Fermi paradox

The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high a priori likelihood of its existence, and by extension of obtaining such evidence. As a 2015 article put it, "If life is so easy, someone from somewhere must have come calling by now."

Italian-American physicist Enrico Fermi's name is associated with the paradox because of a casual conversation in the summer of 1950 with fellow physicists Edward Teller, Herbert York, and Emil Konopinski. While walking to lunch, the men discussed recent UFO reports and the possibility of faster-than-light travel. The conversation moved on to other topics, until during lunch Fermi blurted out, "But where is everybody?" (although the exact quote is uncertain).

There have been many attempts to resolve the Fermi paradox,''If the Universe Is Teeming with Aliens ... WHERE IS EVERYBODY?: Seventy-Five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life, Second Edition''
Stephen Webb, foreword by Martin Rees, Heidelberg, New York, Dordrecht, London: Springer International Publishing, 2002, 2015. such as suggesting that intelligent extraterrestrial beings are extremely rare, that the lifetime of such civilizations is short, or that they exist but (for various reasons) humans see no evidence.

Chain of reasoning

The following are some of the facts and hypotheses that together serve to highlight the apparent contradiction:

* There are billions of stars in the Milky Way similar to the Sun.
* With high probability, some of these stars have Earth-like planets in a circumstellar habitable zone.
* Many of these stars, and hence their planets, are much older than the Sun. If the Earth is typical, some may have developed intelligent life long ago.
* Some of these civilizations may have developed interstellar travel, a step humans are investigating now.
* Even at the slow pace of currently envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years.
* Since many of the Sun-like stars are billions of years older than the Sun, the Earth should have already been visited by extraterrestrial civilizations, or at least their probes.
* However, there is no convincing evidence that this has happened.

History

Fermi was not the first to ask the question. An earlier implicit mention was by Konstantin Tsiolkovsky in an unpublished manuscript from 1933. He noted "people deny the presence of intelligent beings on the planets of the universe" because "(i) if such beings exist they would have visited Earth, and (ii) if such civilizations existed then they would have given us some sign of their existence." This was not a paradox for others, who took this to imply the absence of extraterrestrial life. But it was one for him, since he believed in extraterrestrial life and the possibility of space travel. Therefore, he proposed what is now known as the zoo hypothesis and speculated that mankind is not yet ready for higher beings to contact us. That Tsiolkovsky himself may not have been the first to discover the paradox is suggested by his above-mentioned reference to other people's reasons for not accepting the premise that extraterrestrial civilizations exist.

In 1975, Michael H. Hart published a detailed examination of the paradox, one of the first to do so. He argued that if intelligent extraterrestrials exist, and are capable of space travel, then the galaxy could have been colonized in a time much less than that of the age of the Earth. However, there is no observable evidence they have been here, which Hart called "Fact A".

Other names closely related to Fermi's question ("Where are they?") include the Great Silence, and ''silentium universi'' (Latin for "silence of the universe"), though these only refer to one portion of the Fermi Paradox, that humans see no evidence of other civilizations.

Original conversations

In the summer of 1950 at Los Alamos National Laboratory in New Mexico, Enrico Fermi and co-workers Emil Konopinski, Edward Teller, and Herbert York had one or several lunchtime conversations."Where is everybody?": An account of Fermi's question"
, Dr. Eric M. Jones, Los Alamos technical report, March 1985. Jones wrote to Edward Teller on July 13, 1984, Herbert York on Sept. 4, and Emil Konopinski on Sept. 24, 1984.

As three of the men walked to lunch, Teller writes that he has a "vague recollection" to the effect that "we talked about flying saucers and the obvious statement that the flying saucers are not real." Konopinski joined the others while the conversation was in progress. He remembered a magazine cartoon which showed aliens stealing New York City trash cans and added this humorous aspect to the conversation. He writes, "More amusing was Fermi's comment, that it was a very reasonable theory since it accounted for two separate phenomena: the reports of flying saucers as well as the disappearance of the trash cans." And yet, when Eric Jones wrote to the surviving men decades later, only Konopinski remembered that the cartoon had been part of the conversation.

