Beyond “Fermi’s Paradox” V: What is the Aestivation 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 the reason we’re not hearing from aliens is that they’re asleep and waiting for the Universe to get better.

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 high probability estimates for the existence of extraterrestrial intelligence (ETI) and the apparent lack of evidence. But despite seventy years of looking, we still haven’t been able to answer Fermi’s question, leading to multiple proposals as to why this is. Today, we look at the “Aestivation Hypothesis,” which argues that aliens are not dead (or non-existent), they’re just resting!

This theory takes its cue from nature, where certain organisms enter a state of prolonged torpor during particularly hot or dry periods. Similar to hibernation in the winter, these organisms will remain in this state until conditions become cooler and wetter. Applied to the Fermi Paradox, the Aestivation Hypothesis asserts that alien civilizations are largely dormant because they are awaiting better conditions.

Fermi and Drake

At the heart of Fermi’s famous question was a discrepancy that was undeniable in his time, and hasn’t changed despite seventy years of research. On the one hand, there is the assumed likelihood that extraterrestrial intelligence (ETI) is plentiful throughout the Universe. On the other, there’s the lack of hard evidence attesting to their existence.

Assuming that ETIs are likely is not at all farfetched. Based on the sheer size and age of the observable Universe – 93 billion light-years in diameter and 13.8 billion years – scientists have typically treated the existence of extraterrestrial intelligence (ETI) as a foregone conclusion. Statistically speaking, the odds are very much in favor of their being millions of civilizations out there.

Dr. Frank Drake illustrated as much in 1961 during a meeting at the Green Bank Observatory. While addressing fellow astrophysicists and SETI researchers, he presented his famous equation for estimating the number of ETIs in our galaxy that we can communicate with at any given time. The Drake Equation, as it came to be known, was expressed mathematically as:

N = R* x fp x ne x fl x fi x fc x L
  • N is the number of civilizations in our galaxy that we might able to communicate with
  • R* is the average rate of star formation in our galaxy
  • fp is the fraction of those stars which have planets
  • ne is the number of planets that can actually support life
  • fl is the number of planets that will develop life
  • fi is the number of planets that will develop intelligent life
  • fc is the number civilizations that would develop transmission technologies
  • L is the length of time that these civilizations would have to transmit their signals into space
Frank Drake writing his famous equation on a white board. Credit:

While most of these parameters are subject to varying degrees of uncertainty, the point of the equation is clear. Even when figured for conservatively, the results always indicate that there should be at least a few extraterrestrial intelligences (ETIs) in our galaxy that we should be able to communicate with. Unfortunately, despite decades of research and multiple dedicated SETI surveys, Fermi’s Paradox still holds.

As a result, multiple attempts have been made to resolve the Paradox theoretically. The first and perhaps best known is the Hart-Tipler Conjecture, named jointly for astrophysicist Michael Hart and mathematician/cosmologist Frank Tipler. This theory argues that there is no evidence of intelligent life out there because none exists.

Another is the Great Filter Hypothesis, theorized by Oxford economist Robin Hanson, who argued that while simple life may be very common, advanced life was not. In other words, there exists in the Universe some type of “filter” that prevents simple life from reaching the advanced stage and become an ETI that we would be capable of communicating with.

The built-in assumption in both of these cases is that ETIs do not exist, hence why we see no evidence of them. But as Carl Sagan famously remarked when addressing the possible existence of alien intelligence, “the absence of evidence is not the evidence of absence.” As such, many theorists have proposed alternate explanations of how ETIs can exist, but remain undetected by us.

Kardashev Scale (Redux)

This raises another issue, which is the notion that advanced species will be able to harness increasingly large amounts of energy over time. In his 1964 essay, titled “Transmission of Information by Extraterrestrial Civilizations,” Soviet/Russian astrophysicist Nikolai Kardashev proposed a three-tiered scheme for classifying extraterrestrial civilizations based on the amount of energy they could harness.

This scheme came to be known as the “Kardashev Scale” and 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)

Civilizations that fit these Types would be detectable by looking for signs of technological activity (aka. “technosignatures“). For example, a Type I Civilizations could be detectable through Direct Imaging, where astronomers would look for light reflected by massive clouds of satellites (aka. “Clarke Belts“) around the planet. A Type II civilization, meanwhile, would be capable of building a megastructure around its home star.

These civilizations would be capable of building what Freeman Dyson described in 1960 (what has since come to be known as a “Dyson Sphere“). This would allow a civilization to harness all of the energy of its sun while multiplying the amount of habitable space in their system exponentially. A Type III Civilization, meanwhile, could be easily detected by looking for signs of megastructures that encompass an entire galaxy (or parts thereof).

So it possible that the Universe if filled with civilizations ranging from Type I to Type III levels of development, but are not currently engaged in any technological activity? That’s where the concept of “aestivation” comes into play.


The theory was first suggested by research associates Anders Sandberg and Stuart Armstrong – as well as famed astronomer, astrophysicist, and philosopher Milan Cirkovic – from the Future of Humanity Institute (FHI) at the University of Oxford. In their 2017 study titled, “That is Not Dead Which Can Eternal Lie: the Aestivation Hypothesis for Resolving Fermi’s Paradox,” they proposed this as a possible resolution to the Fermi Paradox.

