What are the Fermi Paradox and the Great Filter?

European Southern Observatory (ESO) - via Wikimedia Commons.

The great astronomer Carl Sagan once spoke for wanderers across all borders, ethnicities, races, and religions when he said, “We were wanderers from the beginning. We were bounded only by the earth, and the ocean, and the sky. The frontier was everywhere.” 

Humanity’s addiction with exploration and wonder is intrinsically tied to our nature. Our dauntless exploration is a testament to our nature; to survive, we must explore new worlds, whether they be across the ocean of water or the ocean of space. When the drought was prolonged or the food was scarce, we moved on. We were hunters and foragers, wanderers and survivors, explorers of the unknown, drawn by innate curiosity to life, survival, a better life. We were voyagers, adventurers, explorers, frontiersmen, pioneers. 

As we grew more advanced, we began to voyage the seas. We wandered across the Atlantic Ocean, we explored the Western Pacific, we circumnavigated Africa, we ventured throughout the Americas. Many of our great explorers – Ferdinand Magellan, Zheng He, Marco Polo, Henry the Navigator and John Cabot, for example – explored worlds some once did not know existed. Our excitement with novelty and expedition continued, leading us soon to space. 

On that night in October of 1957, we entered a new chapter of our existence; we were now members of the cosmic arena, much larger than that small arena of our planet. 

As we begin to explore the heavens – and understand their collective magnitude – we begin to wonder if we are alone. The universe is exponentially larger than we had previously thought; should not there be extraterrestrial life somewhere? 

Yet, despite our intense, decades-long search, we have yet to find any evidence of extraterrestrial intelligence. SETI, METI, none of our radio telescopes or optical telescopes, nothing; we have seen nothing; we have heard nothing.

In a universe so vast and massive, it is daunting to imagine that we are alone. Arthur C. Clark testified to that and to a reality where we are not, “Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.” The Fermi Paradox and the Great Filter, perhaps much to our fears, seek to explain why it appears that we are presently alone in the universe.

The Fermi Paradox

For as long as we have known that our planet, our star, are not alone in the universe, we have wondered whether our inhabited planet is the only or one of many in the universe. Nevertheless, we have come up short in that search insofar as we have searched, even though we know that quadrillions – at least – of planets exist in the universe – many of which may have suitable conditions for life. The Italian physicist Enrico Fermi, in an effort to explain our apparent loneliness, realized and propagated an apparent contradiction between the absolute absence of evidence of extraterrestrial life and the high probability of its existence – as observed by the Drake Equation.

The Drake Equation is a mathematical equation devised by American astronomer Dr. Frank Drake. The Drake Equation consists of seven variables, all multiplied by one another to hypothesize the number of intelligent civilizations in the Milky Way galaxy. The variables are the average rate of star formation in our galaxy, the fraction of existing stars that have planets, the average number of planets that may potentially support life per each star that contains planets, the fraction of these planets that could potentially support life that actually do develop life, the fraction of planets with life that develop intelligent life, the fraction of the intelligent species that are capable of sending out strong radio signals, and the length of time for which the intelligent civilizations release those signals. 

All the variables are, as was mentioned earlier, multiplied together to determine – theoretically – the number of intelligent civilizations that are capable of sending out radio signals, in our galaxy. The Drake Equation gives more a subjective estimate than an empirical value, for only two of the variables have accurate estimates – rather than pure inferences, as is evident with the other five. 

Using the Drake Equation, the number of civilizations that potentially exist in the Milky Way ranges from 20 to 50,000,000. Scientists theorize that anywhere from 10,000 to 15.6 million intelligent civilizations exist in the Milky Way alone.

The Fermi Paradox is merely the contradiction evident when we find no evidence of extraterrestrial, despite its accepted high degree of prevalence. The fact that we haven’t found extraterrestrial life before signifies its paradoxical nature. 

An attempt to explain the Fermi Paradox is the Great Filter hypothesis, but – I must first warn – it is not the happiest of hypotheses.

The Great Filter

The most popular explanation of the Fermi Paradox is the Great Filter. It attempts to explain that, rather than the abundance of alien civilizations evidenced by the Drake Equation, there are few or even no other advanced alien civilizations in the entire universe. The Great Filter relies on events – “filtering” events – that are either impossible to pass or nearly so. The events may be so early that life itself is almost impossibly rare, or so late that life can make it to intelligence and technology – maybe even more advanced than us – only to almost always destroy themselves – or be destroyed. The latter begs what is presently among the most important questions in science: if the Great Filter is real, then are we ahead of the filter, behind it, or in it?

The Great Filter proposes that one factor of the seven in the Drake Equation is exceptionally unlikely, or even impossible. For example, we may assume that simple life develops into intelligent life 1% of the time; but if the Great Filter event occurs here, perhaps the chance of the development of intelligent life from simple life is only one in every ten million, or even less than that. Certain factors in the Drake Equation – particularly those already observed on great scales – cannot be considered Great Filter events, nevertheless. The current rate of star formation, the number of planets per star and the number of planets which may be suitable for life, for example, all are proven to be almost ubiquitous in the Milky Way. The Great Filter events – assuming there are such events – must, therefore, deal with the development of life.

