Once physicist Enrico Fermi asked himself: where is everyone? Or, more precisely, "where are all the aliens?" This is how the Fermi paradox arose. When we estimate the size of the universe, the number of Earth-like planets, and a number of other variables (specified in the Drake equation), it becomes clear that there must be tens of thousands or more extraterrestrial civilizations in one galaxy.
And since the galaxy is about 10 billion years old, intelligent worlds have had enough time to communicate with each other. So if aliens statistically exist, where are they? Why can't we find them?
Earth is special
The Unique Earth Hypothesis suggests that the chain of events that created life on our planet was so complex that only a biologically perfect vortex could create it elsewhere.
While there may be planets similar to Earth, none of them may contain exactly the necessary conditions for the development of life. In other words, we have not met any aliens, because they simply do not exist, or there are so few of them and they are so far away that contact is extremely unlikely.
The main factor that makes the Earth hospitable to life is its long period of relatively stable climatic conditions due to the unique orbit and position of the planet. If it weren't for our exact position relative to the Sun or Moon, the planet would be too hot or cold, not enough oxygen, and too unstable conditions to support life outside of bacteria.
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Paleontologist Peter Ward and astronomer Donald Brownlee were the first to present the hypothesis of a unique Earth. And although 15 years have passed since the hypothesis was born, and we have found a mass of Earth-like planets, scientists are still confident that the chances of life on these worlds are extremely low.
Any sentient life has a stumbling block
According to the Great Filter theory, alien life does exist, but intelligent life is unable to advance technologically far enough to invent long-range space communication or space travel.
While our modern spaceships, satellites, and radios suggest that we are close to a point of no return, perhaps we will inevitably face a barrier or disaster that will either destroy us or degrade technology.
We know that catastrophic natural disasters periodically visit Earth, so it is entirely possible that these same events destroy worlds everywhere, sending intelligent life back to the Stone Age before technology can adequately develop. Or perhaps we will destroy ourselves in a nuclear war. Whatever the filter, only bad news awaits us. Not only will we not be able to contact other astronauts, but we will probably die.
There is also a bright side to this theory. Some believe that we are the first to pass through this filter, therefore, most likely, will become the dominant race in space. And this is good.
They left the universe
According to John Smart's futuristic supremacy hypothesis, intelligent extraterrestrial life once existed in our universe, but became so perfect that it moved to better places. To be more precise, aliens have evolved so much that they stopped looking at outer space and focused on inner space.
The concept can be compared to the miniaturization we see in computers. They were originally conceived as huge technologies that fill entire rooms, but then they became smaller (even pocket-sized) while developing complexity and power. For supremacists, intelligent life evolves in much the same way, continually working towards denser and more efficient uses of space, time, energy, and matter.
Eventually, we will live and work on nanoscale until we are so small that we create and live in a black hole outside this space-time continuum.
For Smart and others, black holes are the ultimate destination. They will enable perfect computation and learning, temporary travel, energy extraction and more. Civilizations that do not achieve this are doomed.
Other cosmic beings can also work on their own superiority, or transcendence. In addition, by following Moore's Law, these beings are more likely to achieve superiority before mastering space.
The earth is not as big as we think
Perhaps it would be too proud to believe that aliens are interested in us or our planet. There may be more interesting inhabited worlds, and intelligent creatures would rather spend their time exploring them, rather than on Earth. This theory is completely opposite to the theory of a unique Earth: there is nothing special about our planet.
An extraterrestrial civilization capable of moving or communicating for light years will hardly worry about our existence - we do not talk to flies. In addition, she will definitely have her own smart technologies, so they don't need our meager resources. If they need minerals or other elements, there are plenty of them in space and without Earth.
Plus, no matter how intelligent the creatures are, traveling light years is not an easy feat. What are the chances that they will expend their energy to visit us when there are 8.8 billion Earth-like planets in the entire Milky Way? The followers of this theory argue that to think that the Earth is a tidbit is to suffer the same geocentrism that led to the mistaken persecution of Galileo.
We live in virtual reality
Perhaps one of the most hard-to-read explanations for the Fermi Paradox is the planetarium hypothesis. Our world is a form of "a planetarium in virtual reality, created to give us the illusion that the universe is empty." We did not find any extraterrestrial civilizations because these extraterrestrial civilizations did not include it in the program.
The foundations of this theory are rooted in the ideas of Descartes, who asked the question: "How can we know that the world around us is real, if we are just a brain in a vat, which thinks that it lives in the real world?"
Instead of putting brains in a vat, modern proponents of these ideas think that we are living in a computer simulation created by advanced aliens. These aliens use enough energy to manipulate matter and energy on a galactic scale. Why do they want to watch us like ants? Maybe just for fun, or maybe just to make sure they can.
