Image: This 3.1 x 3.1 arc minute image required 800 exposures taken during the Hubble Space Telescope's 400 orbits around Earth. The total exposure time was 11.3 days (between September 24, 2003 and January 16, 2004). In this very small area of the southern sky located in the constellation of the Fourneau, there are about 10,000 galaxies.
Credit: NASA, ESA and S. Beckwith (STScI) and the HUDF team
Where are they then?
The Moon occupies in the sky about 31 minutes of angle or ½ degree. The ultra-deep field of the sky (image opposite) captured by the Hubble Space Telescope occupies 3.1 minutes in the sky, or about a tenth of the diameter of the Moon.
In this tiny area, astronomers have counted about 10,000 galaxies. Some of these galaxies already existed when the 13.77 billion year old universe was only 800 million years old.
There would therefore be about 2000 billion galaxies in our observable Universe.
If we consider that the number of stars present in our galaxy is only 100 billion, there would be 2000 billion x 100 billion stars in our universe. This number (200,000,000,000,000,000,000,000) is unimaginable.
We now know that the presence of planets around a star is relatively frequent. And if there was only one planet around each star, then the number of planets would also be unimaginable (2x1023).
The physical conditions are the same everywhere in the Universe. Galaxies contain stars, stars have planets, planets are made up of chemical elements generated by stars. Even if the conditions necessary for the emergence of an advanced civilization are unpredictable, contingent, the number of existing planets allows us to think that many civilizations have developed somewhere in the universe. It would be surprising if nature, having structured itself in the same way at all scales, found the way to life only on our planet.
The question posed in 1950 by Enrico Fermi during an informal conversation stems from this observation.
"Are we the only intelligent and technologically advanced civilization in the Universe?" That is, if there were extraterrestrial civilizations, their representatives should already be there. Where are they?
No scientific evidence has been detected since the advent of technology, no probes, no spacecraft, no radio transmissions and therefore no irrefutable traces.
However, it is difficult to believe that life only exists on Earth because the basic building blocks of life have been detected in the interstellar medium (prebiotic molecules, amino acids, peptides). And wherever there is liquid water, there is the possibility of life even in places where the energy of the sun does not penetrate like in our abysses.
Isn't the tenacity of life the proof that it is present everywhere in the Universe, patiently awaiting a favorable context to pursue its path towards intelligence?
It took a universe to be born, galaxies to merge, stars to die to generate all the chemical elements, a stellar system to stabilize in a protected area of agalaxy for intelligent life to appear on a planet, ours, or 13.61 billion years (age of the Milky Way). And we don't 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, one could immediately conclude that there is no paradox, we are alone because we are the first.
Let us adopt the principle of mediocrity and leave this anthropic vision of extraterrestrial life. Let's try to scientifically analyze the possibilities that we are not alone!
The Rare Earth Hypothesis
Image: the Earth photographed by Apollo 8 between 12/21 and 12/27/1968.
Among the 100 billion star systems in the Galaxy, there are probably many Earth-like planets. But the Rare Earth hypothesis holds that the emergence of intelligent life on our planet is contingent because it required a very long and unlikely sequence of astrophysical, geological, biochemical, biological, Darwinian, paleontological 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 barriers that are difficult to cross, there are, for example, photosynthesis, the appearance of DNA, the passage from prokaryotes to eukaryotes, endosymbiosis, the passage to multicellular life, sexual reproduction, the appearance of intelligent beings, the discovery of science, the transition to a technologically advanced civilization.
All these barriers have only been crossed once on Earth in 4.543 billion years (age of the Earth) which makes the sequence extremely unlikely.
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 says that the Earth is any planet, in any planetary system, located in a commonplace region of any galaxy.
If the emergence of life as we see it on Earth is unlikely, this does not mean that we are alone in the universe. The complexity of our existence may only be in its infancy, much more complex, unimaginable lives can exist.
NB: The expression "Rare Earth" comes from the book Rare Earth: Why Complex Life Is Uncommon in the Universe (2000), a study co-written by geologist and paleontologist Peter Ward, and astronomer and astrobiologist Donald E. Brownlee.
The insurmountable barrier
Image: Artist's impression of the Milky Way showing the position of the Sun in the Orion Arm, a protected area of our Galaxy's habitable zone.
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, with the help of other planets, at 30 km/s or 1/10000 of the speed of light. At this speed, a probe would travel for ∼40 000 years before arriving in the vicinity of Proxima Centauri.
