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

― Arthur C. Clarke


By Jesus Padilla

The Fermi paradox, is a term coined after physicist Enrico Fermi established the contradiction between the incipient evidence and the high probability estimations behind the existence of extraterrestrial life (or advanced extraterrestrial civilizations). The basic principles made by physicists Enrico Fermi and Michael H. Hart are the following:

  • There are billions of stars in our galaxy that are similar to the Sun, and many of these stars are billions of years older than the Solar system.
  • With high probability, some of these stars have a planet like ours, and if the Earth is common, some may have developed intelligent life at some extent.
  • Some of these civilizations may have developed interstellar travel, a step the Earth is way far to accomplish at big scale.
  • Even at the slow pace of currently envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years.

According to the previous points, there should be evidence now that the Earth has been visited by extraterrestrial beings. According to Fermi there was not formal science-based evidence of this, leading him to establish the question of “Where is everybody?” 

There have been many attempts to explain the Fermi paradox, primarily either suggesting that intelligent extraterrestrial life is non existent, extremely rare or proposing reasons that such civilizations have not contacted or visited Earth.

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Italian-born physicist Enrico Fermi explaining a problem in physics, circa 1950. Photo Credit: Encyclopaedia Britannica

The first principle of the Fermi paradox is a function of the scale where an estimation of 200–400 billion stars are in the Milky Way (2–4 × 1011) and 70 sextillion (7×1022) are in the observable universeEven if intelligent life occurs on only a small percentage of planets around these stars, there might still be a great number of potential intelligent civilizations, and if the percentage were high enough it would produce a significant number of existing civilizations in the Milky Way. This assumes the mediocrity principle, by which the Earth is a typical or a “common” planet.

The second aspect of the paradox is based on probability: given intelligent life ability to overcome scarcity and its predetermined behavior to search for new environments, therefore is the possibility that at least some civilizations would be quite technologically advanced, seek for new resources in outer space and colonize their own or surrounding star systems.

The scary conclusion so far, is that apparently there is no conclusive or significant evidence on Earth (yet), or elsewhere in the known observable universe of other intelligent beings after 13.8 billion years of the universe history!, so there is a mismatch between the odds of their existence vs their non existence, at least in the same space-time of ours that demands specific answers, in consequence a big bunch of hypotheses have arisen. Some examples of possible explanations are that intelligent life is really rare, or maybe our assumptions about the general development or behavior of intelligent species are flawed, or more radically that our current scientific understanding of the nature of the universe itself is quite far to be completed or assimilated due to the intrinsic nature of  the “human intelligence” that could be quite limited vs other E.T. living beings.

The Fermi Paradox remind me recent declarations made by Neil deGrasse Tyson (STARMUS Festival 2016) where as per their own words said the likelihood of an encounter are really, really low.  In the case of the existence of “A sufficiently intelligent civilization would have positively no interest in us at all” stated. “In the same way as when you’re walking down a street and there’s a worm there.”. And even if you wanted to kill all the worms, he continued, you’d soon get bored and do something else. Maybe we will never get visited since as per Tyson´s declarations “our biggest protection against being killed by alien civilizations is their conclusion there’s no intelligent civilization on Earth” . “Suppose in fact intelligence has come to the galaxy. Who are we to then decide that we are intelligent? We define our intelligence. Of course we’re intelligent because we define it”.

Neil deGrasse Tyson and Jill Tarter at the STARMUS 2016 Festival, Canary Islands. Tenerife. Image Credit: Victor R. Ruiz Wikicommons.

Continuing with The Fermi paradox, it can be analyzed from two different perspectives. The first is asking what´s going on the neighborhood, “Why are no aliens or their artifacts found here on Earth, or in the Solar System?” If interstellar travel is possible, even the “slow” kind nearly within the reach of Earth technology, then it would only take from 5 million to 50 million years to colonize the galaxy. This is relatively brief on the space-time scale, let alone a cosmological one. Since there are many stars older than the Sun, and since intelligent life might have evolved earlier elsewhere, the question then becomes why the galaxy has not been colonized already. Even if colonization is impractical or undesirable to all alien civilizations, large-scale exploration of the galaxy could be possible by interstellar devices. These might leave detectable artifacts in the Solar System, such as old extraterrestrial devices or evidence of mining activity, but none of these have been observed.