You will be surprised, but philosophers and physicists take this idea very seriously. They say that we would rather be artificial intelligence in a fabricated world than have our own mind. In addition, we are more likely to find a simulation, since we will definitely notice a failure in the system or develop an adequate experiment to confirm this theory.
We live in the outer space
While intelligent alien life may exist, our planets may be too far apart for communications to be justified or possible. The earth may be so far from other inhabited worlds that we may simply not be seen.
If that's not enough to make you feel lonely, some would argue that most other worlds are relatively close to each other and interact while we are outside of this pangalactic party.
The roots of this idea go back to a mathematical theory known as percolation, which describes how things pull together in a random environment. Taking percolation theory as a basis, the universe naturally formed in the form of large clusters and several smaller regions. Other sentient beings live in large clusters, and the Earth is isolated.
Instead of trying to make contact with these creatures, some like Stephen Hawking feel that we need to hide. Hawking says that if we receive an extraterrestrial signal, we need to be extremely careful not to respond until we evolve. Otherwise, we may repeat the fate of the Native Americans after the arrival of Columbus.
We haven't found their signals yet
Scientists like Frank Drake and Carl Sagan argue that "the absence of evidence is not evidence of the absence of aliens." The hunt for extraterrestrial life is severely limited by the lack of government funding that is needed to enable scientists to afford the vast arsenal of tools and resources for tracking aliens.
Historically, the Search for Extraterrestrial Intelligence (SETI) program has relied only on rented radio telescopes and other equipment that it can use for a limited time. These obstacles have made real progress nearly impossible.
There is good news for those who think contact with extraterrestrial life is a good idea. The Allen Telescope Array, a radio telescope specifically designed to search for extraterrestrial life, began operations in 2007.
This giant telescope, made up of 42 separate 6-meter telescopes, was built in large part thanks to Microsoft co-founder Paul Allen. After numerous setbacks, he finally started doing his important work. If something tries to contact the Earth, he will know about it.
We cannot recognize their signals
Even if other planets are hospitable to life, can creatures there develop similarly to life on Earth? Maybe they are so different from us that we are unable to distinguish their signals?
This is quite possible if we think about, for example, bats, which can visualize sound waves, although we only see light. Perhaps aliens and we are equipped with a fundamentally different set of senses.
As cosmologist and astrophysicist Lord Martin Rees noted, “They can look into our faces, but we just don't recognize them. The problem is that we are looking for something that is very similar to us, has similar math and technology. I suspect that life and mind can exist in forms that we cannot even imagine."
Things get even more complicated when trying to connect with a highly developed race that uses other methods of communication (neutrinos, for example, or gravitational waves) that are inaccessible to our understanding of technology. Likewise, our primitive radio may be nothing more than white noise to them. If aliens and humans are not alike, it is unlikely that we will be able to contact them and resolve the Fermi paradox - provided that we continue to anthropomorphize aliens and wait for them to contact us.
Superorganisms are inherently suicidal
Medea's hypothesis, invented by paleontologist Peter Ward, states that humans and other superorganisms carry the seeds of self-destruction. In a way, this ties in well with the Great Filter theory, as it assumes that we will die before we can contact an extraterrestrial civilization.
The hypothesis is named after Medea from Greek mythology, who killed her children. In our case, the planet is Medea, and we are her offspring. We don't want to die, but Mother Earth demands it. Extinction is built into our biology to ensure that we eliminate ourselves before we create a total imbalance on Earth. After humans become an incurable plague on the planet, we will do something to ensure our own demise.
Ward believes that nearly all previous mass extinctions were caused by living organisms. For example, he blames the two Snowball Earth periods millions of years ago on plants that have spread so wildly that they have absorbed excessive amounts of carbon dioxide.
This led to a global cooling, and, consequently, to the death of plants. Likewise, if humans are to blame for the current climate change, there is no guarantee that our own species will be able to survive.
In short, our inner self-destructive clock will stop long before we get a chance to interact with aliens.
They walk among us
Sounds like science fiction, but many people believe that aliens live and work around us. For example, former Canadian Defense Minister Paul Hellayer was interviewed in 2014, in which he claimed that there are about 80 different types of alien life on Earth.
Some of them, including the Scandinavian blondes, look almost identical to humans. Another group, the "Short Grays", are more alien-like and mostly hiding from humans.
Hellyer is not alone in his beliefs. Physicist Paul Davis of Arizona State University and Dr. Robert Trundle of Northern Kentucky University also believe aliens are walking among us.
Such people have already resolved their Fermi paradox - aliens exist and, whether people want it or not, they are among us. You have the right to choose any of the ten explanations.