Fermi paradox

Fermi paradox, contradiction between the seemingly high likelihood for the emergence of extraterrestrial intelligence and the lack of evidence for its existence. The paradox has two broad forms: (1) Why has Earth not already been visited? and (2) Why is there no evidence for extraterrestrial intelligence at all?

The Fermi paradox emerged from a conversation between physicists Enrico Fermi, Edward Teller, Emil Konopinski, and Herbert York at Los Alamos in the summer of 1950 about flying saucers and the likelihood of faster-than-light interstellar travel. The conversation moved on to other subjects, but Fermi brought the discussion back to aliens with, as Teller put it, “the quite unexpected question ‘Where is everybody?’” York recalled later that Fermi argued that when considering various probabilities such as those of Earthlike planets, intelligent life, and the lifetime of intelligent civilizations, Earth should already have been visited many times by aliens.

If one considers that Earth has existed for 4.5 billion years and the Milky Way Galaxy is about 100,000 light-years in diameter, Fermi’s puzzlement that Earth has not been visited becomes clear. Assuming that a civilization could achieve speeds of one-hundredth the speed of light, the whole galaxy could be explored or even colonized in about 10 million years, enough time for that to have happened many times over in Earth’s history. With 100 billion stars in the Milky Way and about 10 percent of them likely to have a habitable planet, the emergence of intelligent life would have to be extremely improbable for there to be no evidence of extraterrestrial visitation at all. York recalled that Fermi came up with three hypotheses: (1) interstellar travel was impossible; (2) if it was possible, alien civilizations had judged it not worth the effort; or (3) technological civilizations did not last long enough. Other hypotheses have posited that an interstellar civilization may not completely colonize the Galaxy or that interstellar civilizations may bypass the Sun, preferring to colonize near longer-lived low-mass stars.

The paradox has expanded since Fermi’s original question to encompass not only that Earth has not been visited by aliens but also that there is no communication from or evidence of extraterrestrial intelligence. The negative results of the search for extraterrestrial intelligence (SETI) that has taken place since 1960 have been dramatically called the Great Silence.

In his arguments for the likelihood of intelligent civilizations, Fermi anticipated the Drake equation, which states the number of intelligent civilizations N is N = R*f_pn_ef_lf_if_cL.The factor R* is the mean rate of star formation in the Galaxy, f_p the fraction of stars with planetary systems, n_e the number of planets in such systems that are ecologically suitable for the origin of life, f_l the fraction of such planets on which life in fact develops, f_i the fraction of such planets on which life evolves to an intelligent form, f_c the fraction of such worlds in which the intelligent life-form invents high technology capable at least of interstellar radio communication, and L the average lifetime of such advanced civilizations. The first three numbers are the best known, with R* = 10 per year and f_pn_e = 0.1. The uncertainty increases progressively with each factor on the right-hand side of the equation. If, for example, to be optimistic, every planet capable of producing life produces intelligent life-forms capable of interstellar radio communication, f_lf_if_c = 1 and L = 10,000 years, then N = 10,000, but, if the chance of life, intelligent life, and interstellar radio communication is 0.01 for each, f_lf_if_c = 10⁻⁶ and L = 1,000 years, then N = 0.001 and humanity is alone in the Galaxy.

There are many explanations for the Great Silence. Even though life developed just a few hundred million years after Earth’s formation, perhaps that is unusual, and thus f_l is not near 1 but a much lower number. The genus Homo has only been around for about 3 million years; if one divides that lifetime by that of Earth, f_i is about 1/1,000. Homo habilis made the first tools about 2.6 million years ago; radio has only existed for a little more than a century. Taking the ratio of those two times suggests that f_c is 1/26,000. Civilizations may just destroy themselves as soon as it is technologically feasible. Such arguments were summed up by American economist Robin Hanson as the “Great Filter,” that some step in the chain of circumstances from the formation of a habitable planet to galactic colonization is exceedingly improbable.

Some have used the Fermi paradox to criticize SETI projects as a waste of time and money, since humanity is likely alone in the universe. Astronomers working in the SETI field respond that hardly any SETI has been done at all. American astronomer Jill Tarter and collaborators have said that when considering the vast number of stars, radio frequencies, and other signal parameters, deducing if extraterrestrial intelligence exists from the results of small-scale SETI projects is like deducing if fish exist by dipping a glass into the ocean.

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