Fermi Paradox: Are We Alone?

Everything around our planet is incredibly old. Although much about it remains unknown, the Big Bang is thought to have happened approximately 15 billion years ago (Sneden, 2001). A significant amount of time like this is difficult for any human being to process. The Earth was formed about 10.5 billion years later (Jacobsen, 2003). And four and a half billion years following the formation of the Blue Planet, evolution gave rise to humans: biological beings that are aware of their surroundings, capable of complex art expressions and technological innovations. One of the questions that have always fascinated humans is whether they are the lone species in this universe. In the vast vacuum of space, there is an incredible number of planets that might sustain life, many of which are much older than Earth (Hart, 1995). Yet, as far as the public is aware, no other biological beings have ever been observed outside of Earth. Statistically speaking, it is highly unlikely that out of the numerous planets existing in the universe, Earth is the only one where biological entities rule (de Magalhães, 2016; Hart, 1995). This improbability is known as the Fermi Paradox.


The Fermi Paradox, strange as it may sound, was neither proposed by Fermi nor is it a paradox. It originates from a question Fermi once asked: “where is everybody?”. And 27 years later, in 1975, Fermi was mistakenly cited as the person who came up with this paradox (Gray, 2015). Furthermore, it would be misguided to categorise the Fermi paradox as a paradox, because there is no intrinsic contradiction between the statements “life might exist elsewhere” and “other life has not been observed” (Gray, 2015). Nonetheless, a lot of work has been done to give answers to this seeming paradox. For years, there have been many possible solutions to the problem, some perhaps more disturbing than others. If life does exist on other planets, it is probable that the universe harbours civilisations that are much older and more advanced than humanity. Merely 300 years ago, there was no electricity, no machines, and no internet on this planet. Civilisation back then could not speculate how vastly different modern society would become. Similarly, it would be difficult for humans nowadays to predict and comprehend other civilisations that are thousands, millions, or perhaps, billions of years older. This, however, did not stop scientists and astronomists from trying to solve the Fermi paradox.


Figure 1: Physicist Enrico Fermi (Patton, 2015).

Two decades ago, Ward and Brownlee (2000) published a book about the possibility that Earth is the only planet that harbours life. This theory is termed the “Rare Earth Hypothesis”. Interestingly, it implies that humans, as curious beings, will always keep looking for something that does not exist. If this hypothesis holds true, something about Earth, and about humans, has to be unique in the universe. A crucial prerequisite for life that is often mentioned is the presence of water in liquid form. This suggests that a planet can only be a certain distance away from its star in order to be habitable (Kasting, 1997). If it is further away, the temperature will be too cold that water would turn into ice; if it is too close, the temperature will be too high, and water would only exist in gaseous form. The area in which water can exist in liquid state in a star system is called the habitable zone (Kasting, 1997). Meanwhile, many planets identified by astronomists over the past decades are in fact found within this habitable zone (Kasting, 1997). So in this regard, Earth cannot be considered unique. In other words, if Earth is indeed special in its own way, scientists have yet to reach a consensus on what makes it so.


The next possible solution to the Fermi paradox is the “Zoo hypothesis” (de Magalhães, 2016). This theory contains a number of different scenarios with a shared outline: other civilisations know that humans exist, but choose not to interfere. This is most likely a civilisation that is more advanced with a much longer history (de Magalhães, 2016; Hart, 1995). This civilisation might not be bothered about humans on Earth, similar to how humans are not bothered about what ants do in their spare time. On the other hand, it is also possible that a civilisation would destroy every other competing civilisation when it becomes too advanced. This would make messages that humans have been sending out over the past decades particularly dangerous, as they might reveal our location in the universe (de Magalhães, 2016).


Lastly, and most notably, there is the “Great Filter hypothesis” suggesting that it is highly improbable for life to develop unlimitedly (Bostrom, 2008). Simply put, somewhere along the process of evolution, there lies a Great Filter which is unimaginably difficult to cross. Possible examples of this filter include the change from unicellular to multicellular life, or the rise of sexual reproduction (Bostrom, 2008). Although the Great Filter has not been identified, it is clear that if such a filter has already been crossed on Earth, it should only have been crossed once. This does not go for photosynthesis, the formation of limbs, the ability to fly or to see. A situation for which this is the case is the emergence of life. This “life”, however, is vastly different to how humans know it now. In 1929, Haldane theorised that life started from a mixture of chemicals that were not alive, termed “primordial soup” (Haldane, 1929). Shortly afterwards, in 1953, Miller showed that inorganic materials can, under certain conditions, form organic materials. This heavily supports the primordial soup theorem (Miller, 1953).

Figure 2: A schematic overview of the Great Filter hypothesis. The yellow bars display common filters that are relatively easily passed. The red bar displays the Great Filter. The blue bar shows the unknown (Modified from Shapira, 2020).