The second perspective is asking the question around interstellar space (long distance related) as “Why we don´t see signs of intelligence elsewhere in the universe?” This version does not assume interstellar travel per se, but includes other galaxies or space bodies as well. For distant galaxies, travel times may well explain the lack of alien visitors to our neighborhood , but a sufficiently advanced civilization could potentially be observable over a significant fraction of the size of the observable universe. Even if such civilizations are scarce, the scale argument indicates they should exist somewhere at some point during the history of the universe, and since they could be detected from far away over a considerable period of time, many more potential sites for their origin are within range of our observation, but again there is no evidence yet. It is unknown whether the paradox is stronger for our galaxy or for the universe as a whole.


Hypothetical proposals to explain the paradox

1) Extraterrestrial life is really rare or could be non-existent. 

There is a group of scientists which argue that intelligent extraterrestrial life is (nearly) impossible where the conditions needed for the evolution of life, or at least the evolution of biological complexity are rare or even unique to Earth. Under this principle, there is the called “rare Earth hypothesis”, a rejection of the mediocrity principle where life as we conceived it is “Earth exclusive” since it is really rare and unusual.

The Rare Earth hypothesis argues that the evolution of biological complexity requires a big bunch of fortuitous circumstances, such as a galactic habitable zone, a central star and planetary system having several “special conditions” such as the circumstellar habitable zone, a right sized terrestrial planet, the advantage of a giant guardian like Jupiter and a large natural satellite, conditions needed to ensure the planet has a magnetosphere and plate tectonics, the chemistry of the lithosphere, atmosphere, and oceans, the role of “evolutionary pumps” such as massive glaciation and rare bolide impacts, and whatever led to the appearance of the eukaryote cell, sexual reproduction and the Cambrian explosion. This explanation could open the argument that “this is something that happens all the time? or is one in a life event in the history of the universe?” Mind-blowing…

A diagram that shows the habitable zone limits around stars and its correlation by star type. Including Solar System planets (Venus, Earth, and Mars) and significant exoplanets such as TRAPPIST-1d, Kepler-186f and the nearest one Proxima Centauri b. Image Credit: Chester Harman.

2) No other intelligent species have arisen

It is possible that even if complex life is common, intelligence (and consequently civilizations) is not. While there are remote sensing techniques that could perhaps detect life-bearing planets without relying on the signs of technology, none of them has any ability to tell if any detected life is intelligent. This is sometimes referred to as the “algae vs. alumnae” problem. An experiment that might support the appearance of the essential elements to make life happen “everywhere” is the Miller–Urey experiment which was a chemical experiment that recreated the conditions thought at the time (1952) to be present on the early Earth, and tested the chemical origin of life under those conditions. The experiment supported the hypothesis that certain conditions on the primitive Earth favoured chemical reactions that synthesized more complex organic compounds from simpler inorganic precursors. Considered to be the classic experiment investigating abiogenesis. This experiment is not a direct indication that if life exists, therefore in consequence might be intelligent beings as a rule for the overcoming evolution of the developed species.

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Stanley L. Miller, professor of chemistry at UC San Diego, circa 1981. Miller was the first to demonstrate that organic molecules necessary for life could be recreated in a laboratory flask. Photo credit: UCSD Publications Office

3) Intelligent alien species lack advanced technology

It may be that while alien species with intelligence exist, they are primitive or have not reached the level of technological advancement necessary to communicate. Along with non-intelligent life, such civilizations would be also very difficult for us to detect, short of a visit by a probe, a trip that would take hundreds of thousands of years with current technology. To skeptics, the fact that in the history of life on the Earth only one species has developed a civilization to the point of being capable of spaceflight and radio technology, lends more credence to the idea that technologically advanced civilizations are rare in the universe.

4) It is the nature of intelligent life to destroy itself

Establishes that technological civilizations may usually or invariably destroy themselves shortly after developing technology such as communication or aerospace devices. Reasons of annihilation could be attributed to several wars, accidental or on purpose environmental contamination, resource depletion, climate change or by designed artificial intelligence that turns against their creators. This general theme is explored both in fiction and in scientific hypothesizing. In 1966, Carl Sagan speculated that technological civilizations will either tend to destroy themselves within a century of developing interstellar communicative capability or master their self-destructive tendencies and survive for billion-year timescales. 

