Beyond “Fermi’s Paradox” III: What is the Great Filter?

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 there is something in the Universe that prevents life from reaching the point where we would be able to hear from it.

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. Seventy years later, we still haven’t answered that question, which has led to many theories as to why the “Great Silence” endures. A popular one is that there must be “Great Filter” that prevents life from reaching an advanced stage of development.


The term was coined by economist Robin Hanson, who is also a research associate at Oxford University’s Future of Humanity Institute (FHI). In 1996, Hanson published an essay titled “The Great Filter – Are We Almost Past It?” where he proposed that there must be something that prevents non-living matter from coming together to form living organisms (abiogenesis) and reaching a high level of development on the Kardashev Scale (see below).

This possibility, he claimed, does not bode well for humanity. As he summarized his argument:

“Humanity seems to have a bright future, i.e., a non-trivial chance of expanding to fill the universe with lasting life. But the fact that space near us seems dead now tells us that any given piece of dead matter faces an astronomically low chance of begating such a future. There thus exists a great filter between death and expanding lasting life, and humanity faces the ominous question: how far along this filter are we?”

Another great description was offered by Nick Bostrom, a philosopher who also hails from the FHI. As he described it in his 2008 essay, “Where Are They? Why I Hope the Search for Extraterrestrial Intelligence Finds Nothing“:

“The Great Filter can be thought of as a probability barrier. It consists of [one or] more highly improbable evolutionary transitions or steps whose occurrence is required in order for an Earth?like planet to produce an intelligent civilization of a type that would be visible to us with our current observation technology.”

Gotta Love the Drake!

When Fermi posed his famous question, scientists were already operating under the assumption that life must be plentiful in the Universe. This should come as no surprise given the sheer size and age of the Universe, which measures 93 billion light-years in diameter (that’s just the “observable” part) and has been around for an estimated 13.8 billion years. With that space is an estimated 2 trillion galaxies!

Frank Drake writing his famous equation on a white board. Credit:

Our own galaxy, meanwhile, measures between 170,000 and 200,000 light-years in diameter and contains between 200 and 400 billion stars. Even if we were to assume that only 1% of those stars had planets, that 1% of those planets could support life, that 1% of them produced intelligent life, and that 1% of this life blossomed into an advanced civilization, that still means 2,000 civilizations could be out there!

This argument was formalized about a decade later by American astronomer and SETI researcher Dr. Frank Drake. During a meeting at the Green Bank facility in 1961, a number of scientists met to discuss the use of radio telescopes to search for signals that appeared to be artificial in origin. In preparation for this meeting, Drake prepared an equation that summed up what SETI researchers were working with.

The Drake Equation, and it would thereafter be called, can be summarized mathematically as follows:

N = R* x fp x ne x fl x fi x fc x L

Where N is the number of civilizations in our galaxy that we could communicate with, R* is the average rate of star formation, fp is the fraction of stars that have planets, ne is the number of planets that can support life, fl is the number that will develop life, fi is the number that will develop intelligent life, fc is the number technologically-advanced civilizations, and L is the time they would have to transmit signals into space.

The Drake Equation, a mathematical formula for the probability of finding life or advanced civilizations in the universe. Credit: University of Rochester

As Dr. Drake would later say of the equation’s creation:

“As I planned the meeting, I realized a few day[s] ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it’s going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy. This was aimed at the radio search, and not to search for primordial or primitive life forms.”

A Question of Scale

The Kardashev Scale takes its name from the Soviet and Russian astrophysicist Nikolai Kardashev who proposed that extraterrestrial civilizations could be classified based on the amount of energy it’s able to harness. In his 1964 essay, titled “Transmission of Information by Extraterrestrial Civilizations,” he proposed a three-tiered scheme – the “Kardashev Scale” – that 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)

In his 1973 book, titled The Cosmic Connection: An Extraterrestrial Perspective, famed science communicator Carl Sagan claimed that there should be a Type 0 included on the scale since humanity had not yet achieved a Type I level of development. As Sagan put it:

“A Type I civilization is able to muster for communications purposes the equivalent of the entire present power output of the planet Earth – which is now used for heating, electricity, transportation, and so on; a large variety of purposes other than communication with extraterrestrial civilizations. By this definition the Earth is not yet a Type I civilization… Our present civilization would be classed as something like Type 0.7.”

Based on these parameters, a Type I civilization would have not only grown to occupy the entire surface area of its planet but would have also colonized Low Earth Orbit (LEO). Such a civilization could be identified by exoplanet-hunters by looking for clouds of satellites around the planet (aka. Clarke Belts), which would be visible during planetary transits.

A Type II civilization, according to Kardashev, is best exemplified as one that is capable of building a megastructure around its home star (i.e. a Dyson Sphere). This would allow the civilization to harness all of the energy produced by its sun, as well as multiplying the amount of habitable space in its home system exponentially.