While the emergence of life serves as an example where the Great Filter is behind the current civilisation, a more disturbing possibility is that the filter still lies ahead of humanity. What this would imply is that life at its current stage is prevalent in the universe, but civilisations never, or barely, advance to a scale where they can be detected or seen by other civilisations (de Magalhães, 2016). This would likely be a stage where the civilisation is able to travel between solar systems, i.e., an interstellar species. One theory is that asteroid impacts, for example, are so common that eventually any life on a planet would be wiped out by such an event before the stage of interstellar travel can be reached (Bostrom, 2008).


Another even more alarming theory is that technological advancements like nuclear weapons and artificial intelligence would progress to ultimately form the Great Filter (Hart, 1995; Jiang et al., 2022). For the first time in the history of mankind, humans have the power to destroy themselves. What the Great Filter hypothesis could imply is that such power inherently leads to a certain doomsday scenario. This frightening thought is the reason why scientists are worried about finding life on Mars (Bostrom, 2008). If any form of life is found on the Red Planet, it means that the probability of the Great Filter lying behind humanity is smaller. On the contrary, if no life has ever roamed Mars, it becomes more likely that the filter occurs at the early stages of life, or that the emergence of life is the filter itself. The latter would then imply that life on Earth is truly unique in the universe.


Figure 3: The estimated number of nuclear warheads per country (Statista, 2022).

To this day, the answer to the question “are we alone?” has not yet been solved. This article offers an overview of the most prominent solutions to the Fermi Paradox. It is possible that Earth is simply a unique creation of this universe, or alternatively, there could still be a Great Filter present ahead of us. Although this branch of science does not offer any practical insights into medicine, physics, or computers, it presents a whole new point of view on humanity and on Earth. If humans are indeed the only advanced species the universe has ever seen, we should strive to work together in a cooperative manner, rather than commit ourselves to religious or territorial conflicts. And if Earth is special in its ability to sustain life, we should learn to appreciate and preserve it for as long as we can. After all, to satisfy our curiosity, humans will never stop the search for extraterrestrial life. Regardless whether we could ever find them, let us hope that these forms of life are so primitive, that the Great Filter is before them, but behind us.


Bibliographical References

Bostrom, N. (2008). Where are they? Why I hope the search For extraterrestrial life finds nothing. MIT Technology Review, May/June issue, 72–77. www.nickbostrom.com


de Magalhães, J. P. (2016). A direct communication proposal to test the zoo hypothesis. Space Policy, 38, 22–26. https://doi.org/10.1016/j.spacepol.2016.06.001


Gray, R. H. (2015). The Fermi paradox is neither Fermi’s nor a paradox. Astrobiology, 15(3), 195–199. Mary Ann Liebert Inc. https://doi.org/10.1089/ast.2014.1247


Haldane, J. B. S. (1929). Origin of life. The Rationalist Annual, 148, 3–10.


Hart, M. H. (1995). Explanation for the absence of extraterrestrials on earth. Quarterly Journal of the Royal Astronomical Society, 16, 128–135.


Jacobsen, S. B. (2003). How old is planet earth? Science, 300, 1513–1514. https://doi.org/10.1126/science.1080682


Jiang, J. H., Rosen, P. E., Lu, K., Fahy, K. A., & Obacz, P. (2022). Avoiding the “great filter”: extraterrestrial life and humanity’s future in the universe. http://arxiv.org/abs/2210.10582


Kasting, J. F. (1997). Habitable zones around low mass stars and the search for extraterrestrial life. Origins of Life and Evolution of the Biosphere : The Journal of the International Society for the Study of the Origin of Life, 27(1–3), 291–307. http://www.ncbi.nlm.nih.gov/pubmed/9150578


Miller, S., L. (1953). A production of amino acids under possible primitive earth conditions. Science, 117, 528-529.


Sneden, C. (2001). The age of the universe. Nature, 409(6821), 673–675. https://doi.org/10.1038/35055646


Ward, P. D., & Brownlee, D. (2000). Rare earth: why complex life is uncommon in the universe. Copernicus New York, N. From https://link.springer.com/book/10.1007/b97646

Visual Sources

Connolly, J. (2017). Are We Alone in the Universe?. [image] https://www.freckle.com.au/news/2017/9/28/are-we-alone-in-the-universe


Patton, P. (2015). Enrico Fermi and Extraterrestrial intelligence. [image] https://phys.org/news/2015-04-enrico-fermi-extraterrestrial-intelligence.html


Shapira, L. (2020). The Great Filter of Startups. [image] https://medium.com/bloated-mvp/the-great-filter-of-startups-efcef25dc051


Statista (2022). Estimated Number of Nuclear Warheads Worldwide in 2022, by Country and Deployment Status. [image] https://www.statista.com/statistics/696410/size-and-composition-of-nuclear-arsenals-around-the-world/




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Sten de Schrijver

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