ventured that there must be something that is preventing life from emerging, others have tried to find explanations for how life could be plentiful, but incommunicative.

Great Filter or Prime Directive?

As noted in a previous installment, the Great Filter Hypothesis was proposed in a 1996 study by economist Robin Hanson, a research associate at Oxford University’s Future of Humanity Institute (FHI). As he argued, the disparity identified in the Fermi Paradox must mean that there is something at work in the Universe that prevents life from evolving to the point that it can become advanced.

Alternately, many proposed resolutions have argued that ETIs are common in our Universe, but are either unable to communicate with humanity, or are deliberately hiding their presence from us. The suggested motivations for this have ranged from self-preservation (i.e. intelligent species are dangerous) to the idea that more advanced species must avoid interfering with less advanced species.

If this is beginning to sound familiar, it’s probably because this thinking is in line with the “Prime Directive” from the Star Trek franchise. For those who are unfamiliar, this directive (aka. Starfleet General Order No. 1) upholds the principle that Starfleet officers must refrain from contacting a species too early in their development or interfering with that development in any way – even at the expense of their lives.

Aside from being featured in Star Trek and a number of other science fiction novels, television shows, movies, and franchises, the principle of non-interference is also the subject of scholarly and scientific debate. At it’s core, the Zoo Hypothesis accepts the existence of ETIs and that the time between the emergence between the first civilizations in the Milky Way and all subsequent ones would be enormous.

Kardashev Strikes Again!

This once again raises the issue of a civilization’s long-term development, which brings us back to the work of Nikolai Kardashev and the famous Scale that bears his name. In his 1964 essay, titled “Transmission of Information by Extraterrestrial Civilizations,” he proposed a three-tiered scheme – the “Kardashev Scale” – for characterizing a civilization based on the amount of energy it was able to harness. It consisted of the following:

  • Type I civilizations, (aka. planetary civilizations) are those that can use and store all of the energy available on its planet (~4×1012 watts)
  • Type II civilizations (aka. stellar civilizations) are those that are capable of using and controlling the energy of its entire star system (~4×1026 watts)
  • Type III civilizations (aka. galactic civilizations) are those that can control the energy of an entire galaxy (~4×1037 watts)

Among SETI researchers, the notion that there could be Type I and Type II civilizations within our galaxy seems entirely rational. For starters, the Milky Way galaxy contains between 200 and 400 billion stars, and current estimates say that there could be 6 billion Earth-like planets orbiting within their respective suns habitable zones.

In addition, the Milky Way is almost as old as the Universe itself – 13.51 billion years – whereas that the Solar System is only 4.6 billion years old and “modern” humans (aka. homo sapiens) have only existed for only a tiny fraction of Earth’s history (200,000 years). Based on this, it stands to reason that humanity is not the first intelligent species to emerge in our galaxy and that there could be many ETIs that have existed for eons.

The timeline of life’s evolution on Earth would also suggest that complex, technologically-dependent life takes a very long time to develop. Therefore, the time between the emergence of the first civilizations in our galaxy and all subsequent civilizations would be on the order of eons. In fact, researchers have conducted Monte Carlo statistical simulations that show that the interval would be similar in length to geologic epochs on Earth.

We therefore have to ask, how would a species that emerged millions of years ago react to the existence of species like humanity? Would they be interested in contacting us and sharing information, or would they choose to hide their existence? Herein lies the core principle of the Zoo Hypothesis, which sets it in opposition to classical SETI theory.


The term Zoo Hypothesis was coined in 1973 by John A. Ball, a Harvard astrophysicist and scientist with MIT’s Haystack Observatory, in a study of the same name. Here, Ball addressed the most commonly-proposed resolutions to the Fermi Paradox, and also identified some common assumptions made by classical SETI researchers.

Among these assumptions is the belief that life will emerge wherever conditions are right, that there many places in the Universe where favorable conditions would exist, and that ETIs exist and we are unaware of “them”. However, another inherent assumption that Ball identified is the notion that “they” – these intelligent species that are likely older and more advanced than humanity – want to talk to us.

In rebuttal, Ball argued the antithesis to this notion, which he termed the “zoo hypothesis”:

“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.

“The zoo hypothesis predicts that we shall never find them because they do not want to be found and they have the technological ability to ensure this. Thus this hypothesis is falsifiable, but not, in principle, confirmable by future observations.”

