Enrico Fermi was a Nobel Prize-winning physicist who predicted the existence of neutrinos and helped create the first artificial nuclear reactor, an early feat of the Manhattan Project. He was known to pose simple but seemingly unanswerable questions—termed "Fermi questions"—to his colleagues and students, like "How many atoms of Caesar's last breath do you inhale with each lungful of air?"

In 1950,^{[note 1]} Fermi visited Los Alamos National Laboratory in New Mexico and, while walking to the Fuller Lodge for lunch, conversed with fellow physicists Emil Konopinski, Edward Teller, and Herbert York about reports of flying saucers and the feasibility of faster-than-light travel. When the conversation shifted to unrelated topics at the lodge, Fermi blurted a question variously recalled as: "Where is everybody?" (Teller), "Don't you ever wonder where everybody is?" (York), or "But where is everybody?" (Konopinski). According to Teller, "The result of his question was general laughter because of the strange fact that, in spite of Fermi's question coming out of the blue, everybody around the table seemed to understand at once that he was talking about extraterrestrial life."

Fermi was not the first to note the paradox. In his 1686 book Conversations on the Plurality of Worlds, Bernard Le Bovier de Fontenelle—later the secretary of the French Academy of Sciences—constructs a dialogue in which Fontenelle's claims of "intelligent beings exist in other worlds, for instance the Moon" are refuted by a character who notes that "If this were the case, the Moon's inhabitants would already have come to us before now." This may have inspired a similar discussion in Jules Verne's 1865 novel Around the Moon, which has also been identified as an early conceptualization of the Fermi paradox.

Another early formulation of the Fermi paradox was presented and dissected in the 1930s writings of Russian rocket scientist Konstantin Tsiolkovsky. Although his rocketry work was embraced by the materialist Soviets, his philosophical writings were suppressed and unknown for most of the 20th century. Tsiolkovsky noted that critics refute the existence of advanced extraterrestrial life as such civilizations would have visited humanity or left some detectable evidence. He posed a solution to the paradox: humanity is quarantined by aliens to protect its independent cultural development, which resembles the zoo hypothesis proposed by John Ball.

The Fermi question first appeared in print in a footnote of a 1963 paper by Carl Sagan. Two years later, Stephen Dole noted the dilemma at a symposium—"If there are so many advanced forms of life around, where is everybody?"—but did not attribute it to Fermi. A chapter of Intelligent Life in the Universe, co-authored by Sagan and Iosif Shklovsky, was headlined with the Fermi-attributed "Where are they?" The Fermi question also appeared in NASA's 1970 Project Cyclops report, a 1973 book by Sagan, and a 1975 article in JBIS Interstellar Studies by David Viewing that first described it as a paradox.

Later that year, Michael Hart published a detailed examination of the paradox in the Quarterly Journal of the Royal Astronomical Society. Hart, who concluded that "we are the first civilization in our Galaxy", proposed four broad categories of solutions to the paradox: those that are physical (a space travel limitation), sociological (aliens choose not to visit Earth), temporal (aliens have not had time to travel to Earth), or that extraterrestrials have already visited. His paper sparked significant interest in the paradox among academics and even politicians, with a discussion held in the House of Lords. A seminal response—"Extraterrestrial intelligent beings do not exist"—was written by Frank Tipler, who argued that, if an advanced extraterrestrial civilization existed, their self-replicating spacecraft should have already been detected in the Solar System. The term "Fermi paradox" was coined in a 1977 article by David Stephenson and was widely adopted.

The popularization of the Fermi paradox damaged search for extraterrestrial intelligence (SETI) efforts, and Senator William Proxmire cited Tipler when he spurred the termination of the federally funded NASA SETI program in 1981. According to Robert Gray, the paradox may contribute to a "de facto prohibition on government support for research in a branch of astrobiology".

Fermi did not publish anything regarding the paradox, with Sagan once suggesting the quote to be apocryphal.^{[note 3]} Scientists like Robert Gray have criticized its attribution to Fermi, and alternative terms like the "Hart–Tipler argument" or "Tsiolkovsky–Fermi–Viewing–Hart paradox" have been proposed. According to Gray, the current understanding of the paradox misinterprets Fermi's question and subsequent discussion, which was challenging the feasibility of interstellar travel rather than the existence of advanced extraterrestrial life.

