We must conclude from the work of those who have studied the origin of life， that given a planet only approximately like our own， life is almost certain to start. Of all the planets in our own solar system we arc now pretty certain the Earth is the only one on which life can survive. Mars is too dry and poor in oxygen，Venus far too hot， and so is Mercury， and the outer planets have temperatures near absolute zero and hydrogen-dominated atmospheres. But other suns， stars as the astronomers call them， are bound to have planets like our own， and as the number of stars in the universe is so vast， this possibility becomes virtual certainty. There are one hundred thousand million stars in our own Milky Way alone， and then there are three thousand million other Milky Ways， or Galaxies， in the universe. So the number of stars that we know exist is estimated at about 300 million million million.

　　Although perhaps only 1 per cent of the life that has started somewhere will develop into highly complex and intelligent patterns， so vast is the number of planets that intelligent life is bound to be a natural part of the universe. If then we are so certain that other intelligent life exists in the universe， why have we had no visitors from outer space yet ？ First of all， they may have come to this planet of ours thousands or millions of years ago， and found our then prevailing primitive state completely uninteresting to their own advanced knowledge.Professor Ronald Bracewell， a leading American radio-astronomer， argued in Nature that such a superior civilization， on a visit to our own solar system， may-have left an automatic messenger behind to await the possible awakening of an advanced civilization. Such a messenger， receiving our radio and television signals， might well re-transmit them back to its home-planet， although what impression any other civilization would thus get from us is best left unsaid.

　　But here we come up against the most difficult of all obstacles to contact with people on other planets――the astronomical distances which separate us. As a reasonable guess， they might， on an average， be 100 light years away. （A light year is the distance which light travels at 186，000 miles per second in one year， namely 6 million million miles.） Radio waves also travel at the speed of light， and assuming such an automatic messenger picked up our first broadcasts of the 1920's， the message to its home planet is barely halfway there. Similarly， our own Present primitive chemical rockets， though good enough to orbit men， have no chance of transporting us to the nearest other star， four light years away， let alone distances of tens or hundreds of light years.

　　Fortunately， there is a 'uniquely rational way' for us to communicate with other intelligent beings， as Walter Sullivan has put it in his excellent recent book， We are not alone. This depends on the precise radio-frequency of the 21-cm wavelength， or 1420 megacycles per second. It is the natural frequency of emission of the hydrogen atoms in space and was discovered by us in 1951； it must be known to any kind of radio-astronomer in the universe.

　　Once the existence of this wave-length had been discovered， it was not long before its use as the uniquely recognizable broadcasting frequency for interstellar communication was suggested. Without something of this kind， searching for intelligences on other planets would be like trying to meet a friend in London without a Pre-arranged rendezvous and absurdly wandering the streets in the hope of a chance encounter

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