Not everyone was content to leave the search for extraterrestrial intelligence in the hands of radio astronomers waiting for incoming alien signals. Scientists critical of an exclusive reliance upon radio astronomy proposed more dynamic ways of learning about extraterrestrial civilizations.
Travel through interstellar space was an alternative but never a very popular one among scientists. As early as 1959, physicist John R. Pierce of the Bell Telephone Laboratories considered the relativistic implications of interstellar space travel. He concluded it was doubtful that a spaceship could carry enough fuel to accelerate it to velocities close to the speed of light. Nobel Laureate Edward Purcell discussed the physics of interstellar travel in 1961 and dismissed trips beyond the solar system as not feasible. "All this stuff about traveling around the universe in space suits," he wrote,"belongs back where it came from, on the cereal box."12 Purcell proposed two-way radio communication with the inhabitants of other worlds. He argued that it was simpler and cheaper to exchange ideas than objects with our neighbors in space.
The German radio astronomer Sebastian von Hoerner, who worked at Green Bank Observatory with Frank Drake, discussed "manned" interstellar travel in 1962 and drew pessimistic conclusions. In the future, he said, space travel would be limited to our planetary system, and extraterrestrials, facing similar problems, would travel through space in their planetary systems. The problem was time dilation, discussed by Einstein in his special theory of relativity. For example, space travelers moving near the speed of light would travel for 27.3 years according to their reckoning while 1,550 years elapsed on Earth. Under the circumstances, von Hoerner advised that communication with extraterrestrials by radio signals was the preferred method.
In 1963, when most scientists doubted the possibility of interstellar space travel, Carl Sagan embraced the notion enthusiastically. He advocated direct physical contact among galactic communities by means of relativistic interstellar flight. Sagan admitted the drawbacks of time dilation but believed they could be overcome by slowing down the metabolic rate of space crew members. Or, since the inhabitants ofthe home planet belonged to very long-lived civilizations, successive generations could maintain records of their space ventures.
These records would contain the departure time, destination, and anticipated time of arrival of spacecraft that began their voyages thousands of years earlier. Spacefaring societies, in communication with one another, might even share information about exploring spacecraft to eliminate unnecessary duplication. The situation, Sagan said, resembled post-Renaissance seafaring nations and their colonies before the coming of fast clipper or steam ships. Sagan concluded that "other civilizations, aeons more advanced than ours,"13 were already traveling between the stars.
Sagan recalculated Drake's equation to determine the number of advanced technological civilizations in the Milky Way Galaxy likely to engage in space travel. He based his calculations on two assumptions. First, that life exists elsewhere in the Galaxy. Second, that levels of intelligence and manipulative ability are of great adaptive value in an organism's evolution.
The lifetimes of extraterrestrial civilizations remained a troublesome problem. The possibility that a nuclear war between the Soviets and the West could destroy all life on Earth caused Sagan to pause in his calculations. He then postulated the lifetimes of technological civilization to be in a range of less than one hundred to more than 100 million years.
Sagan finally determined that technological civilizations in the Galaxy numbered about one million. Therefore, one out of a thousand stars in the heavens had a civilized planet in its vicinity. The closest such civilization would be within several hundred light years of the Earth. A civilization located at this distance was within reach of Sagan's relativistic interstellar spaceships.
Sagan used the results of his solution to Drake's equation to determine the frequency of contact among civilizations within the Galaxy. Sagan's new set of calculations was based on his belief that when a civilization found interstellar spaceflight feasible, it immediately developed the necessary technology no matter how difficult or expensive the undertaking. The scientific advantages gained by a society's contact with other space communities justified the investment in interstellar travel.
Sagan concluded that each star in the Galaxy received a random visit from another galactic civilization at least once every one hundred thousand years. At this visit, space explorers observed which of the star's planets were likely to develop intelligent life. Once a technological civilization appeared in a planetary system, then the frequency ofvisits increased to one every several thousand years.