Self-annihilation may also be viewed in terms of the second law of thermodynamics: Insofar as life is an ordered system that can sustain itself against the tendency to follow disorder or chaos, the “external transmission” or interstellar communicative phase may be the point at which the system becomes unstable and self-destructs in order to balance the universal entropy.

The mushroom cloud of the atomic bombing of the Japanese city of Nagasaki on August 9, 1945, rose some 11 mi (18 km) above the bomb’s hypocenter. Photo Credit: Wikipedia

5) It is the nature of intelligent life to destroy others

Another hypothesis is that an intelligent species beyond a certain point of technological capability will destroy other intelligent species as they appear. The idea that something, or someone might be destroying intelligent life in the universe has been explored in the scientific literature. A species might undertake such extermination driven by expansion purpose, paranoia in order to self-defense or triggered by aggression from the counter part.

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British Astronomer Edward R. “Ted” Harrison. Photo Credit: American Astronomical Society.

In 1981, cosmologist Edward Harrison argued that such behavior would be an act of prudence: an intelligent species that has overcome its own self-destructive tendencies might view any other species bent on galactic expansion as a threat. It has also been suggested that a successful alien species would be a “super-predator”, such the humans. Another possibility invokes the “tragedy of the commons” and the anthropic principle: the first life-form to achieve interstellar travel will necessarily (even if unintentionally) prevent competitors arising, and humans simply happen to be first.

6) Periodic extinction by natural events

New life might commonly die out due to runaway heating or cooling on their fledgling planets. On Earth, there have been numerous major extinction events that destroyed the majority of complex species alive at the time; the extinction of the dinosaurs is the best known example.

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Diagram that shows the “Chicxulub” meteorite impact crater in Yucatán, México. 65 million years ago this event marked the sudden extinction of the dinosaurs as well as the majority of life on Earth. Photo Credit: NASA.

These are thought to have been caused by events such as impact from a large meteorite, massive volcanic eruptions, or astronomical events such as gamma-ray bursts. It may be the case that such extinction events are common throughout the universe and periodically destroy intelligent life, or at least its civilizations, before the species is able to develop the technology to communicate with other intelligent species. Using extinct civilizations such as Easter Island as models, a study conducted in 2018 posited that climate change induced by “energy intensive” civilizations may prevent sustainability within such civilizations, thus explaining the paradoxical lack of evidence for intelligent extra-terrestrial life.

7) Inflation hypothesis and the youngness argument

Cosmologist Alan Guth proposed a multiverse solution to the Fermi paradox. This hypothesis uses the synchronous gauge probability distribution, with the result that young universes exceedingly outnumber older ones (by a factor of e for every second of age). Therefore, averaged over all universes, universes with civilizations will almost always have just one, the first to develop. However, Guth notes “Perhaps this argument explains why SETI has not found any signals from alien civilizations, but I find it more plausible that it is merely a symptom that the synchronous gauge probability distribution is not the right one.”

7) Intelligent civilizations are too far apart within space-time. 

NASA’s conception of the Terrestrial Planet Finder. Image Credit: NASA


It may be that non-colonizing technologically capable alien civilizations exist, but that they are simply too far apart for meaningful two-way communication. If two civilizations are separated by several thousand light-years, it is possible that one or both cultures may become extinct before dialogue can be established. Human searches may be able to detect their existence, but communication will remain impossible because of large distances. It has been suggested that this problem might be mitigated somewhat if contact/communication is made through a Bracewell probe. In this case at least one partner in the exchange may obtain meaningful information. Alternatively, a civilization may simply broadcast its knowledge, and leave it to the receiver to make what they may of it. This is similar to the transmission of information from ancient civilizations to the present, and humanity has undertaken similar activities like the Arecibo message, which could transfer information about Earth’s intelligent species, even if it never yields a response or does not yield a response in time for humanity to receive it. It is also possible that archaeological evidence of past civilizations may be detected through deep space observations.