The theories and principles in the Drake equation are closely related to the Fermi paradox.^[45] The equation was formulated by Frank Drake in 1961 in an attempt to find a systematic means to evaluate the numerous probabilities involved in the existence of alien life. The equation is:

${\displaystyle N=R_{*}\cdot f_{\mathrm {p} }\cdot n_{\mathrm {e} }\cdot f_{\mathrm {l} }\cdot f_{\mathrm {i} }\cdot f_{\mathrm {c} }\cdot L,}$

where ${\displaystyle N}$ is the number of technologically advanced civilizations in the Milky Way galaxy, and is asserted to be the product of

The fundamental problem is that the last four terms (${\displaystyle f_{l}}$, ${\displaystyle f_{i}}$, ${\displaystyle f_{c}}$, and ${\displaystyle L}$) are entirely unknown, rendering statistical estimates impossible.^[46]

The Drake equation has been used by both optimists and pessimists, with wildly differing results. The first scientific meeting on the search for extraterrestrial intelligence (SETI), which had 10 attendees including Frank Drake and Carl Sagan, speculated that the number of civilizations was roughly between 1,000 and 100,000,000 civilizations in the Milky Way galaxy.^[47] Conversely, Frank Tipler and John D. Barrow used pessimistic numbers and speculated that the average number of civilizations in a galaxy is much less than one.^[48]

Almost all arguments involving the Drake equation suffer from the overconfidence effect, a common error of probabilistic reasoning about low-probability events, by guessing specific numbers for likelihoods of events whose mechanism is not understood, such as the likelihood of abiogenesis on an Earth-like planet, with estimates varying over many hundreds of orders of magnitude. An analysis that takes into account some of the uncertainty associated with this lack of understanding has been carried out by Anders Sandberg, Eric Drexler and Toby Ord.^[46] They suggest that there is "a substantial ex ante probability of there being no other intelligent life in our observable universe".

Dennis Danielson and Christopher Graney have argued that optimistic interpretations of the Drake equation are an artefact of history rather than of science. They show that, in the 17th and 18th centuries, the presumption that planets suitable for organic life are abundant became established in the absence of scientific evidence to support it, or even despite the scientific evidence then available against it. The presumption endured into the present day despite evidence for planetary and stellar diversity that has accumulated since the late 19th century. Thus, in their view, what is seen as the Fermi Paradox is simply science overcoming a historical presumption that never adequately recognized the importance of discoveries about, for example, the nature of planets or the origin of life.^[49][50]

The Great Filter, a concept introduced by Robin Hanson in 1996, represents whatever natural phenomena that would make it unlikely for life to evolve from inanimate matter to an advanced civilization.^[51][3] The most commonly agreed-upon low probability event is abiogenesis: a gradual process of increasing complexity of the first self-replicating molecules by a randomly occurring chemical process. Other proposed great filters are the emergence of eukaryotic cells^{[note 5]} or of meiosis or some of the steps involved in the evolution of a brain-like organ capable of complex logical deductions.^[52]

Astrobiologists Dirk Schulze-Makuch and William Bains, reviewing the history of life on Earth, including convergent evolution, concluded that transitions such as oxygenic photosynthesis, the eukaryotic cell, multicellularity, and tool-using intelligence are likely to occur on any Earth-like planet given enough time. They argue that the Great Filter may be abiogenesis, the rise of technological human-level intelligence, or an inability to settle other worlds because of self-destruction or a lack of resources.^[53] Paleobiologist Olev Vinn has suggested that the great filter may have universal biological roots related to evolutionary animal behavior.^[54]

In 2021, the concepts of quiet, loud, and grabby aliens were introduced by Hanson et al. The proposed "loud" aliens expand rapidly in a highly detectable way throughout the universe and endure, while "quiet" aliens are hard or impossible to detect and eventually disappear. "Grabby" aliens prevent the emergence of other civilizations in their sphere of influence, which expands at a rate near the speed of light. The authors argue that if loud civilizations are rare, as they appear to be, then quiet civilizations are also rare. The paper suggests that humanity's existing stage of technological development is relatively early in the potential timeline of intelligent life in the universe, as loud aliens would otherwise be observable by astronomers.^[55][56]

Earlier in 2013, Anders Sandberg and Stuart Armstrong examined the potential for intelligent life to spread intergalactically throughout the universe and the implications for the Fermi Paradox. Their study suggests that with sufficient energy, intelligent civilizations could potentially colonize the entire Milky Way galaxy within a few million years, and spread to nearby galaxies in a timespan that is cosmologically brief. They conclude that intergalactic colonization appears possible with the resources of a single planetary system and that intergalactic colonization is of comparable difficulty to interstellar colonization, and therefore the Fermi paradox is much sharper than commonly thought.^[57]

Critics such as David Kipping have contended that the "Grabby Aliens" model is reliant on unproven assumptions, lacking enough scientific rigor to be empirically falsifiable, and suggested other explanations for the proposed earliness of humans such as planets in M-dwarf systems being uninhabitable. Robin Hanson has responded to these criticisms.^[58]