When the Great Filter may occur

The Great Filter hypothesis, formulated by Robin Hanson, proposes nine possible filtering events. All the events are dependent upon all past events in the list.

  • The right star system – which must have habitable planets and elements capable of forming organic compounds (RULED OUT BY OBSERVATIONAL DATA.

  • Development of reproductive molecules (RNA, DNA).

  • Development of prokaryotic, unicellular life.

  • Development of eukaryotic, unicellular life.

  • Development of sexual reproduction or an equivalent form of biogenesis.

  • Development of multicellular life.

  • Development of a tool-using, intelligent species.

  • A newly-advanced civilization advancing toward the colonization explosion – the rapid colonization of surrounding planets, asteroids and moons.

  • An intelligent species reaching the colonization explosion.

All except the first are candidates for the Great Filter event. So far, we have yet to find significant evidence of two through nine that may disprove its potential to be the event. With the second event, we have seen the building blocks of DNA and RNA – nucleic acids – but we have yet to see DNA or RNA; therefore, everything from two and on can be considered candidates for the event.

Existentiality of a Great Filter Event

The timing of the Great Filter event – or events – is incredibly important; not only does it indicate the probable maximum advancement for the prospect of life on another planet, but it also leads us to question whether we have passed the Great Filter or not. Applying the Great Filter to human civilization, we find three primary considerations:

  • The Great Filter event is behind us; we are one of the first.

  • The Great Filter event, or events, are ahead of us; we are one of many, but we haven’t yet experienced our own Great Filter event

  • We are currently engaged in a Great Filter event – our very existence is in grave danger.

The last two are the most horrifying possibilities; and, unfortunately, it is much more likely that the Great Filter is ahead of us than it is behind us.

Insofar as we have searched for life beyond our planet, we have come up short. We have found no evidence that the universe is not, other than us, completely dead. It is very likely, because we have found no advanced colonies of intelligent species existing throughout the galaxy, that the Great Filter occurs in the ninth step. 

The number of celestial bodies capable of harboring life as advanced as what is developed in the seventh event is further evidence that life itself – even with tool-bearing species – is not so hard to achieve; humans, for example, are not even the only tool-bearing species on earth alone. The development of life itself seems quite probable, especially considering the number of planets and moons we have found to already have water; although, of course, we cannot be empirically certain.

Yet, as we found earlier, the vehicles for steps two through eight are evident virtually everywhere in the galaxy. Therefore, step nine – assuming that steps two through eight are common –  is the Great Filter event; we are currently at step eight and closing in on nine. If the Great Filter is, indeed, a real phenomenon, then the event is ahead of us – and very near.

It is often postulated that a Great Filter so late in the steps must be anthropogenic – originating from human action (self-inflicted). We know now that there is a 19% chance humans will become extinct in the 21st century, by far the highest probability of extinction in human history; and, unsurprisingly, every major cause for our extinction is anthropogenic. 

The reality that the risk of our extinction is so high right now may be an indicator of a potential Great Filter event preceding step nine. Yet, before we lose our minds, 19% is nowhere near high enough for this century to be that of the Great Filter event; perhaps, however, we are missing a much, much likelier event, a self-infliction or external injury that quickly exterminates all of us.

We are probably around 200-300 years away from being a multiplanetary species, fulfilling the requirements for a step nine civilization. If the Great Filter event precedes step nine or occurs early on in step nine, then our chances of extinction in the next few centuries are extraordinarily high; yet, as we know, the chance in even the next five centuries is only 50% – still exceedingly high, but much too low to support a Great Filter event in the near future.

Inversely, if the earlier steps were probable, then the galaxy would be teeming with life; so far, we have yet to detect life beyond earth.

We still have yet to find any evidence supporting that events two through eight are likely. It is also possible – perhaps just as much so – that the Great Filter is behind us. While it appears – appearance means nothing – that we are behind the Great Filter event, there is no evidence that we are not ahead of it, either.

Wrapping it up

We, of course, need more data to understand whether the Great Filter even exists and when it will happen. For now, we must ensure that if it is ahead of us, we are prepared to fight it. Considering the likely cause of our extinction is war and irresponsibility, we must act to be responsible and peaceful. We must continue to push democracy and peace throughout our planet in the hopes of achieving step nine, thereby reaching the colonization explosion and carrying our people into a newer, less limited domain. No country will be able to counteract a Great Filter event; we need all of you. Ultimately, when the Great Filter hits, perhaps we will realize just how frivolous our institutions are, how pointless our countries, nations, races, divisions are; maybe then will we realize that our needless classifications are severe hindrances rather than natural consequences of human society. As always, take care and stay curious, my friends.

If you have any questions, comments, or corrections, please comment on this post or email learningbywilliam@gmail.com with your concerns. Thank you.


(n.d.). Retrieved from https://www.youtube.com/watch?v=uA5XuOIilYc

Drake equation. (2022, July 05). Retrieved from https://en.wikipedia.org/wiki/Drake_equation

Fermi paradox. (2022, July 19). Retrieved from https://en.wikipedia.org/wiki/Fermi_paradox

Great Filter. (2022, July 15). Retrieved from https://en.wikipedia.org/wiki/Great_Filter