An attempt to make future contact with technological advanced civilizations through a probe (The Voyager): The Golden Disc. Image Credit: NASA/JPL

A related speculation by Sagan and Newman suggests that if other civilizations are out there transmitting and exploring, their signals and probes simply have not arrived yet. However, there is a position from those which support the paradox which establish that this is unlikely, since it requires that humanity presence in space-time must be in a very “special moment”, while the Milky Way is in transition from empty to full. This is a tiny fraction of the lifespan of a galaxy under ordinary assumptions and calculations resulting from them, so the likelihood that we are in the midst of this transition is considered low in the paradox.


8) Lack of resources to spread physically throughout the galaxy

Many speculations about the ability of an alien culture to colonize other star systems are based on the idea that interstellar travel is technologically possible. While the current understanding of physics rules out the possibility of faster-than-light travel, it appears that there are no major theoretical barriers to the construction of “slow” interstellar ships, even though the engineering required is considerably beyond our present capabilities. This idea underlies the concept of the Von Neumann and the Bracewell probe as a potential avenue to find extraterrestrial intelligence.

It is possible, however, that present scientific knowledge cannot properly gather and assess the feasibility and costs of such interstellar colonization. Theoretical barriers may not yet be understood, and the resources needed may be so great as to make it unlikely that any civilization could afford to attempt it, at least as per our current condition where we are putting more and more attention to save our planet instead of looking out there.

Even if interstellar travel and colonization are possible, they may be difficult, leading to a colonization model based on percolation theory. Colonization efforts may not occur as an unstoppable rush, but rather as an uneven tendency to “percolate” outwards, within an eventual slowing and termination of the effort given the enormous costs involved and the expectation that colonies will inevitably develop a culture and civilization of their own. Colonization may thus occur in “clusters,” with large areas remaining uncolonized at any one time.

If exploration, or backup from a native system disaster is the primary motive for expansion, then it is possible that mind uploading and similar technologies may reduce the desire to colonize by replacing physical travel with much less-expensive communication. Therefore, the first civilization may have physically explored or colonized the galaxy, but subsequent civilizations find it cheaper, faster, and easier to travel by contacting existing civilizations rather than physically exploring or traveling themselves. This leads to little or no physical travel at the current era, and only directed communications, which are hard to see except to the intended receiver.

9) Human beings have not existed long enough

Humanity’s ability to detect intelligent extraterrestrial life has existed for only a very brief period—from 1937 onwards, if the invention of the radio telescope is taken as the dividing line—and Homo sapiens is a geologically recent species. The whole period of modern human existence to date is a very brief period on a cosmological scale, and radio transmissions have only been propagated since 1895. Thus, it remains possible that human beings have neither existed long enough nor made themselves sufficiently detectable to be found by extraterrestrial intelligence.

The dawn of the “Radio Era”: Marconi’s first transmitter incorporating an antenna circa 1895. It consisted of an elevated copper sheet connected to a Righi spark gap powered by an induction coil with a telegraph key to switch it on and off to spell out text messages in Morse code. Photo Credit: americanradiohistory.com

10) We are not listening properly

There are some assumptions that underlie the SETI programs that may cause searchers to miss signals that are occurring. Extraterrestrials might, for example, transmit signals that have a very high or low data rate, or employ unconventional frequencies, which would make them hard to distinguish from background noise. Signals might be sent from non-main sequence star systems that we search with lower priority; current programs assume that most alien life will be orbiting Sun-like stars.

The Very Large Array in Socorro, New Mexico, an interferometric array formed of 27 parabolic dish telescopes. Image Credit: Hajor

The greatest challenge is the sheer size of the radio search needed to look for signals (effectively spanning the entire observable universe), the limited amount of resources committed to SETI, and the sensitivity of modern instruments. SETI estimates, for instance, that with a radio telescope as sensitive as the Arecibo Observatory, Earth’s television and radio broadcasts would only be detectable at distances up to 0.3 light-years, less than 1/10 the distance to the nearest star. A signal is much easier to detect if the signal energy is limited to either a narrow range of frequencies, or directed at a specific part of the sky. Such signals could be detected at ranges of hundreds to tens of thousands of light-years distance. However, this means that detectors must be listening to an appropriate range of frequencies, and be in that region of space to which the beam is being sent. Many SETI searches assume that extraterrestrial civilizations will be broadcasting a deliberate signal, like the Arecibo message, in order to be found.

