Elizabeth Howell reports that the Fermi Paradox seeks to answer the question of where the aliens are. Given that our star and Earth are part of a young planetary system compared to the rest of the universe — and that interstellar travel might be fairly easy to achieve — the theory says that Earth should have been visited by aliens already. As the story goes, Italian physicist Enrico Fermi, most famous for creating the first nuclear reactor, came up with the theory with a casual lunchtime remark in 1950. The implications, however, have had extraterrestrial researchers scratching their heads in the decades since.
"Fermi realized that any civilization with a modest amount of rocket technology and an immodest amount of imperial incentive could rapidly colonize the entire galaxy," the Search For Extraterrestrial Intelligence (SETI) Institute in Mountain View, California, said on its website. "Within ten million years, every star system could be brought under the wing of empire. Ten million years may sound long, but in fact it's quite short compared with the age of the galaxy, which is roughly ten thousand million years. Colonization of the Milky Way should be a quick exercise."
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Fermi reportedly made the initial remark, but he died in 1954. Publication fell to other people, such as Michael Hart, who wrote an article titled "An Explanation for the Absence of Extraterrestrials on Earth" in the Royal Astronomical Society (RAS) Quarterly Journal in 1975. (Some say this is the first such paper to explore the Fermi paradox, although this claim is a bit hard to prove.)
"We observe that no intelligent beings from outer space are now present on Earth," Hart wrote in the abstract. "It is suggested that this fact can best be explained by the hypothesis that there are no other advanced civilizations in our galaxy." He noted, however, that more research in biochemistry, planetary formation and atmospheres was needed to better narrow down the answer (more).
While Hart was more of the opinion that we were the only advanced civilization in the galaxy (he argued that in Earth's history, somebody could have visited us already unless they started their journey less than two million years ago), he outlined four arguments exploring the paradox:
1) Aliens never came because of a physical difficulty "that makes space travel infeasible," which could be related to astronomy, biology or engineering.
2) Aliens chose never to come to Earth.
3) Advanced civilizations arose too recently for aliens to reach us.
4) Aliens have visited Earth in the past, but we have not observed them.
The argument has been challenged on many grounds. "Maybe star travel is not feasible, or maybe nobody chooses to colonize the galaxy, or maybe we were visited long ago and the evidence is buried with the dinosaurs — but the idea has become entrenched in thinking about alien civilizations," wrote Fermi paradox researcher Robert H. Gray in a 2016 Scientific American blog post.
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