Of all the problems facing modern astronomers, perhaps the most fascinating is: 'Can intelligent life exist elsewhere?' Since the Earth is an unimportant planet moving round an unimportant star, it would be a pride on our part to suppose that we are the only intelligent beings in the universe. But to obtain proof is difficult. The main trouble is that our neighbor worlds, the bodies in the Solar system appear to be unsuitable for advanced life-forms. The Moon may be ruled out at once it has hardly any atmosphere. Venus(金星) is little better the surface temperature is extremely high and the atmosphere is mostly carbon dioxide. Mars with a very thin atmosphere and a severe shortage of water, may well support simple plant life but there seems no hope of finding animals, while the attractive Martians(火星人) of the story-tellers have long since been given up. Of course this has not stopped the flow of bright ideas for communicating with the supposed people on Mars. In the early nineteenth century the great mathematician Gauss suggested planting treepatterns in Siberia(西伯利亚), so that the Martians would see them and replay suitably. Following up this idea, the Austrian scientist Karl Littrow proposed digging very wide ditches in the Sahara, triangular in patterns, and then filling them with petrol or some substance so that, when lit, the ditches would present Martian observers with a 'flaming triangle' which would show the existence here of intelligent minds. Even better were the plans of Charles Cross, a French writer of the 1870's, who wanted to build a large mirror to reflect the sun's rays and concentrate them on the surface of Mars, thereby making a vast burning-glass. By swinging the mirror around, Cross explained it would be practicable to write words in the Martian deserts simply by burning the sand. For many years he bombared(轰击) the French government with literature about this plan and was very disappointed when no official interest was shown. The opinion of the writer is that ______.