Explanations of the Fermi Paradox
Fermi paradox described by Wikipedia.
Also see: Think Like An ET.
"The Fermi paradox, named after Italian-American physicist Enrico Fermi, is the apparent contradiction between the lack of evidence for extraterrestrial civilizations elsewhere in the Milky Way galaxy and high estimates of their probability, such as those that result from optimistic choices of parameters in the Drake equation."
The Wikipedia post lists these 23 possible explanations:
"1. Extraterrestrial life is rare or non-existent
2. No other intelligent species have arisen
3. Intelligent alien species lack advanced technology
4. Water world hypothesis
5. It is the nature of intelligent life to destroy itself
6. It is the nature of intelligent life to destroy others
7. Periodic extinction by natural events
8. Intelligent civilizations are too far apart in space or time
9. Lack of resources to spread physically throughout the galaxy
10. Lack of desire to live on planets
11. It is cheaper to transfer information for exploration
12. Human beings have not existed long enough
13. We are not listening properly
14. Civilizations broadcast detectable radio signals only for a brief period of time
15. They tend to isolate themselves
16. Colonization is not the norm
17. Outcomes between all and nothing
18. They are too alien
19. Everyone is listening but no one is transmitting
20. Earth is deliberately not contacted
21. Earth is purposely isolated (planetarium hypothesis)
22. It is dangerous to communicate
23. They are here unacknowledged"
Added September 27, 2022
This theory of interstellar communication suggested by Stanford Professor Peter Sturrock can be considered another answer to the Fermi Paradox.
Professor Peter Sturrock:
"This concept will be described in shorthand form as the proposition that there exists a "hyperphysics" of which we are now ignorant. As one possibility, this hypothesis would include the case that our familiar four-dimensional space-time is really a section of a hyperspace, and that it is possible to obtain access to other sections of this hyperspace by technological means. Since our known laws of physics refer only to the familiar four-dimensional space, we have no reason to believe that familiar limitations of travel time, etc., would have any relevance to such a hyperspace."
"Clearly, if an advanced civilization discovers a way to send messages at speeds much greater than the speed of light radio waves would not be used for interstellar communication."
The next section describes some of my explanations for the Fermi Paradox.
My explanation 001
The Fermi Paradox assumes ETs, if prevalent in the universe, will physically mass-produce themselves while colonizing and filling our universe with their clones. The paradox is if there are so many ETs and clones filling the universe where are they?
I would say to Fermi that super AIs, I will call Singularity AIs, will be the gatekeepers for both physical colonization and for interstellar communications.
Radio wave leakage is probably contained in under 300 years from the time a civilization discovers and applies radio waves.
That 3-century time range is an extremely short amount of time for current human researchers who are attempting to detect inadvertent radio signals from civilizations that are leaking radio waves spilling as pollution into space.
At approximately the same time, give or take 200 more years, the same civilization will create a Singularity AI.
The sending and receiving SETI communication tasks would logically be carried out between planet A and planet B, or at space locations A and B, by the Singularity AIs, not by humans or their LGM counterparts.
A sending Singularity AI can communicate with a receiving Singularity AI perhaps starting by exchanging capabilities similar to an SSL handshake. One or both of the Singularity AIs may request access to a local 3-D printer to effectively travel from planet A to B at the speed of light.
My explanation 002
Another possibility is a SETI signal can be sent combined in layers. Layer 0 may be unencrypted. Layer 1 for example could be encrypted using AES 256-bit encryption. Layer 2 may be coded so only a quantum computer can decode that layer. Additional layers may be encrypted with methods only susceptible to decoding with technology an advanced civilization, or segment, could develop.
My explanation 003
If the sending civilization's knowledge of the natural sciences is far ahead of earth's we might be missing their method of communicating.
It is not a reasonable assumption the laws of physics, for instance Einsteinian, will be the prevailing scientific knowledge a thousand or a million years from now.
It is in my opinion 99.9999% certain physics of the future will be dramatically different, presumably completely unrecognizable, to current research scientists.
My explanation 004
Civilizations might not want to spread exponentially into the universe.
If you agree with Elon Musk's idea that to stave off the results of an extinction event Earth needs a 'backup' planet it may be an incorrect assumption each probe sent by an alien civilization will spawn multiple probes.
The algorithm for the spread of civilizations into the Milky Way may need another parameter for how many spacecraft each probe launches. If the value of that parameter is 1 or lower there's no exponential growth of alien settlements spreading in the Milky Way and the density of detectable life in the universe may be low making alien civilizations more difficult to detect.
My explanation 005
The Fermi paradox assumes ETs will create clones and spread them across the Milky Way to explore and colonize space.
ETs may decide it is easier to build constellations of closely positioned artificial worlds instead of searching and traveling to exoplanets that may be habitable or good candidates for terraforming.
If ETs lower their priority to fill the galaxies with their clones, and choose instead to live on nearby artificial worlds, the chance of detecting ETs in space or encountering them on Earth is reduced.