Teller writes that he thinks Fermi directed the question to him: "How probable is it that within the next ten years we shall have clear evidence of a material object moving faster than light?" Teller answered one in a million. Fermi said, "This is much too low. The probability is more like ten percent." Teller also writes that ten percent was "the well known figure for a Fermi miracle."

Herb York does not remember a previous conversation, although he says it makes sense given how all three later reacted to Fermi's outburst.Of the three surviving men, only Emil Konopinski clearly remembered that Fermi's lunchtime exclamation was connected to a previous conversation which had occurred on the same day. In 1984, he wrote, "I do have a fairly clear memory of how the discussion of extra-terrestials got started—"

After sitting down for lunch, and when the conversation had already moved on to other topics, Fermi suddenly blurted out, "Where is everybody?" (Teller's letter), or "Don't you ever wonder where everybody is?" (York's letter), or "But where is everybody?" (Konopinski's letter).

Teller wrote, "The result of his question was general laughter because of the strange fact that in spite of Fermi's question coming from the clear blue, everybody around the table seemed to understand at once that he was talking about extraterrestrial life."

Herbert York wrote, "Somehow (and perhaps it was connected to the prior conversation in the way you describe, even though I do not remember that) we all knew he meant extra-terrestrials."

Emil Konopinski merely wrote, "It was his way of putting it that drew laughs from us."

Regarding the continuation of the conversation, York wrote in 1984 that Fermi "followed up with a series of calculations on the probability of earthlike planets, the probability of life given an earth, the probability of humans given life, the likely rise and duration of high technology, and so on. He concluded on the basis of such calculations that we ought to have been visited long ago and many times over."

Teller remembers that not much came of this conversation "except perhaps a statement that the distances to the next location of living beings may be very great and that, indeed, as far as our galaxy is concerned, we are living somewhere in the sticks, far removed from the metropolitan area of the galactic center."

Teller wrote "maybe approximately eight of us sat down together for lunch."Teller wrote: "This incident I have clearly in mind and I believe it was on the same occasion where the other question arose which you have mentioned. This late point, however, I am not certain of." Both York and Konopinski remember that it was just the four of them.York wrote, "At a luncheon in the Lodge which included just four people, Fermi, Teller, Emil Konoinski and myself, Fermi said, virtually apropos of nothing:"Konopinski wrote, "I think there was only the four of us just as Herb York remembers it."

Fermi died of cancer in 1954. However, in letters to the three surviving men decades later in 1984, Dr. Eric Jones of Los Alamos was able to partially put the original conversation back together. He informed each of the men that he wished to include a reasonably accurate version or composite in the written proceedings he was putting together for a previously held conference entitled "Interstellar Migration and the Human Experience".''Interstellar Migration and the Human Experience''
edited by Ben R. Finney, Eric M. Jones, University of California Press, 1985.

Jones first sent a letter to Edward Teller which included a secondhand account from Hans Mark. Teller responded, and then Jones sent Teller's letter to Herbert York. York responded, and finally, Jones sent both Teller's and York's letters to Emil Konopinski who also responded. Furthermore, Konopinski was able to later identify a cartoon which Jones found as the one involved in the conversation and thereby help to settle the time period as being the summer of 1950.

Basis

The Fermi paradox is a conflict between the argument that scale and probability seem to favor intelligent life being common in the universe, and the total lack of evidence of intelligent life having ever arisen anywhere other than on Earth.

The first aspect of the Fermi paradox is a function of the scale or the large numbers involved: there are an estimated 200–400 billion stars in the Milky Way (2–4 × 10¹¹) and 70 sextillion (7×10²²) in the observable universe. Even if intelligent life occurs on only a minuscule percentage of planets around these stars, there might still be a great number of extant civilizations, and if the percentage were high enough it would produce a significant number of extant civilizations in the Milky Way. This assumes the mediocrity principle, by which Earth is a typical planet.

