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Where Are They?
Nobel prize-winning Italian physicist Enrico Fermi asked the question "Where are they?" regarding the question of life existing elsewhere in the universe. If one assumes the mediocrity principle - that the development of life on earth is typical in comparison to the rest of the universe - we should see evidence of other life. The Fermi paradox is the contradiction between estimates of the number of extraterrestrial civilizations and a corresponding lack of evidence of these civilizations - both physical and radio (the Great Silence).
It's obvious that civilizations can exist - we are one. Why not others?
- The universe is extremely old. Its current age is estimated at approximately 13.73 billion years
- The number of stars in the visible universe is extremely large - approximately 5 x 1022.
- Some of these stars will have habitable planets which develop intelligent life that can produce radio signals.
- Interstellar travel is possible, and some civilizations desire to colonize stars.
More intelligent estimates of the Drake Equation have been possible with recent advances in astronomy and astrobiology. One estimate in the July 2000 edition of Scientific American suggests the number of civilizations that have existed in our galaxy in the past is 12 billion, with 1,000 of them still transmitting radio evidence of their existence.
Despite those assumptions, limited radio searches of the skies for nearly 50 years have found no evidence of extraterrestrial life. And, it would take only one civilization that desires so between 5 and 50 million years to colonize the entire galaxy - a blink of an eye compared to the age of the universe. So where are they?
Clearly there must be something wrong with assumptions that have been made. Some possible resolutions to the Fermi Paradox are:
A Great Filter drastically limits the number of civilizations in the universe. Life faces existential risks which are difficult to overcome. For example, the development of the ability to communicate by radio occurs at roughly the same time as the development of nuclear weapons. Professor Nick Bostrom claims that the lack of evidence of extraterrestrial life is a positive sign for the outcome of our own existence.
One or more of our assumptions is wrong. Perhaps we're not searching in the right way. Before the invention of radio, for example, the only way to search for life would have been optically. Perhaps neutrinos or some other communication method is used instead.
Transcendence - Life passing through a singularity stage may discover a way to exit the universe, or find more meaningful ways of existing. Maybe we are boring and they have no interest in making contact or communicating with us.
Perhaps we are living in a simulation.
The Zoo hypothesis - we are being isolated intentionally.
- Perhaps abiogenesis occurs less frequently than current assumptions and we really are alone.
What do you think?
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