Imagine that we manage to reach 10% of the speed of light in the centuries to come. It would still take 40 years to visit the nearest star knowing that the diameter of the Milky Way measures ∼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 are 530 million years old, the first mammals appeared 125 million years ago, the first primates, 56 million years, the first hominids, 10 million years and Homo sapiens, 300 000 years. 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 dazzling acceleration allows us to think that a few centuries will be enough to reach the first nearby stars in a few years with another technology (propulsion by nuclear fusion or thanks to antimatter).
As soon as the first star is reached, humans could return to the next nearest star in a few thousand years and so on. At this rate, even if certain civilizations do not adopt the concept of colonization, it would take only a few tens of millions of years to colonize the part of the Milky Way which is located in its habitable zone.
But the impassable barrier constituted by the speed limit of light obliges us to abandon intergalactic journeys which are measured in billions of years.
The distances between the galaxies are so great that communications would be impossible and our curiosity would not be satisfied. We will probably never leave our Galaxy.
Intelligent Life Extinction
Image: a gamma-ray burst could have contributed to the extinction of the Ordovician-Silurian (∼445 Ma). Credit NASA Gamma-ray burst light bubble.
Another reason why we don't pick up any traces of intelligent life could be the too 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 disasters can prevent or delay this event.
In the last 500 million years, there have been at least 5 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 (depletion of resources, self-destruction, climate change) or very close cosmic events (gamma bursts, supernova explosions) can slow down the advent of a technologically advanced civilization.
After all, we've only been able to emit electromagnetic signals for about a hundred years, which is a really short time compared to the age of the Earth. Our very weak electromagnetic signals only reached the nearest stars, those less than 100 light-years away.
Life has invaded virtual worlds
Image: Parallel worlds with artificial intelligence indistinguishable from reality end up producing their own simulations!!!
It is possible that all civilizations will reach advanced digital technology and eventually realize that interstellar travel is useless and uninteresting, that the distances are too great, that the energy required is too expensive and that in any case their eventual descendants having left the Earth will disappear forever into the dark areas of the Milky Way.
On the other hand, machines that can contain all the information of a human body would be much more interesting. The exploration of physical worlds become useless, the new civilizations prefer to live exclusively in their own simulations.
By creating new civilizations populated by avatars, they transform the real, imperfect, degenerating and corruptible world 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 sensible reality.
It is likely that by dint of living in much larger virtual worlds where everything works better, that the real world ends up becoming uninteresting and sublimates into a virtual world.
The border between real and virtual has become porous, all these new complex civilizations end up getting lost in the virtual and immersive parallel worlds of metaverses that are much easier and faster to create.
If the extraterrestrials have abandoned the conquest of space in favor of virtual reality, we will never hear from them.
This could be one of the solutions to the Fermi paradox.
They are there outside our cave
Image: The allegory of the cave is a metaphorical representation of the ignorance that characterizes the human condition. The allegory features men chained and immobilized in a cave since birth. They have their backs to the entrance and have never seen any real objects, they have only seen the shadows of objects projected onto the cave wall.
The extraterrestrials are already there but we don't see them!
With only a million years in advance an extraterrestrial civilization could no longer interfere with matter. If they no longer use electromagnetic waves, we cannot sense their presence their advanced technology would make them indistinguishable.
In the allegory of the cave, the characters of Plato (427-347 BC) are prisoners of their point of view, seeing only the shadows of objects. The prisoners immobilized and not seeing the entrance to the cave, consider the projected shadows as objects of reality. Of the outside world, they only know the faint gleam that manages to penetrate to them. Plato's cave represents the place of confinement, ignorance and appearances.
We human beings have all been conditioned since childhood by our material environment, which is perceived by our senses and which becomes part of us like a second skin.
But our terrestrial environment obliges us to live in a vile world that of physiological needs and animal conditions essential to survival.
Like the prisoners of the cave, we live in a sentient world that may just be 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 sensible world to the intelligible world (sense and intellect, shadow and light). The cave symbolizes the sensitive world which is only appearance.
Our knowledge is not complete enough and it is possible that, like Plato's characters, we only see one facet of reality. Maybe we should colonize our inner space and forget about expensive interstellar travel.
To get out of 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 light and many dazzles will be essential to achieve knowledge. Only then can we answer the Fermi paradox.
Are we alone or accompanied!!!