Based on this, Ball asserted that it is highly unlikely that there are many species in the Milky Way that are as far along in their development as humanity. Statistically speaking, it is far more likely (he argued) that the majority of life is either early in its development (consistent with life on Earth eons ago) or highly-advanced (comparable to what life on Earth will be like eons from now).

Using human history as a template, Ball considered that the reason ETIs would avoid contact with humanity might involve a reverential attitude towards life and evolution, as well as a desire to avoid committing any harmful interference. The example of wildlife sanctuaries comes into play here, places where species are allowed to develop naturally without (or with minimal) contact from humans.

The Karl G. Jansky Very Large Array (VLA) in Socorro County, New Mexico, at night. Credit: NRAO/AUI/NSF; J. Hellerman

Given the likely gap that would exist between civilizations that emerged first in our galaxy and those emerging since then it makes sense that the older civilization would be in a position to monitor, control, or manipulate any species that emerged after them. However, such an approach would limit the evolutionary outcomes of younger species, which is why an advanced species might want to avoid it.

Alternately, it could be that an advanced species would want to hold off contact until they judge a younger species to be ready for the cultural, social, and psychological impact this would create. Once again, Star Trek’s “Prime Directive” comes to mind since, in this fictional universe, no first contacts were to be made until a civilization developed warp capability.


Like the Planetarium Hypothesis and similar proposed resolutions to the Fermi Paradox, the Zoo Hypothesis has drawn its share of criticism for being highly speculative and largely untestable. As Ball argued, the hypothesis is falsifiable insofar as the discovery of an ETI (living or dead) would prove it to be wrong. However, until such time as that discovery is made, the hypothesis cannot be proven.

Second, a number of critics have emphasized how the hypothesis assumes a great deal about the mentality and sociology of alien civilizations. For starters, astrophysicist Dr. Duncan Forgan showed that given the limitations imposed by the speed of light and the size of the Milky Way, it would be impossible for an advanced ETI to impose galactic hegemony and a “no contact policy” over the course of eons.

In another study, he questioned the idea that ETIs would form a unified “Galactic Club” with a common agenda and protocols. A more likely assumption, he claimed, is that ETIs would form “galactic cliques” that would have different agendas. Forgan has also pointed out that it would take only one dissident species (or one small group with a spacecraft) to violate the no contact rule.

But perhaps the strongest critic of Ball’s hypothesis was Ball himself! In the same study where he proposed that humanity was in a sort of alien-imposed quarantine, he expressed the following:

“The zoo hypothesis seems to me to be pessimistic and psychologically unpleasant. It would be more pleasant to believe that they want to talk with us, or that they would want to talk with us if they knew that we are here. However the history of science contains numerous examples of psychologically unpleasant hypotheses that turned out to be correct.”

Good point! And while Ball’s hypothesis doesn’t provide a tidy resolution to Fermi’s big question, it does help to bring it into sharper focus. It has also been picked up by proponents of “active SETI” – aka. Messaging Extraterrestrial Intelligence (METI) – who see it as a reason why we should be sending messages to space and inviting ETIs to make contact – similar to the Arecibo Message, the Pioneer Plaques, and the Voyager Golden Records.

Naturally, this raises the counter-argument that advertising our existence to the Universe could invite unwanted visitors, such as hostile ETIs or Berserker probes! But that’s a whole other can of worms and a debate for another day! In the meantime, Fermi’s question remains unresolved and all we can do is keep looking, listening, and waiting.

We have written many interesting articles about the Fermi Paradox, the Drake Equation, and the Search for Extraterrestrial Intelligence (SETI) here at Universe Today.

Here’s Where Are All the Aliens? The Fermi Paradox, Where Are The Aliens? How The ‘Great Filter’ Could Affect Tech Advances In Space, Why Finding Alien Life Would Be Bad. The Great Filter, Where Are All The Alien Robots?, How Could We Find Aliens? The Search for Extraterrestrial Intelligence (SETI), and Fraser and John Michael Godier Debate the Fermi Paradox.

Want to calculate the number of extraterrestrial species in our galaxy? Head on over to the Alien Civilization Calculator!

And be sure to check out the rest of our Beyond Fermi’s Paradox series:

Astronomy Cast has some interesting episodes on the subject. Here’s Episode 24: The Fermi Paradox: Where Are All the Aliens?, Episode 110: The Search for Extraterrestrial Intelligence, Episode 168: Enrico Fermi, Episode 273: Solutions to the Fermi Paradox.