新概念英語第四冊（美音版）Are there strangers in space?

[00:00.55]Lesson 43 [00:03.12]Are there strangers in space? [00:11.44]What does the 'uniquely rational way' for us to communicate with other intelligent beings in space depend on? [00:22.80]We must conclude from the work of those who have studied the origin of life, [00:28.36]that given a planet only approximately like our own, life is almost certain to start. [00:36.13]Of all the planets in our solar system, we are now pretty certain the Earth is the only one on which life can survive. [00:46.40]Mars is too dry and poor in oxygen, Venus far too hot, and so is Mercury, [00:54.04]and the outer planets have temperatures near absolute zero and hydrogen-dominated atmospheres. [01:02.12]But other suns, start as the astronomers call them, are bound to have planets like our own, and as is the number of stars in the universe is so vast, [01:14.38]this possibility becomes virtual certainty. [01:18.33]There are one hundred thousand million starts in our own Milky Way alone, [01:24.73]and then there are three thousand million other milky ways or galaxies, in the universe. [01:32.54]so the number of stars that we know exist is now estimated at about 300 million million million. [01:43.29]Although perhaps only 1 percent of the life that has started somewhere will develop into highly complex and intelligent patterns, [01:52.47]so vast is the number of planets, that intelligent life is bound to be a natural part of the universe. [02:00.95]If then we are so certain that other intelligent life exists in the universe, why have we had no visitors from outer space yet? [02:10.93]First of all, they may have come to this planet of ours thousands or millions of years ago, [02:18.52]and found our then prevailing primitive state completely uninteresting to their own advanced knowledge. [02:26.42]Professor Ronald Bracewell, a leading American radio astronomer, [02:32.01]argued in Nature that such a superior civilization, on a visit to our own solar system, [02:40.13]may have left an automatic messenger behind to await the possible awakening of an advanced civilization. [02:48.54]Such a messenger, receiving our radio and television signals, [02:53.48]might well re-transmit them back to its home-planet, [02:57.84]although what impression any other civilization would thus get from us is best left unsaid. [03:06.46]But here we come up against the most difficult of all obstacles to contact with people on other planets [03:14.05]-- the astronomical distances which separate us. [03:18.51]As a reasonable guess, they might, on an average, be 100 light years away. [03:26.43](A light year is the distance which light travels at 186, 000 miles per second in one year, namely 6 million million miles.) [03:38.43]Radio waves also travel at the speed of light, [03:42.04]and assuming such an automatic messenger picked up our first broadcasts of the 1920's, [03:48.75]the message to its home planet is barely halfway there. [03:53.48]Similarly, our own present primitive chemical rockets, [03:58.30]though good enough to orbit men, have no chance of transporting us to the nearest other star, [04:06.05]four light years away, let alone distances of tens or hundreds of light years. [04:13.85]Fortunately, there is a 'uniquely rational way' for us to communicate with other intelligent beings, [04:22.33]as Walter Sullivan has put it in his excellent book, [04:26.34]We Are not Alone. [04:28.60]This depends on the precise radio frequency of the 21-cm wavelength, or 1420 megacycles per second. [04:41.42]It is the natural frequency of emission of the hydrogen atoms in space and was discovered by us in 1951; [04:50.31]it must be known to any kind of radio astronomer in the universe. [04:55.85]Once the existence of this wave-length had been discovered, [04:59.84]it was not long before its use as the uniquely recognizable broadcasting frequency for interstellar communication was suggested. [05:09.30]Without something of this kind, [05:11.49]searching for intelligences on other planets would be like trying to meet a friend in London without a pre-arranged rendezvous [05:19.41]and absurdly wandering the streets in the hope of a chance encounter.

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