In 1959, Giuseppe Cocconi and Philip Morrison wrote an article called “Searching for Interstellar Communications” and published it in Nature. In the article, they suggest that the way to communicate with extraterrestrial life is by using radio waves since the galaxy is transparent to them. It was the first time scientists had proposed a specific strategy for searching for extraterrestrial intelligence.
Soon after Cocconi and Morrison’s article was published, Frank Drake came up with an equation that estimates the number of technological civilization that may exist in the galaxy. The equation is written as
N = R* x fp x ne x fl x fi x fc x L
N is the number of civilizations in the Milky Way, R* stands for the rate of formation of stars suitable for the development of intelligent life, ne is the number planet per solar system that is suitable for life, and L stands for the length of time that the intelligent civilization needs to emit detectable signals into space. Fp, fl, fi and fc refer to different fractions, from that of the stars with planetary systems to that of civilizations that release detectable signs of their existence into space. Depending on the values that we adopt for each variable in Drake’s’ equation, the number of intelligent civilizations in the Milky Way can vary from less than a thousand to about a billion. However, even though there is a debate on the exact values of the variables in the equation, Drake’s equation is generally accepted as a tool for estimating how many intelligent civilizations may exist in the galaxy.
Currently, there is no hard evidence supporting the existence of extraterrestrial life. Therefore, in his essay, “An Explanation for the Absence of Extraterrestrial on Earth,” after rejecting all other hypotheses, Michael H. Hart claims that the best hypothesis that can explain this fact is that that there are no other advanced civilizations in the Galaxy. Hart groups the hypotheses that assume there is life in the galaxy into four main categories: physical explanations, sociological explanations, temporal explanations, and past-visit explanations.
Physical explanations say that extraterrestrial visitors have never arrived on Earth because they are physically or intelligently incapable. Nonetheless, using mathematical calculations and the example of Apollo and Skylab missions, Hart concludes that neither the time of travel nor the energy required would render a visit impossible. Therefore, Hart rejects the physical explanation. Sociological explanations include any hypothesis that the extraterrestrials have chosen not to visit the Earth. Hart also rejects this proposal, saying that the social structure and the interest in space travel should have changed throughout their history. Holding the same belief for thousands or even millions of years is not plausible. In addition, Hart believes that if the hypothesis is true, scientists would be able to find evidence of extraterrestrial life in the space, but the lack of proof indicates the argument is not valid. Even if the physical and sociological explanations are rejected, temporal explanations, which say that extraterrestrial life has not yet had the time to reach the Earth, may still be valid. However, since our galaxy is at least 1010 years old, there has been ample time for extraterrestrials to visit. Hart argues that the temporal explanation is plausible but highly unlikely. There is also the possibility that extraterrestrial beings have come, with some perhaps leaving after their visits and some perhaps staying unnoticed among us. Hart questions these hypotheses and believes further study is needed to prove them. For now, Hart concludes there is simply no extraterrestrial life.
- Jiaxuan Liu