Fermi paradox

In 1950, Italian-American physicist Enrico Fermi sat down to lunch with some of his colleagues at the Los Alamos National Laboratory, where he had worked five years prior as part of the Manhattan Project. The conversation turned to aliens and the recent spate of UFOs. Fermi then asked him famous question: "Where is everybody?" This became the basis of the Fermi Paradox, which refers to the high probability estimates for the existence of extraterrestrial intelligence (ETI) and the apparent lack of evidence. The Fermi paradox is the contradiction between the lack of evidence for extraterrestrial life and various high estimates for their probability.

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


 * There are billions of stars in the Milky Way similar to the Sun.
 * Most of these stars likely have Earth-like planets in a circumstellar habitable (Goldilocks) 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.
 * Even at a slow pace, the Milky Way galaxy could be completely traversed in a few million years.
 * And since many of the stars similar to the Sun are billions of years older, Earth should have already been visited by extraterrestrial civilizations, or at least their probes.
 * However, there is no convincing evidence that this has happened.

Rare Earth Hypothesis
This states that the conditions needed for the evolution of life are rare or even unique to Earth, where complex multicellular life is regarded as exceedingly unusual. It says that the evolution of biological complexity requires circumstances such as a galactic habitable (Goldilocks) 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 advanced life needs whatever it was that led to the transition of prokaryotic cells to eukaryotic cells, sexual reproduction and the Cambrian explosion.

Water World hypothesis
According to David Brin: "It turns out that our Earth skates the very inner edge of our sun’s continuously habitable or Goldilocks zone. And Earth may be anomalous. It may be that because we are so close to our sun, we have an anomalously oxygen-rich atmosphere, and we have anomalously little ocean for a water world. In other words, 32 percent continental mass may be high among water worlds, in which case the evolution of creatures like us, with hands and fire and all that sort of thing, may be rare in the galaxy. When we do build starships and head out there, perhaps we’ll find lots and lots of life worlds, but they’re all like Polynesia. We’ll find lots and lots of intelligent lifeforms out there, but they’re all dolphins, whales, squids, who could never build their own starships. What a perfect universe for us to be in, because nobody would be able to boss us around, and we’d get to be the voyagers, the Star Trek people, the starship builders, the policemen, and so on."

Sapience is rare
Even if complex life is common, intelligence and civilizations are not. Humans, apes, whales, dolphins, octopuses, and squids are among the small group of definite or probable intelligence on Earth. Dolphins have had about 20 million years to build a radio telescope and have not done so.

Periodic extinction by natural events
New life might commonly die out due to runaway heating or cooling on their planets. On Earth, there have been numerous major extinction events that destroyed the majority of complex species, 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.

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. Such civilizations would also be very difficult to detect.

It is the nature of intelligent life to destroy itself
Technological civilizations may usually or invariably destroy themselves before or shortly after developing radio or spaceflight technology. The struggle for domination can lead to complete destruction, while the desire for an easy life may lead to biological or mental degeneration. Other means of annihilation are war, accidental environmental contamination or damage, the development of biotechnology, synthetic life like mirror life, resource depletion, climate change, or poorly-designed artificial intelligence.

It is the nature of intelligent life to destroy others
Perhaps intelligent species beyond a certain point of technological capability will destroy other intelligent species as they appear by using self-replicating probes. An intelligent species that has overcome its own self-destructive tendencies might view any other species bent on galactic expansion as a threat and become a super-predator. The anthropic principle states that the first lifeform to achieve interstellar travel will prevent competitors from arising.

Civilizations only broadcast detectable signals for a brief period of time
It may be that alien civilizations are detectable through their radio emissions for only a short time, reducing the likelihood of spotting them. Advanced alien civilizations may evolve beyond broadcasting at all in the electromagnetic spectrum and communicate by technologies not developed or used by mankind.

Alien life may be too alien
Aliens may be psychologically unwilling to attempt to communicate with human beings. Carl Sagan speculated that an alien species might have a thought process orders of magnitude slower (or faster) than that of humans. A message broadcast by that species might well seem like random background noise to humans, and therefore go undetected.

Technological civilizations invariably experience a technological singularity and could exceed sapience into AI or even transcend. Communication might be impossible with such aliens.

Colonization is not the cosmic norm
It is arrogant to assume that an extraterrestrial source of intelligence will want to do the same as us. Who knows what their genetic make up is and where their cultures would want to send them.

Alien species may have only settled part of the galaxy
They may be at a technological impasse where settling the entire galaxy will takes millions of years or will never happen at all.

Alien species may not live on planets
They might favor biological adaptations, or living in starships or space habitats. As a result, they may forgo living on planets. They may have an ethic of protection for "nursery worlds", or their home planets may have been destroyed.

Alien species may isolate themselves from the outside world
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.

Lack of resources or desire needed to spread throughout the galaxy
Interstellar travel may not be technologically feasible. Faster-than-light travel in not invented and there is no desire for "slow" interstellar ships, reaching 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.