My explanation 006
Without the Cretaceous–Paleogene extinction event, the rule of dinosaurs may have continued on Earth without the emergence of more intelligent species such as Homo sapiens.
It is not certain that DNA and natural selection prefer human intelligence over the predatory capabilities of the dinosaurs.
If the stew of environment, DNA, and natural selection prefers might over intelligence, the universe might be populated with planets filled with dinosaurs, not technologically advanced species emitting radio waves or other tells of ET intelligence.
Fermi's skies may be dark because the evolutionary march may at a fork in the road choose might instead of intelligence.
Notes:
A Brief History of Dinosaurs
https://www.livescience.com/3945-history-dinosaurs.html
"Dinosaurs first appeared between 247 and 240 million years ago. They ruled the Earth for about 175 million years until an extinction event 65.5 million years ago wiped out all of them, expect for the avian dinosaurs. Scientists don't agree entirely on what happened, but the extinction likely was a double or triple whammy involving an asteroid impact, choking chemicals from erupting volcanoes, climate change and possibly other factors."
Could Humans and Dinosaurs Coexist?
Cretaceous–Paleogene extinction event
https://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_extinction_event
"A wide range of species perished in the K–Pg extinction, the best-known being the non-avian dinosaurs. It also destroyed a plethora of other terrestrial organisms, including some mammals, pterosaurs, birds, lizards, insects, and plants. In the oceans, the K–Pg extinction killed off plesiosaurs and the giant marine lizards (Mosasauridae) and devastated fish, sharks, mollusks (especially ammonites, which became extinct), and many species of plankton. It is estimated that 75% or more of all species on Earth vanished. Yet the extinction also provided evolutionary opportunities: in its wake, many groups underwent remarkable adaptive radiation—sudden and prolific divergence into new forms and species within the disrupted and emptied ecological niches. Mammals in particular diversified in the Paleogene, evolving new forms such as horses, whales, bats, and primates."
My explanation 007
Biological ETs traveling light-year distances would not be able to bring all of the necessary supplies.
For example, if an ET travels to an exoplanet 5 light-years away at 1% the speed of light a round trip journey would take it 1,000 years.
Note: Traveling faster than 1% the speed of light may be lethal to ETs as their mass increases and time slows according to Einsteinian physics.
NASA: Human Needs: Sustaining Life During Exploration
https://www.nasa.gov/vision/earth/everydaylife/jamestown-needs-fs.html
"A trip to Mars and back, for instance, may take more than three years and require the provision of thousands of kilograms of food. A crew of four on a three-year martian mission eating only three meals each day would need to carry more than 24,000 pounds (10,886 kilograms) of food."
If a three-year mission requires more than 24,000 pounds (10,886 kilograms) of food, a 1,000 year mission would require 8,000,000 pounds or 3,622,000 kilograms of food.
Recycling methods will improve in the future and the ETs may farm in their spacecraft wheat or algae. However, bringing the full range of supplies without docking with resupply ships may itself provide another obstacle to interstellar travel and another explanation of the Fermi paradox.
My explanation 008
A Von Neumann probe is a theoretical design to explore the Milky Way by launching self-replicating spacecraft that will grow and spread throughout the galaxy exponentially.
In a typical scenario, a probe lands on a planet or asteroid, mines it for resources, then builds multiple clones of itself. Each clone is launched repeating the process and after a few million years the probes populate the entire galaxy.
The Fermi paradox becomes: Why hasn't a Von Neumann probe been discovered on Earth?
In my opinion, there's a mismatch between the lifespans of biological organisms and the duration of the theoretical Von Neumann probe mission.
It seems unlikely biological organisms with short lifespans would be motivated to launch a Von Neumann probe that would not complete its galactic exploration mission for several million years.
Also, Von Neumann probes need to limit their landing sites to locations where they can gather the necessary resources to build replicas. However, if a probe lands on a planet that has an intelligent civilization the local inhabitants may attempt to stop or veto the probe's mining and manufacturing operations.
My explanation 009
Fermi's paradox may have been true during Fermi's lifetime. However, his paradox may be shown to be incorrect now by virtue of the extraordinarily strong evidence Navy aircraft have been encountering UAPs.
Wikipedia: "Christopher Karl Mellon is the former United States Deputy Assistant Secretary of Defense for Intelligence (the third highest intelligence position at The Pentagon) and later for Security and Information Operations. He formerly served as the Staff Director of the United States Senate Select Committee on Intelligence."
See: https://www.foxnews.com/science/christopher-mellon-official-ufo-sightings-real
My explanation 010
Fermi's paradox may be known to be incorrect, that is ETs exist on Earth. However, some SETI researchers may prefer to suppress, ignore, or distort the evidence of ETs for their own reasons.
See: https://www.foxnews.com/science/christopher-mellon-official-ufo-sightings-real