国外精美pptppt课件模板248-PPT精选(同名1219).ppt

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7、 extraterrestrial intelligence,myself among them,have attempted to estimate the number of advanced technical civilizations-defined operationally as societies capable of radio astronomy-in the Milky Way Galaxy.Such estimates are little better than guesses.They require assigning numerical values to qu

8、antities such as the numbers and ages of stars;the abundance of planetary systems and the likelihood of the origin of life,which we know less well;and the probability of the evolution of intelligent life and the lifetime of technical civilizations,about which we know very little indeed.v When we do

9、the arithmetic,the sorts of numbers we come up with are,characteristically,around a million technical civilizations.A million civilizations is a breathtakingly large number,and it is exhilarating to imagine the diversity,lifestyles and commerce of those million worlds.But the Milky Way Galaxy contai

10、ns some 250 billion stars,and even with a million civilizations,less than one star in 200,000 would have a planet inhabited by an advanced civilization.Since we have little idea which stars are likely candidates,we will have to examine a very large number of them.Such considerations suggest that the

11、 quest for extraterrestrial intelligence may require a significant effort.v Despite claims about ancient astronauts and unidentified flying objects,there is no firm evidence for past visitation of the Earth by other civilizations.We are restricted to remote signaling and,of the long-distance techniq

12、ues available to our technology,radio is by far the best.Radio telescopes are relatively inexpensive;radio signals travel at the speed of light,faster than which nothing can go;and the use of radio for communication is not a short-sighted or anthropocentric activity.Radio represents a large part of

13、the electromagnetic spectrum and any technical civilization anywhere in the Galaxy will have discovered radio early-just as in the last few centuries we have explored the entire electromagnetic spectrum from short gamma rays to very long radio waves.Advanced civilizations might very well use some ot

14、her means of communication with their peers.But if they wish to communicate with backward or emerging civilizations,there are only a few obvious methods,the chief of which is radio.v The first serious attempt to listen for possible radio signals from other civilizations was carried out at the Nation

15、al Radio Astronomy Observatory in Greenbank,West Virginia,in 1959 and 1960.It was organized by Frank Drake,now at Cornel University,and was called Project Ozma,after the princess of the Land of Oz,a place very exotic,very distant and very difficult to reach.Drake examined two nearby stars for a few

16、weeks with negative results.Positive results would have been astonishing because as we have seen,even rather optimistic estimates of the number of technical civilizations in the Galaxy imply that several hundred thousand stars must be examined in order to achieve success by random stellar selection.

17、v Since Project Ozma,there have been six or eight other such programs,all at a rather modest level,in the United States,Canada and the Soviet Union.All results have been negative.The total number of individual stars examined to date in this way is less than a thousand.We have performed something lik

18、e one tenth of one percent of the required effort.v However,there are signs that much more serious efforts may be mustered in the reasonably near future.Besides,hand in hand with the recent spectacular advances in radio technology,there has been a dramatic increase in the scientific and public respectability of the entire subject of extraterrestrial life.A clear sign of the new attitude is the Viking missions to Mars,which are to a significant extent dedicated to the search for life on another planet.