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If even a very small fraction of the hundred billion stars in the galaxy are home to technological civilizations which colonize over interstellar distances, the entire galaxy could be completely colonized in a few million years. The absence of such extraterrestrial civilizations visiting Earth is the Fermi paradox.
Criticism of the Drake equation follows mostly from the observation that several terms in the equation are largely or entirely based on conjecture. Thus the equation cannot be used to draw firm conclusions of any kind.
One reply to such criticisms is that even though the Drake equation currently involves speculation about unmeasured parameters, it was not meant to be science, but intended as a way to stimulate dialogue on these topics.
1) Do you have any proof of there not (being any, sic) planets outside our own that are inhabited by intelligent beings?
NASA's awesomely named deep-space observatory, the Kepler Planet Hunter, has been designed to scour space for planets less massive than Earth. The latest data, released on Feb. 2, is an astronomer's dream come true: The Planet Hunter has tagged 1,235 objects as possible planets.
catalogued at The Extrasolar Planets Encyclopaedia exoplanet.eu... and are by no means nearing exhaustion, because of selection bias, current searches are limited to truly huge planets in close orbits to the parent star. Within a few years, we may be closer to finding Earth-sized planets in a proper orbit for life support, but that is currently just beyond our detection threshold.
623 planetary systems and 777 planets
"Many compound classes detected in [the meteorite] Murchison are also present in terrestrial living systems, raising the possibility that extra terrestrial organic matter may have seeded the origins of life on Earth."
The Hubble Deep Field image contains 10,000 galaxies. At 2.5 arcminutes, that's roughly 1/2,000,000 of the visible universe in any direction. So if we multiply 3,000,000,000 x 10,000 x 2,000,000 we get 6 sextillion or 6,000,000,000,000,000,000,000 planets in the visible universe.
If 1991 VG is a man-made object, why has no one been able to identify which one in the past 21 years?
Socratic Question 1) why is it necessary for alien life to visit Earth, when there may be literally thousands of other candidates for them to visit, closer to their location?
If there are colonies in other galaxies, there is no way for us to prove their existence. No contact leads us to doubt their existence. The same could be said of unicorns and dragons. Without proof, even though a person may want to believe, there is only the imagination.
The Fermi Paradox is the apparent contradiction between the high probability extraterrestrial civilizations' existence and the lack of contact with such civilizations.
Under our current understanding pf physics, there are real barriers to technological development, especially in the area of space travel. That is not to say that such barriers can not be overcome, but that it is by no means a given that they can be.
that the absense of evidence is NOT the evidence of absense