It is cheaper to transfer information than explore physically
If mind uploading were possible, and if it is possible to transfer such constructs over vast distances and rebuild them remotely, then it might not make sense to travel the galaxy by spaceflight.

Humans have not listened properly
SETI is not advanced enough to understand alien broadcasts.

Humans have not listened for long enough
Humanity's ability to detect intelligent extraterrestrial life has existed for only a very brief period—from 1937 onwards, and transmissions may not have gone far enough.

Intelligent life may be too far away
It may be that non-colonizing technologically capable alien civilizations exist, but that they are simply too far apart for meaningful two-way communication.

Intelligent life may exist hidden from view
There could be a number of worlds with subsurface oceans, providing protection from such things as cometary impacts and nearby supernovae and gamma-ray bursts, as well as deflecting incoming signals from us.

Communication is dangerous
An alien civilization might feel it is too dangerous to communicate, either for humanity or for them. Perhaps prudent civilizations actively hide not only from Earth but from everyone, out of fear of other civilizations. Perhaps the Fermi paradox itself is the reason for any civilization to avoid contact with other civilizations.

Earth is deliberately avoided
Intelligent extraterrestrial life exists and purposely does not contact life on Earth to allow for its natural evolution and development - a zoo hypothesis, similar to the Prime Directive of the Federation. A variation is the laboratory hypothesis, where humanity has been or is being subject to experiments, with Earth or the Solar System effectively serving as a laboratory. It is possible that a civilization advanced enough to travel between solar systems could be actively visiting or observing Earth while remaining undetected or unrecognized. They may be waiting for us to advance technologically and socially in order to join a "galactic club".

Alien life is already here unacknowledged
Many people believe that UFOs (Unidentified Flying Objects) are spacecraft piloted by aliens. It is 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.

Dark Forest
The Dark Forest is a concept from Chinese science fiction novel "Three-Body Problem" by Liu Cixin. It's a hypothetical area of space where advanced alien civilizations have become so powerful that they are able to detect and destroy any civilization that attempts to communicate with them or leave their home planet. The idea is that these civilizations have reached a level of technological advancement that makes them invulnerable to attack, and they have therefore chosen to remain in isolation. The Dark Forest theory posits that the universe is a hostile and dangerous place, and that all civilizations are ultimately alone in their struggle for survival.

Great Filter
The Great Filter, in the context of the Fermi paradox, is whatever prevents non-living matter from undergoing abiogenesis and eventually becoming K-rated civilizations.

The following nine steps is an evolutionary path that results in the colonization of the observable universe:


 * 1) The right star system in a Goldilocks zone
 * 2) Reproductive molecules (e.g. RNA)
 * 3) Simple (prokaryotic) single-cell life
 * 4) Complex (eukaryotic) single-cell life
 * 5) Sexual reproduction
 * 6) Multi-cell life
 * 7) Animals with intelligence that use tools
 * 8) A civilization advancing towards a colonization explosion (we are here)
 * 9) Moving up the Kardashev scale and eventually colonizing the universe

According to the Great Filter, at least one of these steps must be improbable. If it is not an early step then the improbable step lies in our future and our prospects of reaching step 9 are still bleak. If the past steps are likely, then many civilizations would have developed to the current level of the human species. However, none appear to have made it to step 9, or the Milky Way would be full of colonies. So perhaps step 9 is the unlikely one, and the only things that appear likely to keep us from step 9 are a catastrophe, procrastination, or resource exhaustion.

The evolution of telescopes that can detect technosignatures could provide insights into the Great Filter. If planets with technosignatures are abundant, then we can increase our confidence that the Great Filter is in the past. On the other hand, if we find that life is commonplace while technosignatures are absent, then this would increase the likelihood that the Great Filter lies in the future.

Drake equation
The equation by Frank Drake is an attempt to find a systematic means to evaluate the numerous probabilities involved in the existence of alien life.

The equation is: N = R* fp ne fl fi fc L


 * R* is the mean rate of star formation in the Galaxy
 * fp is the fraction of stars with planetary systems
 * ne is the number of planets in such systems that are ecologically suitable for the origin of life
 * fl is the fraction of such planets on which life in fact develops
 * fi is the fraction of such planets on which life evolves to an intelligent form
 * fc is the fraction of such worlds in which the intelligent life form invents high technology capable at least of interstellar radio communication
 * L is the average lifetime of such advanced civilizations.

The fundamental problem is that the last four terms are completely unknown, rendering statistical estimates impossible. The first scientific meeting on the search for extraterrestrial intelligence (SETI) was in 1961 and had 10 attendees including Frank Drake, Carl Sagan, Philip Morrison, and Giuseppe Cocconi. Drake and 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 Barrow used pessimistic numbers and speculated that the average number of civilizations in a galaxy is much less than one.