The second aspect of the Fermi paradox is the argument of probability: given intelligent life's ability to overcome scarcity, and its tendency to colonize new habitats, it seems possible that at least some civilizations would be technologically advanced, seek out new resources in space, and colonize their own star system and, subsequently, surrounding star systems. Since there is no significant evidence on Earth, or elsewhere in the known universe, of other intelligent life after 13.8 billion years of the universe's history, there is a conflict requiring a resolution. Some examples of possible resolutions are that intelligent life is rarer than is thought, that assumptions about the general development or behavior of intelligent species are flawed, or, more radically, that current scientific understanding of the nature of the universe itself is quite incomplete.

The Fermi paradox can be asked in two ways.See Hart for an example of "no aliens are here", and Webb for an example of the more general "We see no signs of intelligence anywhere". The first is, "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.Crawford, I.A.
"Where are They? Maybe we are alone in the galaxy after all"
, ''Scientific American'', July 2000, 38–43, (2000). This is relatively brief on a geological 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 probes. These might leave detectable artifacts in the Solar System, such as old probes or evidence of mining activity, but none of these have been observed.

The second form of the question is "Why do we see no signs of intelligence elsewhere in the universe?". This version does not assume interstellar travel, but includes other galaxies as well. For distant galaxies, travel times may well explain the lack of alien visits to Earth, 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 rare, 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 human observation. It is unknown whether the paradox is stronger for the Milky Way galaxy or for the universe as a whole.

Drake equation

The theories and principles in the Drake equation are closely related to the Fermi paradox. 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 presented as follows:

$N = R_* f_p n_e f_l f_i f_c L$

Where $N$ is the number of technologically advanced civilizations in the Milky Way galaxy, and $N$ is asserted to be the product of
* $R_*$, the rate of formation of stars in the galaxy;
* $f_p$, the fraction of those stars with planetary systems;
* $n_e$, the number of planets, per solar system, with an environment suitable for organic life;
* $f_l$, the fraction of those suitable planets whereon organic life actually appears;
* $f_i$, the fraction of habitable planets whereon ''intelligent'' life actually appears;
* $f_c$, the fraction of civilizations that reach the technological level whereby detectable signals may be dispatched; and
* $L$, the length of time that those civilizations dispatch their signals.

The fundamental problem is that the last four terms ($f_l$, $f_i$, $f_c$, and $L$) are entirely unknown, rendering statistical estimates impossible.

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. 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. 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 yet understood, such as the likelihood of abiogenesis on an Earth-like planet, with current likelihood 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, and suggests "a substantial ''ex ante'' probability of there being no other intelligent life in our observable universe".

Great Filter

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. 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 cellsEukaryotes also include plants, animals, fungi, and algae. or of meiosis or some of the steps involved in the evolution of a brain capable of complex logical deductions.

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.

Empirical evidence

There are two parts of the Fermi paradox that rely on empirical evidence—that there are many potential habitable planets, and that humans see no evidence of life. The first point, that many suitable planets exist, was an assumption in Fermi's time but is now supported by the discovery that exoplanets are common. Current models predict billions of habitable worlds in the Milky Way.

The second part of the paradox, that humans see no evidence of extraterrestrial life, is also an active field of scientific research. This includes both efforts to find any indication of life, and efforts specifically directed to finding intelligent life. These searches have been made since 1960, and several are ongoing.See, for example, the SETI Institute
The Harvard SETI Home Page
, o
The Search for Extra Terrestrial Intelligence at Berkeley

Although astronomers do not usually search for extraterrestrials, they have observed phenomena that they could not immediately explain without positing an intelligent civilization as the source. For example, pulsars, when first discovered in 1967, were called little green men (LGM) because of the precise repetition of their pulses. In all cases, explanations with no need for intelligent life have been found for such observations,Pulsars are now attributed to neutron stars, and Seyfert galaxies to an end-on view of the accretion onto the black holes. but the possibility of discovery remains. Proposed examples include asteroid mining that would change the appearance of debris disks around stars, or spectral lines from nuclear waste disposal in stars.

Electromagnetic emissions

Radio technology and the ability to construct a radio telescope are presumed to be a natural advance for technological species, 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, , would not be able to detect non-directional radio signals even at a fraction of a light-year away, but other civilizations could hypothetically have much better equipment.

