The Fermi paradox is named after physicist Enrico Fermi and refers to the apparent contradiction between the lack of evidence for and various high probability estimates of the existence of extraterrestrial civilizations elsewhere in the Milky Way galaxy. The basic points of the argument were more fully developed in a 1975 paper by Michael H. Hart and include: There are billions of stars in the galaxy that are similar to the Sun, and many of these stars are billions of years older than the Solar system. With high probability, some of these stars have Earth-like planets, and if the Earth is typical, some may have already developed intelligent life. Some of these civilizations may have developed interstellar travel, a step the Earth is investigating now. Even at the slow pace of currently envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years. According to this line of reasoning, the Earth should have already been visited by extraterrestrial aliens, or at least their probes. Fermi's name is linked because of a casual conversation in the summer of 1950 with fellow physicists Edward Teller, Herbert York, and Emil Konopinski. While walking to lunch, the men discussed recent UFO reports and the possibility of faster-than-light travel. The conversation moved on to other topics, until during lunch Fermi suddenly said, "Where are they?" or alternatively, "Don't you ever wonder where everybody is?" or alternatively, "But where is everybody?" (the exact quote is uncertain). Despite the jump in topic, two of his three lunch companions remember immediately knowing that Fermi was referring to potential extraterrestrials. Furthermore, York remembers that Fermi "followed up with a series of calculations on the probability of earthlike planets, the probability of life given an earth, the probability of humans given life, the likely rise and duration of high technology, and so on. He concluded on the basis of such calculations that we ought to have been visited long ago and many times over." There have been many attempts to explain the Fermi paradox, primarily either suggesting that intelligent extraterrestrial beings are extremely rare or proposing reasons that such civilizations have not contacted or visited Earth.
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