If then we are so certain that other intelligent life exists in the universe, why have we had no visitors from outer space yet?

outer space ^{n. [U] the area outside the Earth’s atmosphere containing the stars and planets 外层空间}

• After you press the button, the elevator then takes you to outer space. ^{按下按钮后，电梯就会带你去外太空。}
• Visiting outer space is still a far dream for anyone who isn't an astronaut. ^{对任何不是宇航员的人来说，参观外太空仍然是一个遥远的梦想。}
• The seeds are influenced by the radiation and low gravity in outer space. ^{种子在外太空会受到辐射和低重力的影响。}

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.

prevailing ^{adj. existing or most common at a particular time（某时期）普遍的，盛行的，流行的}

• the attitude towards science prevailing at the time
• It takes little experience on Pacific islands to be aware that there are prevailing winds. ^{在太平洋岛屿上不需要什么经验就能知道那里有盛行风。}
• This reflects attitudes and values prevailing in society. ^{这反映出社会上普遍流行的态度和价值观。}

prevail ^{v. if a belief, custom, situation etc. prevails, it exists among a group of people at a certain time 普遍存在，盛行，流行}

• the economic conditions which prevail in England and Wales
• I admired the creativity which prevailed among the young writers.
• The documentary depicted the then prevailing medieval customs in great detail.^{这部纪录片详细描绘了当时普遍存在的中世纪习俗。}

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.

radio astronomer ^{射电天文学家}

• One radio astronomer actually left Paris to go off and to observe an eclipse. ^{一位射电天文学家离开巴黎，去观看月食。}
• In 10,000 hours you could have learned enough to become a radio astronomer or engineer. ^{一万个小时后，你学到的知识足以让你成为一名射电天文学家或工程师。}

Nature: a British weekly journal which deals with all areas of science. It is a very respected journal, and many important new discoveries are first reported in articles in Nature. ^{英国《自然》杂志}

await ^{v. (fml.) to wait for sth. 等候}

• Several men are awaiting trial for robbery.
• They were awaiting the birth of their first child.
• The seed bank was established to await the possible need for specific genes in the future.^{建立种子库是为了等待未来对特定基因的可能需求。}

awakening ^{n. the moment when you first realize or experience sth. 觉醒；认识的提高；（某种情感或能力的）激发}

• The book explores his spiritual awakening after his long journey across Asia. ^{这本书探讨了他在漫长的亚洲之旅后的精神觉醒。}
• the awakening of public concern about the environment

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.

may / might / could well … ^{（某事）可能（发生或是真的）}

• Another patient might equally well complain that her neighbours were combining to slander her and persecute her, and yet one might be cautious about believing this statement.
• Over the years, one may well become an authority on one's hobby and will very probably be asked to give informal talks to little gatherings and then, if successful, to larger audiences.
• ... giant hovercraft liners could span the Atlantic; and the railway of the future may well be the 'hovertrain', riding on its air cushion over a single rail...

thus ^{adv. in this way, like this 以此方式，如此，这样}

• calculate the area of the triangle thus formed
• Hold the wheel in both hands, thus.
• The apparatus thus described, however, records only the horizontal component of the wave movement, which is, in fact, much more complicated.

be left unsaid ^{if sth. is left unsaid, you do not say it although you might be thinking it 不说；被搁置在一边（不提）}

• Some things are better left unsaid.
• It’s better to leave some things unsaid.
• He got up slowly leaving the lunch unfinished.
• Please excuse me if I have left any of your questions unanswered.
• It is then left completely undisturbed with plenty of food and water so that it can absorb the atmosphere of its new home and see that nothing particularly alarming is happening to its companions.
• Though they were both angry, much was left unsaid to avoid a permanent rift. ^{尽管两人都很生气，但为了避免产生永久的隔阂，很多话都没有说出口。}
• The details of their private disagreement were best left unsaid during the public meeting.^{他们私人分歧的细节在公开会议期间最好不要说。}