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" "Imagination will often carry us to worlds that never were. But without it, we go nowhere. Skepticism enables us to distinguish fancy from fact, to test our speculations. The Cosmos is rich beyond measure — in elegant facts, in exquisite interrelationships, in the subtle machinery of awe.
Carl Edward Sagan (9 November 1934 – 20 December 1996) was an American astronomer, planetary scientist, cosmologist, astrophysicist, astrobiologist, author, and science communicator. His best known scientific contribution is research on extraterrestrial life, including experimental demonstration of the production of amino acids from basic chemicals by radiation. Sagan assembled the first physical messages sent into space, the Pioneer plaque and the Voyager Golden Record, universal messages that could potentially be understood by any extraterrestrial intelligence that might find them. Sagan argued the hypothesis, accepted since, that the high surface temperatures of Venus can be attributed to, and calculated using, the greenhouse effect. He testified to the US Congress in 1985 that the greenhouse effect will change the earth's climate system.
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From the point of view of a mayfly, human beings are stolid, boring, almost entirely immovable, offering hardly a hint that they ever do anything. From the point of view of a star, a human being is a tiny flash, one of billions of brief lives flickering tenuously on the surface of a strangely cold, anomalously solid, exotically remote sphere of silicate and iron.
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No civilization can possibly survive to an interstellar spacefaring phase unless it limits its numbers.
Any society with a marked population explosion will be forced to devote all its energies and technological skills to feeding and caring for the population on its home planet. This is a very powerful conclusion and is in no way based on the idiosyncrasies of a particular civilization. On any planet, no matter what its biology or social system, an exponential increase in population will swallow every resource. Conversely, any civilization that engages in serious interstellar exploration and colonization must have exercised zero population growth or something very close to it for many generations.