引言

The question of whether we are alone in the Universe has haunted humanity for centuries, but we may now stand poised on the brick of the answer to that question, as the search for radio signals from other intelligent civilizations. This search, often known by the acronym SETI (search for extra-terrestrial intelligence), is a difficult one. Although groups around the world have been searching intermittently for three decades, it is only now that we have reached the level of technology where we can make a determined attempt to search all nearby stars for any sign of life.

A部分

The primary reason for the search is basic curiosity—the same curiosity about the natural world that drives all pure science. We want to know whether life evolves naturally if given the right conditions, or whether there is something very special about the Earth to have fostered the variety of life forms that we see around us on the planet. The simple detection of a radio signal will be sufficient to answer this most basic of all the questions. In this sense, SETI is another cog in the machinery of pure science which is continually pushing out the horizon of our knowledge. However, there are other reasons for being interested in whether life exists elsewhere. For example, we have had civilization on Earth for perhaps only a few thousand years, and the threats of the nuclear war and pollution over the last few decades have told us that our survival may be tenuous. Will we last another two thousand years or will we wipe ourselves out? Since the lifetime of a planet like ours is several billion years, we can expect that, if other civilizations do survive in our galaxy, their ages will range from zero to several billion years. Thus any mere existence of such a civilization will tell us that long-term survival is possible, and gives us some cause for optimism. It is even possible that the older civilization may pass on the benefits of their experience in dealing with threats to survival such as nuclear war and global pollution, and other threats that we haven’t yet discovered.

    A部分：

寻找外星智慧的主要原因是人类对自然世界的好奇心，正是这种好奇心推动了所有纯粹科学的发展。我们想知道，在给予适当条件的情况下，是否会自然演化出生命，或者地球上孕育了我们所看到的多样化生命形式是否有特别之处。简单地探测到一个无线电信号将足以回答这个最基本的问题。从这个意义上说，寻找外星智慧是纯粹科学的一部分，不断推动着我们知识的边界。然而，对于生命是否存在于其他地方的兴趣还有其他原因。例如，地球上的文明可能只存在数千年，过去几十年核战争和污染的威胁告诉我们，我们的生存可能是脆弱的。我们能再撑两千年吗，还是会自我毁灭？由于像我们这样的行星的寿命是几十亿年，我们可以预期，在我们的银河系中如果存在其他文明，它们的年龄将从零到数十亿年不等。因此，任何存在这样的文明都将告诉我们长期生存是有可能的，并给我们一些乐观的原因。甚至可能老一辈的文明会传授他们在应对生存威胁方面的经验，如核战争和全球污染，以及我们尚未发现的其他威胁。

B部分

In discussing whether we are alone, most SETI scientists adopt two ground rules. First, UFOs (Unidentified Flying Objects) are generally ignored since most scientists don’t consider the evidence for them to be strong enough to bear serious consideration although it is also important to keep an open mind in case any really convincing evidence emerges in the future. Second, we make a very conservative assumption that we are looking for a life form that is pretty well like us, since if it differs radically from us we may well not recognize it as a life form, quite apart from whether we are able to communicate with it. In other words, the life form we are looking for may well have two green heads and seven fingers, but it will nevertheless resemble us in that it should communicate with its fellows, be interested in the Universe, live on a planet orbiting a star like our Sun, and perhaps most restrictively, have a chemistry, like us, based on carbon and water.

B部分：

在讨论我们是否独自存在时，大多数寻找外星智慧的科学家遵循两个基本原则。首先，通常忽略不明飞行物（UFOs），因为大多数科学家认为对它们的证据不足以进行严肃考虑，尽管保持开放的态度对于未来出现真正令人信服的证据也很重要。其次，我们做出一个非常保守的假设，即我们正在寻找与我们非常相似的生命形式，因为如果生命形式与我们差异巨大，我们可能无法将其识别为生命形式，更不用说我们能否与其交流了。换句话说，我们正在寻找的生命形式可能拥有两个绿色的头和七个手指，但它仍然与我们相似，因为它应该与同伴交流，对宇宙感兴趣，生活在像太阳那样的恒星周围的行星上，并且可能最具限制性的是，像我们一样基于碳和水的化学构成。

C部分

Even when we make these assumptions, our understanding of other life forms is still severely limited. We do not even know, for example, how many stars have planets, and we certainly do not know how likely it is that life will arise naturally, given the right conditions. However, when we look at the 100 billion stars in our galaxy (the milky way), and 100 billion galaxies in the observable Universe, it seems inconceivable that at least one of these planets does not have a life form on it: in fact, the best educated guess we can make, using the little we do know about the conditions for carbon-based life, leads us to estimate that perhaps one in 100,000 stars might have a life-bearing planet orbiting it. That means that our nearest neighbours are perhaps 100 light years away, which is almost next door in astronomical terms.

