Fermi Paradox - The stars are too far away! As far as we know, we have always been alone. No one has ever called us, no one has ever come to visit our planet. Yet our galaxy is full of stars, each with its planetary system. Billions of celestial bodies are suitable to host life forms, thousands of sentient beings wandering around, and dozens of civilizations that have pushed or are trying to push themselves into space.
But if that is the case, why didn't they come, why didn't they call us? Where is everyone? Italian physicist Enrico Fermi was the first to ask this question, in 1950. Since then scientists have proposed solutions to the paradox that bears his name. One of the best known came from Sagan himself, who said in an article in 1981 that we had to be patient. No one has visited us because they are all too far away; because a species intelligent enough to invent interstellar journeys evolves, it takes time, and more time for that species to spread in so many worlds. Some say that species with good technological abilities, when they emerge, then self-destruct. Others suggest that aliens may have visited us in the past, or that they are avoiding us on purpose. The most pessimistic response is in an article in 1975. In that article astrophysicist Michael Hart declared that the only plausible reason why no life has reached us is that there is no one out there.
The question of how the colonization of the galaxy has played a central role in attempts to solve the Fermi paradox. Italian physicist calculated it himself and then by others that a single space species could populate the galaxy in a few million years. And if we have no evidence of this, according to Hart it means that such civilizations do not exist. In the article "There is no Fermi Paradox", physicist Robert Freitas defines Fermi and Hart's reasoning as nonsense. He compared their logic to that which supports such a discourse: "Lemmings - small hamster-like rodents - reproduce, at a rate of about three litters per year and up to eight pups per litter. Calculation shows that in very few years the total mass of lemmings should equal the mass of the entire Earth's biosphere.
The Earth should then be covered by it... But most of us don't even notice their existence. Have you ever seen a lemming? “. Thinking along these lines would lead us to say that lemmings do not exist. In other words, to a completely crazy conclusion! Many different ideas, but the most reasonable solution to the Fermi paradox is in front of our eyes! The distance between the stars of our galaxy is too high to allow interstellar travel, simple! It is possible that, regardless of the development of Extra-Terrestrials, the distance barrier is hard to overcome. To support this thesis, let's try to take our Solar System as an example: its escape speed (starting from Earth) is 42 km/s.
Only a vehicle launched at this speed would be able to free itself from the gravitational attraction of the Sun. Well, our species has already built and launched several vehicles of this kind. Even if current technology forces us to cheat a little and take advantage of the gravitational aid offered by the planets with the so-called "slingshot effect" to reach the limit speed! The Voyager 1 probe, for example, launched on 5 September 1977. It visited the outer planets of the Solar System and then headed into space. Currently, its distance from the Sun is 152 Astronomical Unit, where 1 AU is the Earth-Sun distance. Unless unlikely, it will drift!
1.6 light-years away from a red dwarf called “Gliese 445”. The only problem is that the probe will take hundreds of thousands of years to reach the encounter. Voyager 1, in fact, is moving away from the Sun at a speed of 17.0 km/s; which may seem enormous in terms of human experience, but which in reality is only 0.000055 c, where "c" is the speed of light, which is known to be just under 300 thousand km/s...
This means that in all these years the probe has only managed to get 21 light-hours away from the Sun! To give an idea of the large difference, the nearest star to our Sun, Proxima Centauri, is 4.22 light-years away... At Voyager, if it traveled in that direction, it would take almost 77,000 years to get there! If we then think that, on average, assuming the existence of about thirty alien civilizations in our galaxy (as suggested by a recent study published in Astrophysical Journal), their average distance from each other would be at least 17,000 light-years... And here, before our eyes and before our minds, real abysses of space and time open up. The enormity of the time required to travel at subluminal speeds so leads many experts to conclude that although interstellar travel is possible theoretically. But impossible from a practical one: either one travels at great speeds, or travel times become very long and impractical for any biological form.
So it is clear that to reach the stars quickly we must build vehicles capable of traveling at near light speed. And even then travel times could belong in relation to a man's life. For example, ignoring the time needed to speed up and decelerate at the beginning and the end of the journey. If we travel speed of 0.1c would take 105 years to reach Epsilon Eridani.
One of the Sun-like stars closest to us (10.5 light-years). And for that, we would have to leave with the certainty of having already identified an Earth-like planet to land on. The passengers of the spaceship would measure a shorter interval due to the time dilation. Although we should ignore the effects of time dilation on space navigators traveling at 0.1 c since it only affects 0.5%. It is true that the closer the speed approaches c, the more the effect is felt. For terrestrial observers, a vehicle bound for Epsilon Eridani with a speed of 0.999 c would reach its destination in 105 years, but for passengers, the journey would only take 171 days. Yes, you read it right, 171 days!