A number of astronomers and observatories have attempted and are attempting to detect such evidence, mostly through the SETI organization. Several decades of SETI analysis have not revealed any unusually bright or meaningfully repetitive radio emissions.

Direct planetary observation

Exoplanet detection and classification is a very active sub-discipline in astronomy; the first possible terrestrial planet discovered within a star's habitable zone was found in 2007. New refinements in exoplanet detection methods, and use of existing methods from space (such as the Kepler and TESS missions) are starting to detect and characterize Earth-size planets, to determine whether they are within the habitable zones of their stars. Such observational refinements may allow for a better estimation of how common these potentially habitable worlds are.

Conjectures about interstellar probes

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. Self-replicating probes could exhaustively explore a galaxy the size of the Milky Way in as little as a million years. 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.

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, though the entirety of the Solar System is vast and difficult to investigate. Attempts to signal, attract, or activate hypothetical Bracewell probes in Earth's vicinity have not succeeded.

Searches for stellar-scale artifacts

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 possible 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.

There have been some attempts to find evidence of the existence of Dyson spheres that would alter the spectra of their core stars. Direct observation of thousands of galaxies has shown no explicit evidence of artificial construction or modifications. In October 2015, there was some speculation that a dimming of light from star KIC 8462852, observed by the Kepler space telescope, could have been a result of Dyson sphere construction. However, in 2018, 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 culprit for causing the dimming.

Hypothetical explanations for the paradox

Rarity of intelligent life

Extraterrestrial life is rare or non-existent

Those who think that intelligent extraterrestrial life is (nearly) impossible argue that 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 assumption, called the rare Earth hypothesis, a rejection of the mediocrity principle, complex multicellular life is regarded as exceedingly unusual.

The rare Earth hypothesis argues that the evolution of biological complexity requires a host of fortuitous circumstances, such as a galactic habitable zone, a star and planet(s) having the requisite conditions, such as enough of a continuous habitable zone, the advantage of a giant guardian like Jupiter and a large moon, 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 perhaps most importantly, advanced life needs whatever it was that led to the transition of (some) prokaryotic cells to eukaryotic cells, sexual reproduction and the Cambrian explosion.

In his book '' Wonderful Life'' (1989), Stephen Jay Gould suggested that if the "tape of life" were rewound to the time of the Cambrian explosion, and one or two tweaks made, human beings most probably never would have evolved. Other thinkers such as Fontana, Buss, and Kauffman have written about the self-organizing properties of life.

Extraterrestrial intelligence is rare or non-existent

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 have any ability to tell if any detected life is intelligent. This is sometimes referred to as the "algae vs. alumnae" problem.

Charles Lineweaver states that when considering any extreme trait in an animal, intermediate stages do not necessarily produce "inevitable" outcomes. For example, large brains are no more "inevitable", or convergent, than are the long noses of animals such as aardvarks and elephants. Humans, apes, whales, dolphins, octopuses, and squids are among the small group of definite or probable intelligence on Earth. And as he points out, "dolphins have had ~20 million years to build a radio telescope and have not done so".Paleontological Tests: Human Intelligence is Not a Convergent Feature of Evolution.
, Charles Lineweaver, Australian National University, Canberra, published in ''From Fossils to Astrobiology'', edited by J. Seckbach and M. Walsh, Springer, 2009.

In addition, Rebecca Boyle points out that of all the species who have ever evolved in the history of life on the planet Earth, only one—we human beings and only in the beginning stages—has ever become space-faring."Galaxy Simulations Offer a New Solution to the Fermi Paradox"
Quanta Magazine "Abstraction Blog," Rebecca Boyle, March 7, 2019. “The sun has been around the center of the Milky Way 50 times," said Jonathan Carroll-Nellenback, astronomer at the University of Rochester.

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 non-avian dinosaurs is the best known example. 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.

Evolutionary explanations

Intelligent alien species have not developed advanced technologies

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 also be very difficult to detect. A trip using conventional rockets would take hundreds of thousands of years to reach the nearest stars.

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.

Another hypothesis in this category is the "Water World hypothesis". According to author and scientist David Brin

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