The Wow! signal represented as “6EQUJ5”. Credit: Big Ear Radio Observatory/NAAPO

A Type III civilization is harder to characterize, but various theorists have argued that a sufficiently advanced ETIs could build megastructures around their entire galaxy, or around the core region of their galaxy in order to harness the energy of its supermassive black hole (SMBH). Regardless, it is fair to say that a civilization capable of harnessing the energy of its entire galaxy would be impossible not to notice.

Where To Draw the Line?

In his essay, Hanson argued that the “filter” must lie somewhere between the point where life emerges on a planet (abiogenesis) and the point where it becomes an interplanetary or interstellar civilization. Using life on Earth and the emergence of humanity as a template, Hanson outlined a nine-step process that life would need to follow to reach the point of becoming a space-faring civilization. These included:

  1. Habitable star system (organics and habitable planets)
  2. Reproductive molecules (e.g. RNA)
  3. Prokaryotic single-cell life
  4. Eukaryotic single-cell life
  5. Sexual reproduction
  6. Multi-cell life
  7. Animals capable of using tools
  8. Industrial civilization
  9. Wide-scale colonization

In accordance with the Great Filter hypothesis, at least one of these steps must be improbable. Either life has a difficult time emerging from inorganic materials early on, or the odds of catastrophic failure increase as a species becomes more and more advanced. Either of these possibilities have significant consequences for the human race.

If the filter is in an early step, then the existence of complex life forms (including humans) is a rarity and we beat the odds just by being here. If, on the other hand, the filter is located at a later step, then many ETIs must have reached our current level of development, but failed to progress farther (for whatever reason). This would mean that the point where extinction becomes likely lies ahead of us.

For that reason, there are many who feel that the discovery of life beyond Earth would not be a reason to celebrate. Or as Nick Bostrom summarized:

“You start with billions and billions of potential germination points for life, and you end up with a sum total of zero extraterrestrial civilizations that we can observe. The Great Filter must therefore be powerful enough — which is to say, the critical steps must be improbable enough — that even with many billions rolls of the dice, one ends up with nothing: no aliens, no spacecraft, no signals, at least none that we can detect in our neck of the woods.”

In any case, it is a foregone conclusion that no species in our galaxy has reached the ninth step. Otherwise, the evidence of its existence would be everywhere. So it’s entirely possible that intelligent species don’t make the transition from step eight to step nine. So it is entirely possible that intelligent species don’t make it from step eight to step nine – i.e. Type I to a Type II civilization).

Possible Resolutions

In the end, the Great Filter and the Fermi Paradox are inseparable and attempts to resolve one invariably intrude on the other. For example, placing the filter at an early stage in Hanson’s nine-step process would be to conclude that humanity has found no evidence of intelligent life because it does not exist – which is the very basis of the Hart-Tipler Conjecture.

On the other hand, if it is the case that intelligent life exists out there, but the conditions under which it developed are rare, then we are left with the inevitable conclusion that we simply haven’t found any evidence yet. This is the exact logic that lies behind the Rare Earth Hypothesis, which is another possible resolution to the Fermi Paradox.

Or, as already noted, it could be that it is the nature of intelligent life to destroy itself as a result of technological advancement. This could be the result of nuclear war, climate change, the development of artificial intelligence, or other means. As Sagan and Shklovsky summarized in their 1966, The Quest for Extraterrestrial Intelligence:

“The temptation is to deduce that there are at most only a few advanced extraterrestrial civilizations – either because we are one of the first technical civilizations to have emerged, or because it is the fate of all such civilizations to destroy themselves before they are much further along.”

Photo of the central region of the Milky Way. Credit: UCLA SETI Group/Yuri Beletsky, Carnegie Las Campanas Observatory

Using humanity as a template, one could argue that the many existential threats we face are typical of civilizations at our level of development. There are many other proposed resolutions, such as ETIs are not non-existent or dead, but hibernating – aka. the Aestivation Hypothesis. There’s the theory that they could be avoiding contact so they don’t interfere with our evolution (the Zoo Hypothesis), or to protect themselves!

It has even been suggested that humanity has been deliberately isolated by an ETI (aka. the Planetarium Hypothesis) so they can study us more closely. Of course, these are theories that – like the Fermi Paradox itself- cannot be resolved until evidence is found for the existence of extraterrestrial intelligence. As famed scientist and SF author Arthur C. Clarke once said:

“Two possibilities exist: Either we are alone in the universe or we are not. Both are equally terrifying.”

We have written many interesting articles about the Great Filter, the Fermi Paradox, the Search for Extraterrestrial Intelligence (SETI), and related concepts 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.

Matt Williams

Matt Williams is a space journalist and science communicator for Universe Today and Interesting Engineering. He's also a science fiction author, podcaster (Stories from Space), and Taekwon-Do instructor who lives on Vancouver Island with his wife and family.

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