If it were possible to reach speeds under the speed of light, for the travelers the crossing, would only take a fraction of a second and it would be possible to travel to the farthest galaxies within a human lifetime, even if this would mean losing any temporal reference with their home planet. We are very far from having developed technologies capable of accelerating a ship at relativistic speeds, and as if that were not enough, there are problems for which it is also difficult to theorize a solution: a spaceship that was capable of reaching those speeds, to give one an example would be bombarded by the tiny dust particles of the interstellar medium such on the subject the structure to intensity of radiation hundreds of times greater than that produced by a nuclear reactor. But, other civilizations could be millions of years ahead of us. Is it really possible that none of them have already developed interstellar travel technology?
Those who is conceptually optimistic about the existence of intelligent life in space say that the fact that aliens are not here is not proof of their non-existence, but rather that interstellar travel presents objectively insoluble problems for any level of technology, and that physics does not offer any convenient shortcuts, such as hypothetical wormholes that would put distant regions of the cosmos in direct communication?
In short, even the most technologically advanced civilizations must resign themselves to colonize their own backyard. Then there is a final consideration: the logic of colonization envisages a technological civilization inspired by a plan of action projected over millions of years to come... Can you imagine a political system that launches, not a five-year plan, not even a ten-year plan, but millions of years, to program the colonization of the galaxy? And assuming that the alleged colonizers would go to found civilizations that would lose all contact with the mother civilization and that this disconnection would be repeated with every newly colonized planet, how could the idea of a common conquest be maintained over time and space? Much more likely, from jump to jump and from generation to generation, the wave would break into ever smaller rivulets, and then it would be completely extinguished.
No... Clearly, if "they" are not here, physically, it is because the distances involved are too demanding, whatever the technological level of civilization. Everything we have talked about so far, but, concerns the Fermi Paradox, which is called into play when you want to stress the lack of physical encounter with an alien civilization. But very often we also hear talk of "Great Silence", that is, the failure to receive a signal of an electromagnetic nature. And also in this case there is no lack of reasons to justify this lack without necessarily blaming it on the non-existence of the interlocutors. There are in fact enormous difficulties also in trying to send interstellar messages. We certainly have no idea what kind of communication technology an evolved alien civilization could have. If Guglielmo Marconi with his primitive radio receiver was somehow transported to the New York of our days, where it only broadcasts in FM, we would deduce that there is no transmission in progress. And in the same way, he would remain unaware of the existence of transmissions using lasers, optical fibers, or geostationary satellites.
The most obvious way to send information to us at present seems to be to use electromagnetic radiation, but how can we exclude that "others" use gravitational waves, particle beams, or hypothetical tachyon beams instead (just to limit ourselves to future technologies based on our current knowledge)? How can we pretend to imagine which communication channels are available to a technical culture that could be millions of years more advanced than ours? Maybe there are so many civilizations in space, committed to communicate with each other through gravitational waves, neutrinos, and tachyons, or maybe they send signals applying techniques we don't even dream about, not because they are outside the laws of physics but because for us they are just as exotic as fiber optic communication would be for Marconi.
Our inability to detect signals of this kind would explain the "great silence", then there is another problem. Not knowing in advance which star is the extraterrestrial civilization we want to contact, the safest option would be to send isotropically, i.e. with the same power in all directions. But, for example, if we wanted to send an isotropic signal up to a distance of 100 light-years, the power required by the transmitter would exceed the Earth's current total electrical capacity! Not to mention the fact that 100 light-years represent little more than the immediate vicinity of our planet... And the further away we wanted to push our signal, the higher the energy requirements would become. Anisotropic transmitter, so, is well beyond our current technological capabilities. Things would change if we knew the exact position of the star to which we wanted to send the message, in which case our own Arecibo could successfully send a signal from one end of the Galaxy to the other; but the problem is that a priori, we don't know where to point the transmitter (and even if we did, as we'll see later, another problem would arise), and a parabola like Arecibo's one emits such a narrow beam that it is extremely unlikely that by pure chance the emitting "beam" would align with a large receiver in some remote point of deep space. Beyond that, the stars do not standstill.
Even if a civilization sends a signal to the point where a star is now, by the time the light reaches that point the star will have moved. This means that the transmitting civilization will also need accurate information about the speed at which the target stars are moving. In short, isotropic transmission - which guarantees to be heard by anyone who is listening within a certain distance - is expensive, while the targeted transmission is much cheaper but uncertain. Is it so any wonder that in the face of so many problems it is very likely that in the galaxy many civilizations only listen and very few or none that send? Besides, who would care to send messages today and receive an answer in 30,000 years?