Description of the image: The allegory of the cave is a metaphorical representation of the ignorance that characterizes the human condition. The allegory depicts men who have been chained and immobilized in a cave since birth. They face away from the entrance and have never seen real objects, only the shadows of objects projected on the cave wall.
In about 3 arcminutes of the ultra-deep field of the sky, astronomers have counted approximately 10,000 galaxies. Some of these galaxies already existed when the 13.77-billion-year-old universe was only 800 million years old. Thus, there are an estimated 2 trillion galaxies in our observable universe. Considering there are about 100 billion stars in our galaxy alone, there would be 2 trillion times 100 billion stars in our universe. We now know that the presence of planets around a star is relatively common. If there were only one planet per star, then the number of planets would also be unimaginable (2 x 10²³). Physical conditions are the same throughout the universe. Galaxies contain stars, stars have planets, and planets are made of chemical elements generated by stars. Even if the conditions necessary for the emergence of an advanced civilization are unpredictable and contingent, the number of existing planets suggests that many civilizations could have developed somewhere in the universe. It would be surprising if nature, having structured itself in the same way at all scales, had only found the path to life on our planet. The question posed by Enrico Fermi in 1950 during an informal conversation stems from this observation.
"Are we the only intelligent and technologically advanced civilization in the Universe?" In other words, if there were extraterrestrial civilizations, their representatives should already be here. So, where are they? No scientific evidence has been detected since the advent of technology—no probes, no spacecraft, no radio transmissions, and thus no irrefutable traces. However, it is hard to believe that life exists only on Earth because the building blocks of life have been detected in the interstellar medium (prebiotic molecules, amino acids, peptides). And everywhere there is liquid water, there is a possibility of life, even in places where the sun's energy does not penetrate, such as in our abysses. Isn't the tenacity of life proof that it is present everywhere in the universe, patiently waiting for a favorable context to continue its path toward intelligence? An entire universe had to be born, galaxies had to merge, stars had to die to generate all the chemical elements, and a stellar system had to stabilize in a protected zone of a galaxy for intelligent life to appear on a planet, ours, which took 13.61 billion years (the age of the Milky Way). And we do not yet have the technology to travel in the Galaxy... Since it takes practically the age of the universe for a civilization capable of leaving its planet to appear, we could immediately conclude that there is no paradox, we are alone because we are the first. Let's adopt the principle of mediocrity and abandon this anthropic view of extraterrestrial life. Let's try to scientifically analyze the possibilities that we are not alone!
Among the 100 billion stellar systems in the Galaxy, there are probably many planets similar to Earth. But the Rare Earth hypothesis argues that the emergence of intelligent life on our planet is contingent because it required a very long and improbable sequence of astrophysical, geological, biochemical, biological, and Darwinian events, which makes it unique. Indeed, the appearance of life as it exists on Earth had to overcome a considerable number of barriers. Among these difficult-to-cross barriers are, for example, photosynthesis, the appearance of DNA, the transition from prokaryotes to eukaryotes, endosymbiosis, the transition to multicellular life, sexual reproduction, the appearance of intelligent beings, the discovery of sciences, and the transition to a technologically advanced civilization. All these barriers were crossed only once on Earth in 4.543 billion years (the age of the Earth), making the sequence extremely improbable.
This hypothesis forces us to abandon the idea that this has happened many times and therefore we would be the first to want to explore and colonize the universe. The Rare Earth hypothesis opposes the principle of mediocrity, which states that the Earth is an ordinary planet, in an ordinary planetary system, located in an ordinary region of an ordinary galaxy. If the emergence of life as we see it on Earth is improbable, it does not mean that we are alone in the universe. The complexity of our existence may just be in its infancy; much more complex, unimaginable lives may exist.
Travel is only possible within the galaxy! The closest star to us is 4.3 light-years away. With current technology (chemical propulsion), we can travel, using other planets for assistance, at 30 km/s or 1/10,000 of the speed of light. At this speed, a probe would travel for about 40,000 years before reaching the vicinity of Proxima Centauri. Imagine we manage to reach 10% of the speed of light in the coming centuries. It would still take 40 years to visit the nearest star, knowing that the diameter of the Milky Way is about 100,000 light-years. However, the Earth is 4.54 billion years old, the first traces of life (cyanobacteria) are 3.48 billion years old, the first fish appeared 530 million years ago, the first mammals 125 million years ago, the first primates 56 million years ago, the first hominids 10 million years ago, and Homo sapiens 300,000 years ago. Agriculture appeared 12,000 years ago, the space age is 65 years old (Sputnik 1 in 1957), and Voyager 1 left the solar system in 2012.
