“If they exist, where are they?” So direct was the Nobel Prize in Physics Enrico Fermi in 1950. Known since then as the Fermi paradox, it raises the contradiction that exists between believing that there are a large number of extraterrestrial civilizations and the objective fact that we have not seen them.
For Fermi, it was obvious that if an intelligent species develops the technology necessary to travel through space, sooner or later it will end up reaching Earth. Since we have no proof of such visits, aliens cannot exist. At this point it is key to calculate how long it would take to travel the entire Galaxy. In 1981, physicist Frank J. Tipler estimated that an advanced civilization would colonize it in 300 million years . Taking into account that the Milky Way is 8,000 million years old, it is incredible that in all that time none of these civilizations has been able to do it.
Now, how can we be sure that they are not around here (forgetting UFOs, a product of our peculiar cultural eccentricity)? For Robert A. Freitas, Jr., an expert in nanorobots and a researcher at the Institute for Molecular Manufacturing in California, it is a mistake to think that there are no extraterrestrial exploration probes just because we have not seen them. Especially since we have not looked for them. Assuming the probes are 1 to 10 meters in size, how much of the 100 billion square kilometers of the Solar System have we explored at that resolution? 0.001%. For this reason, Freitas defends, we must carry out an active search for these artifacts before deciding that there are none .
Of course, if they are really there, why haven’t they made themselves known? Some scientists think they don’t because they don’t want to interfere: this is the zoo hypothesis , first proposed in 1973 by John A. Ball in the journal Icarus. In fact, it had already been advanced by science fiction writers Olaf Stapledon in ” The Star Maker” (1937) and Arthur C. Clarke in ” Childhood’s End” (1953). According to engineer Ronald N. Bracewell , they remain in a discreet background, waiting for us to develop the necessary technological level to belong to their exclusive “Galactic Club” . This idea also appeared in the prestigious journal Science in 1977. There TBH Kuiper and M. Morris argued that among the reasons for avoiding us was the culture shock that such contact would mean. And they added a new twist to the idea of the Galactic Club: they would not contact us until we reached a certain intellectual level and thus prevent the extinction of “the only thing on this planet that could have any value for extraterrestrials”.
This colorful hypothesis that turns aliens into our older brothers has been tempered by Freitas by stating that they do not have to be silent. Although there are probes in our Solar System, they should not have one of their priorities to dedicate themselves to sending little messages: they do their job and simply ignore us. Whether we discover them or not is up to us; ETs are not proactive in making themselves known.
More witty are other proposals to solve the Fermi paradox. One of the funniest is the so-called Old Rent Neighborhood: we live in a rather boring area of the Galaxy, with few interesting things and the ETs prefer to go to more attractive places, such as the galactic center.
Another is the Quarantine Hypothesis: they think we are a very dangerous species and keep us isolated. But the most amazing is the Hypothesis of the deadly probes: there is a very advanced civilization that does not want competitors for control of the Galaxy . To do this they send self-replicating exploration probes and, if they detect a civilization capable of traversing space, the probe looks for a very large comet or asteroid, adheres to it and directs it against the planet. Imagination to the power. Freitas is not daunted by this maelstrom of impossible explanations. As it is obvious that we have no proof that there are genuinely galactic civilizations, he proposes his hypothesis of the disappearance of civilizations with voracious impulses, “a cancer without a purpose of technological exploitation”: they do not exist because some kind of unknown selection mechanism eliminates them.
In the midst of this cosmic sea of nonsense, the result of too much poorly digested science fiction, voices arise that affirm that the Fermi paradox is only for those who believe in extraterrestrial civilizations.
The best analysis on the subject was made by the astronomer Michael H. Hart in an article published in 1975 in the Quarterly Journal of the Royal Astronomical Society. In it he showed that Fermi’s question was not a boutade and made a rigorous analysis of the problem. Is it possible that they have not arrived because space travel is unfeasible? Of course it is not the same as walking in the garden at home, but there are ways to avoid the problem. We have suspended animation, either through cryogenics or other techniques well known from the movie Alien .
It’s obvious that we don’t know how to freeze warm-blooded beings but… who said that ETs should have it? There is also no reason to believe that their life expectancy is similar to ours. Why can’t they live 3,000 years? If so, spending 200 on interstellar travel isn’t too much.
Other alternatives are travel at speeds close to the speed of light , sending automatic probes with frozen embryos ready to develop as soon as a viable planet is found, or generational travel , first proposed in 1929 by the father of ray crystallography X John D. Bernal in his work The World, the Flesh & the Devil : different families embark on a journey that will last centuries and it will be their descendants who finish the mission.
Sociological explanations, such as the zoo hypothesis and its various variants, also make no sense to Hart. They all suffer from the same problem: they assume that all extraterrestrial races, regardless of their biological, psychological, political or social structure, throughout their history, always do the same thing. Is it really believable? Perhaps the inhabitants of Vega III in the year 600,000 a. C. decided to stay home and look at their navel, but that does not imply that in 599,000 a. C. they would like to do the same or that their descendants in 555,000 a. C. follow that path. When has something like this happened on Earth? Let’s realize that what Hart is saying is not that the ETs are going to act like us, but that it is impossible for all possible ET civilizations throughout their history to always act the opposite of how we would.
Similarly, to say that none have had time to arrive is even more rare. Hart’s account is revealing: suppose we are dedicated to sending missions to stars located about 20 light-years from us. Once the colony is installed, you can send your own missions. With no pause between expeditions, the amount of time needed to traverse the Galaxy at a rate of expansion one-tenth that of light is 650,000 years. Let’s be generous and think that they rest between missions so that the expansion time is doubled. Only a civilization that began its exploratory epoch less than 2 million years ago could not have visited us. That they have not come means that the million civilizations that, according to Carl Sagan, populate the Milky Way are all at the same moment in their technological history is truly incredible. What is the conclusion? That there is no Fermi paradox because, obviously, we are (practically) alone in the Galaxy.