This line of reasoning brought Sagan back to the Earth and to several critical questions. If advancing civilizations are monitored periodically, have extraterrestrials visited the Earth in historical times? If so, do historical records show evidence of their visits?
Sagan footnoted his mention of direct contact between extraterrestrials and humans with a reference to Enrico Fermi's cryptic remark, "Where are they?" When Fermi asked this question in 1950, he was referring to aliens which, if they existed, should have been on Earth at that moment. Sagan used Fermi's question to indicate that the possibility of human and alien interaction had been "seriously raised before."14
Fermi was a more subtle and careful thinker than Sagan, and it is not clear precisely what he had in mind when he asked, "Where are they?" Most commentators assume that the Italian physicist asked the question ironically. That is why they refer to it as Fermi's paradox. Fermi's paradoxical inquiry is an ironic statement about the gap between the claim that the universe is teeming with intelligent alien life and our failure to find any clues of extraterrestrials on Earth.
Sagan raised the issue of encounters between humans and extraterrestrials in a 1963 article he published in the journal Planetary and Space Science, a scientific periodical refereed by his peers. The National Aeronautics and Space Administration funded Sagan's research for this article with a grant. Hence, this is a scientific paper, not a sample of Sagan's science fiction or one of his speculative excursions into popular science.
In this article, Sagan warns that there are no reliable reports of alien contacts with humans during the past few centuries. However, contact may have occurred earlier and the evidence modified and distorted by legend and metaphysical or theological speculation. Despite the difficulty of proving such contact, Sagan refers to some ancient documents that he feels deserve notice.
The texts he names preserve the Babylonian account of the origins of Sume-rian civilization in the fourth millennium b.c. According to this account, an event occurred on the shores of the Persian Gulf, near the site of the ancient Sumerian city of Eridu. A strange fish-like creature suddenly appeared to the inhabitants of the region. The creature taught these uncivilized peoples the fundamentals of the arts and sciences, including mathematics. It also instructed them in how to build houses and temples, compile tables of laws, and practice agriculture. Sagan later expanded on this story in his 1966 book Intelligent Life in the Universe.
Sagan admitted that he had no hard evidence to prove that the ancient Sumerians owed their newly acquired civilized ways to fish-like visitors from outer space. He also noted that representatives of galactic civilizations might have visited the Earth as many as 10,000 times during the geological era of Earth's history when no human witnesses were present. In that case, the visiting space travelers may have discarded artifacts, which have since vanished or remain undiscovered on Earth, or erected an automatic base in our solar system.
During his discussion of extraterrestrial contact, Sagan was careful not to commit himself to the unequivocal truth of the story he told. However, Sagan's scientific credentials, the authority given his article by NASA funding and its publication in a scientific journal, and the author's elusive language helped to legitimize and establish his interpretation of the ancient texts. In a different setting, knowledgeable readers would simply dismiss his story of Sumerians and visitors from outer space as nonsense.
No specialist in Sumerian studies ever claimed that Sumerian civilization began when extraterrestrial visitors passed on their superior knowledge to the primitive inhabitants of Lower Mesopotamia. Furthermore, the creation stories of many cultures tell of superior creatures, usually divinities, bringing the gifts of fire, learning, agriculture, or technology to humanity. Did visitors from outer space routinely confer the blessings of civilization upon so many different peoples? Or are we merely uncovering myths held in common around the world?
In 1960 astronomer Thomas Gold argued that space travelers had visited the Earth billions of years earlier than Sagan had supposed. He presented these views in an article entitled "Cosmic Garbage." Gold proposed that these early visitors accidentally introduced life to Earth in the garbage they left behind when they moved on to other parts of the universe. He noted that when space travel becomes common, humans will likewise spread life by carrying microbes to lifeless planets.
Sagan and Gold spoke with the authority of science. Gold, who was at one time associated with the Harvard College Observatory, was chair of the department of astronomy at Cornell University and director of its Center for Radiophysics and Space Research. He convinced Cornell University to hire Carl Sagan and make him director of a new Laboratory for Planetary Science.
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