Anthropic reasoning and the question of why we happen to find ourselves as humans creates a number of potential problems for astrobiology. Walter Barta argues that Hanson's grabby aliens model creates an anthropic dilemma. According to Hanson's model, most observers in our reference class should be grabby aliens themselves. This leads to the question of why we do not find ourselves as grabby aliens, but rather as a species confined to a single planet.^[59]

Radio technology and the ability to construct a radio telescope are presumed to be a natural advance for technological species,^[66] theoretically creating effects that might be detected over interstellar distances. The careful searching for non-natural radio emissions from space may lead to the detection of alien civilizations. Sensitive alien observers of the Solar System, for example, would note unusually intense radio waves for a G2 star due to Earth's television and telecommunication broadcasts. In the absence of an apparent natural cause, alien observers might infer the existence of a terrestrial civilization. Such signals could be either "accidental" by-products of a civilization, or deliberate attempts to communicate, such as the Arecibo message. It is unclear whether "leakage", as opposed to a deliberate beacon, could be detected by an extraterrestrial civilization. The most sensitive radio telescopes on Earth, as of 2019^[update], would not be able to detect non-directional radio signals (such as broadband) even at a fraction of a light-year away,^[67] but other civilizations could hypothetically have much better equipment.^[68][69]

A number of astronomers and observatories have attempted and are attempting to detect such evidence, mostly through SETI organizations such as the SETI Institute and Breakthrough Listen. Several decades of SETI analysis have not revealed any unusually bright or meaningfully repetitive radio emissions.^[70]

Exoplanet detection and classification is a very active sub-discipline in astronomy; the first candidate terrestrial planet discovered within a star's habitable zone was found in 2007.^[71] New refinements in exoplanet detection methods, and use of existing methods from space (such as the Kepler and TESS missions) have detected and characterized Earth-size planets, and determined whether they are within the habitable zones of their stars. Such observational refinements have allowed better estimates of how common these potentially habitable worlds are, typically in the range of 0.5-1.0 potentially habitable planets per star. ^{[72] [73] [74] [75] [76] [77] [78]}

The Hart–Tipler conjecture is a form of contraposition which states that because no interstellar probes have been detected, there likely is no other intelligent life in the universe, as such life should be expected to eventually create and launch such probes.^[80] Self-replicating probes could exhaustively explore a galaxy the size of the Milky Way in as little as a million years.^[12] If even a single civilization in the Milky Way attempted this, such probes could spread throughout the entire galaxy. Another speculation for contact with an alien probe—one that would be trying to find human beings—is an alien Bracewell probe. Such a hypothetical device would be an autonomous space probe whose purpose is to seek out and communicate with alien civilizations (as opposed to von Neumann probes, which are usually described as purely exploratory). These were proposed as an alternative to carrying a slow speed-of-light dialogue between vastly distant neighbors. Rather than contending with the long delays a radio dialogue would suffer, a probe housing an artificial intelligence would seek out an alien civilization to carry on a close-range communication with the discovered civilization. The findings of such a probe would still have to be transmitted to the home civilization at light speed, but an information-gathering dialogue could be conducted in real time.^[81]

Direct exploration of the Solar System has yielded no evidence indicating a visit by aliens or their probes. Detailed exploration of areas of the Solar System where resources would be plentiful may yet produce evidence of alien exploration,^[82][83] though the entirety of the Solar System is relatively vast and difficult to investigate. Attempts to signal, attract, or activate hypothetical Bracewell probes in Earth's vicinity have not succeeded.^[84]

In 1959, Freeman Dyson observed that every developing human civilization constantly increases its energy consumption, and he conjectured that a civilization might try to harness a large part of the energy produced by a star. He proposed a hypothetical "Dyson sphere" as a means: a shell or cloud of objects enclosing a star to absorb and utilize as much radiant energy as possible. Such a feat of astroengineering would drastically alter the observed spectrum of the star involved, changing it at least partly from the normal emission lines of a natural stellar atmosphere to those of black-body radiation, probably with a peak in the infrared. Dyson speculated that advanced alien civilizations might be detected by examining the spectra of stars and searching for such an altered spectrum.^[85][86][87]

There have been attempts to find evidence of Dyson spheres that would alter the spectra of their core stars.^[88] Direct observation of thousands of galaxies has shown no explicit evidence of artificial construction or modifications.^{[86][87][89][90]} In October 2015, there was speculation that a dimming of light from star KIC 8462852, observed by the Kepler space telescope, could have been a result of such a Dyson sphere under construction.^[91][92] However, in 2018, further observations determined that the amount of dimming varied by the frequency of the light, pointing to dust, rather than an opaque object such as a Dyson sphere, as the cause of the dimming.^[93][94]