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Fermi Paradox: Where is Everybody?

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The Fermi paradox is the apparent contradiction between the lack of evidence for alien life and mathematical predictions that alien civilizations probably exist. Models such as the Drake Equation predict that extraterrestrial life exists based on the billions of stars in the galaxy that are similar to the Sun and estimates of the number of Earth-like planets.
Recent scientific estimates of the probable number of advanced alien civilizations in the Milky Way range from 2 to as high as 280,000,000. Given that may of the estimated 40 Billion Earth sized planets in the galaxy are older than the Earth it is thought that some of these civilizations would be more technologically advanced than humans.
A number of theories have been proposed to explain the Fermi Paradox:

Rare Earth Hypothesis

The theory that the conditions for life to occur are so rare that earth is unique in the galaxy or even the universe.

Rare Intelligence Hypothesis

The idea that life is somewhat common in the galaxy but that intelligence at the human level is exceedingly rare.

Rare Technology Hypothesis

The hypothesis that intelligence in the universe is typically more similar to the intelligence of dolphins or elephants whereby lifeforms don't end up developing technology.

Destructive Tendencies

Technologically advanced alien civilizations may have a tendency to destroy themselves by creating existential risks such as powerful weapons.

Natural Disasters

Disasters such as asteroid impacts may be common enough across the galaxy to be an obstacle to life or technological advancement.

Aliens Are Isolationists

Aliens may have a tendency to isolate themselves and hide for one reason or another.

Civilizations Outgrow Radio

The search for extraterrestrial intelligence (SETI) has focused on monitoring electromagnetic radiation that may not be commonly used by advanced alien civilizations.

Aliens Are Too Alien

Alien lifeforms may have features beyond our imagination that make them difficult to detect. For example, they may live beyond our experience of physics or reside in a virtual reality of some type.

Aliens Are Too Slow

The idea that aliens function at a fundamentally different speed than humans, making their signals appear as noise.

Everyone Is Listening No One Is Transmitting

Aliens may simply view transmission as an existential risk and avoid it. Such civilizations would have no reason not to listen.

Humanity Is Too Young

Many of the key technologies for detecting alien signals are less than 100 years old. This is a short period of time to a galaxy estimated to be 13.21 billion years old.

Broadcasters Are Destroyed

The idea that advanced civilizations that broadcast signals are destroyed by some other advanced civilization. According to this theory, only quiet civilizations have survived.

Conspiracy Theories

The theory that aliens are known to governments who have developed elaborate ways to hide this information.

Zoo Hypothesis

The hypothesis that aliens have not contacted us due to some code of conduct. For example, they may have the principle not to contact worlds they consider primitive so as not to disrupt their development.

Simulated Universe

The theory that much of the universe we see past our solar system is simulated to prevent us from discovering what's really there.


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