This rapid acceleration allows us to think that a few centuries will suffice to reach the nearest stars in a few years with another technology (nuclear fusion propulsion or antimatter). As soon as the first star is reached, humans could move to the next nearest star in a few thousand years, and so on. At this rate, even if some civilizations do not adopt the concept of colonization, it would take only a few tens of millions of years to colonize the habitable zone of the Milky Way. But the impassable barrier of the speed of light forces us to abandon intergalactic travel, which would take billions of years. The distances between galaxies are so great that communication would be impossible, and our curiosity would remain unsatisfied. We will probably never leave our galaxy.
Another reason why we detect no trace of intelligent life could be the short lifespan of a civilization. It takes a long time for a biological system to stabilize and for a technologically advanced civilization to appear. It took 4.54 billion years on Earth. Many natural catastrophes can prevent or delay this advent. In the last 500 million years, there have been at least five mass extinctions due to a great glaciation (~445 Ma), major biological crises (~370 Ma and ~250 Ma), volcanic episodes, and asteroid impacts (~200 Ma and ~65 Ma).
But many other natural disasters (resource depletion, self-destruction, climate disruption) or very close cosmic events (gamma-ray bursts, supernova explosions) can slow down the advent of a technologically advanced civilization. We have only been capable of emitting electromagnetic signals for about a hundred years, which is a very short time compared to the age of the Earth. Our very weak electromagnetic signals have only reached the closest stars, those within 100 light-years.
Furthermore, cultural evolution and biological evolution are two distinct processes that interact in complex ways. Is cultural evolution compatible with biological evolution?
Cultural evolution is faster than biological evolution, it can change the environment at a speed to which biological structures can no longer adapt.
It is possible that all civilizations reach advanced digital technology and eventually understand that interstellar travel is useless and uninteresting, that distances are too great, that the necessary energy is too expensive, and that any potential descendants who left Earth would disappear forever into the dark zones of the Milky Way. On the other hand, machines that can contain all the information of a human body would be much more interesting. Physical world exploration becomes unnecessary, and new civilizations prefer to live exclusively in their own simulations. By creating new civilizations populated by avatars, they transform the real world, imperfect, degenerative, and corruptible, into intelligible and perfect virtual worlds.
These worlds, as rich as the real world, end up possessing a degree of reality superior to material and sensory reality. It is likely that by living in much larger virtual worlds where everything works better, the real world becomes uninteresting and sublimates into a virtual world. The boundary between real and virtual becomes porous, and all these new complex civilizations end up getting lost in the immersive parallel virtual worlds of metaverses, which are much easier and faster to create. If extraterrestrials have abandoned space exploration in favor of virtual reality, we will never hear from them. This could be one of the solutions to the Fermi paradox.
Extraterrestrials are already here, but we don't see them! With only a million years of advancement, an extraterrestrial civilization might no longer interfere with matter. If they no longer use electromagnetic waves, we cannot sense their presence; their advanced technology would render them indiscernible. In Plato's allegory of the cave, the characters (427-347 BC) are prisoners of their point of view, seeing only the shadows of objects. The prisoners, immobilized and unable to see the cave's entrance, consider the projected shadows as the real objects. From the outside world, they know only the faint glow that reaches them. Plato's cave represents the place of confinement, ignorance, and appearances. We humans are all conditioned from childhood by our material environment, which is perceived through our senses and integrates into us like a second skin. But our earthly environment forces us to live in a base world, one of physiological needs and the essential animal conditions for survival.
Like the prisoners in the cave, we live in a sensory world that may be nothing more than a falsification of reality. For Plato, the material and physical reality is not the true reality. In the allegory of the cave, he opposes the sensory world to the intelligible world (sense and intellect, shadow and light). The cave symbolizes the sensory world, which is only appearance. Our knowledge is not sufficiently developed, and it is possible that, like Plato's characters, we see only one facet of reality. Perhaps we should colonize our inner space and forget about the too costly interstellar journeys. To emerge from this state of ignorance and access knowledge, many intermediate steps will be necessary, and the path will be very long. As in Plato's cave, our eyes are not accustomed to the light, and many dazzling experiences will be essential to reach knowledge. Only then will we be able to answer Fermi's paradox: Are we alone or accompanied?
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