Thus to detect alien civilizations through their radio emissions, Earth observers either need more sensitive instruments or must hope for fortunate circumstances: that the broadband radio emissions of alien radio technology are much stronger than our own; that one of SETI’s programs is listening to the correct frequencies from the right regions of space; or that aliens are deliberately sending focused transmissions in our general direction.

11) They tend to isolate themselves

It has been suggested that some advanced beings may divest themselves of physical form, create massive artificial virtual environments, transfer themselves into these environments through mind uploading, and exist totally within virtual worlds, ignoring the external physical universe.

It may also be that intelligent alien life develops an “increasing disinterest” in their outside world. Possibly any sufficiently advanced society will develop highly engaging media and entertainment well before the capacity for advanced space travel, and that the rate of appeal of these social contrivances is destined, because of their inherent reduced complexity, to overtake any desire for complex, expensive endeavors such as space exploration and communication. Once any sufficiently advanced civilization becomes able to master its environment, and most of its physical needs are met through technology, various “social and entertainment technologies”, including virtual reality, are postulated to become the primary drivers and motivations of that civilization.

12) They are too alien

Microwave window as seen by a ground-based system. From NASA report SP-419: SETI – the Search for Extraterrestrial Intelligence. Image: SETI


Another possibility is that human theoreticians have underestimated how much alien life might differ from that on Earth. Aliens may be psychologically unwilling to attempt to communicate outside. Perhaps human mathematics is parochial to Earth and not shared by another kind of beings, though others argue this can only apply to abstract math since the math associated with physics must be similar (in results, if not in methods). Physiology might also cause a communication barrier. Carl Sagan speculated that an alien species might have a thought process orders of magnitude slower (or faster) than ours. A message broadcast by that species might well seem like random background noise to us, and therefore go undetected.

Another thought is that technological civilizations invariably experience a technological singularity and attain a post-biological character. Hypothetical civilizations of this sort may have advanced drastically enough to render communication impossible.

13) It is dangerous to communicate

An alien civilization might feel it is too dangerous to communicate, either for us or for them. After all, when very different civilizations have met on Earth, the results have often been disastrous for one side or the other, and the same may well apply to interstellar contact. Even contact at a safe distance could lead to infection by computer code or even ideas themselves. Perhaps prudent civilizations actively hide not only from Earth but from everyone, out of fear of other civilizations.

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Stephen Hawking stated several times about the threats of contacting “on purpose” E.T. civilizations.

Perhaps the Fermi paradox itself—or the alien equivalent of it—is the reason for any civilization to avoid contact with other civilizations, even if no other obstacles existed. From any one civilization’s point of view, it would be unlikely for them to be the first ones to make first contact. Therefore, according to this reasoning, it is likely that previous civilizations faced fatal problems with first contact and doing so should be avoided. So perhaps every civilization keeps quiet because of the possibility that there is a real reason for others to do so. Liu Cixin’s novel The Dark Forest is based upon such a situation.

14) They are here unacknowledged

A significant fraction of the population believes that at least some UFOs (Unidentified Flying Objects) are spacecraft piloted by aliens. While most of these are unrecognized or mistaken interpretations of mundane phenomena, there are those that remain puzzling even after investigation. The consensus scientific view is that although they may be unexplained, they do not rise to the level of convincing evidence.

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Extract from the news on the supposed UFO footage by George Adamski. c. 1952. Photo Credit: Burlington News.

Similarly, it is theoretically possible that SETI groups are not reporting positive detections, or governments have been blocking signals or suppressing publication. This response might be attributed to security or economic interests from the potential use of advanced extraterrestrial technology. It has been suggested that the detection of an extraterrestrial radio signal or technology could well be the most highly secret information that exists.Claims that this has already happened are common in the popular press, but the scientists involved report the opposite experience—the press becomes informed and interested in a potential detection even before a signal can be confirmed.


Author: Jesus Padilla

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