[al:新概念英语（四）] [ar:MP3 同步字幕版（英音）] [ti:Are There Strangers in Space?] [by:更多学习内容，请到chazidian.com搜索“新概念”] [00:01.47]Lesson 43 [00:03.56]Are there strangers in space? [00:12.47]What does the 'uniquely rational way' for us to communicate with other intelligent beings in space depend on? [00:22.11]We must conclude from the work of those who have studied the origin of life, [00:26.87]that given a planet only approximately like our own, life is almost certain to start. [00:34.34]Of all the planets in our solar system, we are now pretty certain the Earth is the only one on which life can survive. [00:43.34]Mars is too dry and poor in oxygen, Venus far too hot, and so is Mercury, [00:50.89]and the outer planets have temperatures near absolute zero and hydrogen-dominated atmospheres. [00:58.38]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, [01:08.11]this possibility becomes virtual certainty. [01:12.33]There are one hundred thousand million starts in our own Milky Way alone, [01:17.34]and then there are three thousand million other milky ways or galaxies, in the universe. [01:23.88]so the number of stars that we know exist is now estimated at about 300 million million million. [01:33.03]Although perhaps only 1 percent of the life that has started somewhere will develop into highly complex and intelligent patterns, [01:41.18]so vast is the number of planets, that intelligent life is bound to be a natural part of the universe. [01:50.00]If then we are so certain that other intelligent life exists in the universe, why have we had no visitors from outer space yet? [01:59.59]First of all, they may have come to this planet of ours thousands or millions of years ago, [02:05.65]and found our then prevailing primitive state completely uninteresting to their own advanced knowledge. [02:12.82]Professor Ronald Bracewell, a leading American radio astronomer, [02:17.07]argued in Nature that such a superior civilization, on a visit to our own solar system, [02:24.69]may have left an automatic messenger behind to await the possible awakening of an advanced civilization. [02:32.69]Such a messenger, receiving our radio and television signals, [02:36.67]might well re-transmit them back to its home-planet, [02:41.12]although what impression any other civilization would thus get from us is best left unsaid. [02:48.95]But here we come up against the most difficult of all obstacles to contact with people on other planets [02:55.71]-- the astronomical distances which separate us. [03:00.06]As a reasonable guess, they might, on an average, be 100 light years away. [03:06.34](A light year is the distance which light travels at 186, 000 miles per second in one year, namely 6 million million miles.) [03:18.70]Radio waves also travel at the speed of light, [03:22.46]and assuming such an automatic messenger picked up our first broadcasts of the 1920's, [03:28.68]the message to its home planet is barely halfway there. [03:33.27]Similarly, our own present primitive chemical rockets, [03:37.29]though good enough to orbit men, have no chance of transporting us to the nearest other star, [03:43.57]four light years away, let alone distances of tens or hundreds of light years. [03:50.39]Fortunately, there is a 'uniquely rational way' for us to communicate with other intelligent beings, [03:57.55]as Walter Sullivan has put it in his excellent book, [04:01.28]We Are not Alone. [04:03.24]This depends on the precise radio frequency of the 21-cm wavelength, or 1420 megacycles per second. [04:14.81]It is the natural frequency of emission of the hydrogen atoms in space and was discovered by us in 1951; [04:23.84]it must be known to any kind of radio astronomer in the universe. [04:29.27]Once the existence of this wave-length had been discovered, [04:32.99]it was not long before its use as the uniquely recognizable broadcasting frequency for interstellar communication was suggested. [04:42.12]Without something of this kind, searching for intelligences on other planets would be like trying to