The study was partly based on previous research conducted by Sandberg and Armstrong in a 2013 study where they extended the Fermi Paradox beyond the Milky Way. Titled “Eternity in Six Hours: Intergalactic Spreading of Intelligent Life and Sharpening the Fermi Paradox,” Sandberg and Armstrong argued that an advanced civilization would be able to colonize a galaxy and even travel between galaxies with relative ease.

Having concluded that in a Universe of about 2 trillion galaxies (according to recent estimates) that has existed for 13.8 billion years, there should be many Type III Civilizations out there (based on the Kardashev Scale). Not only would these species have been able to colonize their respective galaxies in a relatively short amount of time, but have been able to reach the Milky Way by now.

The reason why this is not evidence to us, argued Sandberg and Armstrong, has to do with Landauer’s Principle, which is considered by many to be the basic principle of the thermodynamics of information processing. This rule holds that any logically irreversible manipulation of information (aka. computation) must be accompanied by a corresponding entropy increase (loss of heat) for the information-processing apparatus.

Applied to megastructures like Dyson Spheres, Matrioshka Brains, etc., the level of heat energy and entropy involved would be enormous. Meanwhile, astronomy and cosmology teach us that the Universe is getting steadily cooler over time as star formation slowly dies. At the same time, cosmic expansion causes the wavelength of light to stretch, which causes momentum and energy to be lost.

Eventually, it’s believed that this will result in the “Big Chill” (or “Big Freeze”) scenario, where even the background radiation will cool and the Universe will experience “heat death.” But from a computational point of view, long before that happens, advanced species could be waiting for the Universe to cool so their megastructures are able to function more efficiently.

According to Sandberg and Armstrong, an advanced civilization could (in principle) perform exponentially more irreversible logical operations by transferring entropy to the cosmological background in the future. In fact, by waiting until the background temperature is significantly lower, they estimate that an additional ten nonillion (1030), or ten quadrillion more computations could be performed.

How cosmic expansion is measured. Credit: NASA/ESA/A. Feild (STScI)

It is also possible that aestivation is a means for “early arrivals” to our Universe to skip the long waiting period for other intelligent species to evolve so that when they wake up, they’ll have plenty of people to talk to! Considering that life capable of communicating with the cosmos took 4.5 billion years to evolve here on Earth, this makes a fair degree of sense.

Aestivation can also be interpreted in terms of the Great Filter hypothesis, which ultimately addresses why ETIs would not visible to us. As Hanson himself explained to Universe Today:

“In the three-part taxonomy of Great Filter steps, the Aestivation Hypothesis is hypothesizing a delay step, near the end of the filter. The Filter is ‘what does it take to be visible.’ Aestivation says that they are already visible, they are just choosing not to be.”

Limits and Criticisms

Of course, the Aestivation Hypothesis (much like the Fermi Paradox and the Drake Equation) is based on some assumptions about how ETIs would behave. These include:

  1. There are civilizations that mature much earlier than humanity
  2. These civilizations can expand over sizeable volumes, gaining power over their contents
  3. These civilizations have solved their coordination problems
  4. A civilization can retain control over its volume against other civilizations
  5. The fraction of mature civilizations that aestivate is non-zero
  6. Aestivation is largely invisible

In short, the hypothesis assumes that – given the age of the Universe – enough time has passed for civilizations to emerge that are more advanced than humanity. It is also assumed that they would have become space-faring civilizations, actively colonizing neighboring star systems and possibly even neighboring galaxies.

Lastly, it is assumed that this process would be visible by looking for evidence of megastructures and massive construction processes. This would include smashing up planets for building materials, relocating stars or galaxies, or even consuming gas giants, stars, or (again) entire galaxies to create fuel.

Second, Charles Bennett – a physicist, information theorist and Fellow at the IBM Watson Research Center – along with Hanson and C. Jess Reidel (of the Perimeter Institute for Theoretical Physics) produced a rebuttal paper to the Aestivation Hypothesis in 2019. In it, they argued that Sandberg et al. implicitly assume that computer-generated entropy could only be disposed of by transferring it to the cosmological background.

According to Bennett, Hanson, and Reidel, this is based on a misunderstanding of astrophysics and the physics of computing. While such an argument might apply in the distant future, they argue, it does not apply in the present and renders the aestivation model inaccurate. As they state:

“[O]ur universe today contains vast reservoirs and other physical systems in non-maximal entropy states, and computer-generated entropy can be transferred to them at the adiabatic conversion rate of one bit of negentropy to erase one bit of error. This can be done at any time, and is not improved by waiting for a low cosmic background temperature. Thus aliens need not wait to be active.

This all-sky Fermi view includes only sources with energies greater than 10 GeV. Credit: NASA/DOE/Fermi LAT Collaboration

In the end, the Aestivation Hypothesis is like all other attempts to resolve the Fermi Paradox (and the Drake Equation, for that matter). Far from being a concrete answer, this theory is a thought experiment designed to bring Fermi’s famous question into focus and perhaps provide some testable assertions. In the end, the ultimate goal is to help refine the search for extraterrestrial intelligence (SETI).

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

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.