C部分：

即使在做出这些假设时，我们对其他生命形式的了解仍然非常有限。例如，我们甚至不知道有多少恒星拥有行星，而且我们当然也不知道在适当条件下生命自然产生的可能性有多大。然而，当我们看着银河系（银河系）的1000亿颗恒星和可观测宇宙中的1000亿个星系时，很难想象这些行星中至少没有一个拥有生命形式：事实上，根据我们对碳基生命条件的了解所能做出的最有教养的猜测是，每10万颗星星中可能有一颗携带生命的行星。这意味着我们最近的邻居可能相距100光年，在天文学上几乎是邻居。

D部分

An alien civilization could choose many different ways of sending information across the galaxy, but many of these either require too much energy, or else are severely attenuated while traversing the vast distances across the galaxy. It turns out that, for a given amount of transmitted power, radio waves in the frequency range 1000 to 3000 MHz travel the greatest distance, and so all searches to date have concentrated on looking for radio waves in this frequency range. So far there have been a number of searches by various groups around the world, including Australian searches using the radio telescope at Parkes, New South Wales. Until now there have not been any detections from the few hundred stars which have been searched. The scale of the searches has been increased dramatically since 1992, when the US Congress NASA 110 million per year for ten years to conduct a thorough search for extra terrestrial life. Much of the money in this project is being spent on the developing the special hardware needed to search many frequencies at once. The project has two parts. One part is a targeted search using the world’s largest radio telescopes, the America-operated telescope in Arecibo, Puerto Rico and the French telescope in Nancy in France. This part of the project is searching the nearest 1000 likely stars with high sensitivity for signals in the frequency range 1000 to 3000 MHz. The other part of the project is an undirected search which is monitoring all of space with a lower sensitivity, using the smaller antennas of NASA’s Deep Space Network.

D部分：

外星文明可以选择许多不同的方式来在银河系传递信息，但其中许多方式要么需要过多的能量，要么在穿越银河系的广阔距离时信号会被严重衰减。事实证明，在给定的传输功率下，1000至3000 MHz频率范围内的无线电波传播距离最远，因此迄今为止的所有搜索都集中在这个频率范围内寻找无线电波。目前世界各地的许多团队进行了一系列搜索，包括澳大利亚在新南威尔士州的帕克斯射电望远镜的搜索。迄今为止，从已搜索的几百颗恒星中还没有发现任何信号。自1992年以来，搜索的规模已经大幅增加，当时美国国会拨款每年1.1亿美元，连续十年进行了彻底的外星生命搜索。这个项目的很大一部分资金正在用于开发同时搜索多个频率所需的特殊硬件。该项目有两个部分。一部分是使用世界上最大的射电望远镜进行有针对性的搜索，包括位于波多黎各阿雷西博的美国运营望远镜和法国南锡的法国望远镜。该项目的这个部分以高灵敏度搜索频率范围为1000至3000 MHz的最近1000颗可能的恒星上的信号。该项目的另一部分是一个无定向搜索，使用NASA深空网络的较小天线以较低的灵敏度监测整个太空。

E部分

There is considerable debate over how we should react if we detect a signal from an alien civilization. Everybody agrees that we should not reply immediately. Quite apart from the impracticality of sending a reply over such large distances at short notice, it raises a host of ethical questions that would have to be addressed by the global community before any reply could be sent. Would the human race face the culture shock if faced with a superior and much older civilization? Luckily, there is no urgency about this. The stars being searched are hundreds of light years away, so it takes hundreds of years for their signal to reach us, and a further few hundreds for our reply to reach them. It’s not important, then, if there’s a delay of a few years, or decades. While the human race debates the question of whether to reply, and perhaps carefully drafts a reply.

E部分：

关于如果我们探测到来自外星智慧的信号，如何应对存在相当大的争议。大家都同意我们不应立即回复。除了在短时间内发送回应的不切实际性外，这还引发了一系列伦理问题，全球社区必须在发送任何回应之前解决这些问题。如果面对一个更高级和更古老的文明，人类是否会面临文化冲击？幸运的是，这并没有紧迫性。被搜索的恒星距离数百光年远，所以它们的信号需要数百年才能到达我们，再经过几百年才能收到我们的回应。因此，如果有几年或几十年的延迟并不重要。而人类辩论是否回应，并可能仔细起草回应时。