cases feedback loops are extremely sophisticated, where a facial gesture or slight variation in tone can steer a conversation in an entirely different direction. Consider the difficulties involved in conducting a telephone conversation with only a rudimentary understanding of the language involved. Note how much easier it is when one can read the facial expressions and the visible responses from the other person. Without a shared background we could not even start a conversation. For example, suppose they interrupt our radio or TV programmes? How would we distinguish a message from interference? We would need the equivalent of the Rosetta Stone to decipher it. This is, of course, the philosophical problem of a communicative starting point. Appeals to the universality of science and mathematics have been made but all require some degree of tacit knowledge or prior agreement.

One solution to the joint problems of time lag and the lack of prior agreement was proposed by the Princeton physicist, R.N. Bracewell (1974), who suggested that the advanced civilization might find it expedient to send out probes to the vicinity of the planet they intend to communicate with. When the probe arrives we would have the basis for an exchange of information. On arrival the probe would be activated, first informing them of the presence of life, so that they would be on the alert for a message. But the communication problem remains: how does it attract our attention and how do we respond to it? Maybe there are probes here already, but we lack the ability even to recognize them. Perhaps, as in Kubrick’s 2001, they have left an artefact – on the Moon – such that it will not be found until we have begun space exploration and have reached a technological level which enables us to communicate. Gregory Benford (1990) suggests that an artefact may have been left on the Moon’s far side, or in a stable part of the Earth’s surface, which are relatively unexplored today. Yet without a common framework of discourse, even if the probe signals ‘We are from outer space’, we might not listen.

Bracewell’s suggestion is that an intelligently directed probe would listen to our familiar broadcasts and play them back to us after a time delay, like an echo, corresponding to the time it takes for their broadcast to reach us. For example, the newscaster might say ‘That is the end of the news, good night’, and five minutes later it is repeated. This would certainly make us sit up. As it repeats our broadcast we repeat, the probe returns it, and so on. All that the probe would need is a receiver, an amplifier and a transmitter. Bracewell’s example is based on the story of The Count of Monte Cristo, where the central character is imprisoned in the Château d’If. The next dungeon was 30 feet away but he did not know if it was occupied. He tapped on the stone and the other prisoner tapped back: two taps out, two taps back, and so on as conventions were built up step by step. This, of course, is possible if both share access to a similar code, such as Morse code. But what if they do not? A lot of redundant work would have to be carried out. Certainly the repetitive playback of our broadcasts would alert us to the likelihood that an intelligence was at work, but we would still need a common framework of discourse to proceed further.

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Questions can be raised regarding Bracewell’s proposed long-delayed echoes. Certainly they would attract attention if they disrupted radio and TV programmes, but they could be explained away in terms of natural phenomena. In the 1970s Anthony Lawson, a British electronics engineer, discovered that refractions of radio waves in the ionized region of our upper atmosphere can cause long-delayed echoes. Thus if the probe chose this potentially confusing method it would risk being dismissed as a natural phenomenon. The best thing the probe could do – in the honourable traditions of science fiction movies – is to repeatedly interrupt our broadcasts with its own message, which ought to contain a significant amount of scientific wisdom, such as a solution to a long-standing problem which is presented in a conventional manner.

In some respects, however, the communicative problem is not as difficult as it might seem. If limited to radio signals, like the prisoners tapping the wall, a great deal of background agreement will have to be employed. But this is very much a limitation imposed by the limits of present technology. Simultaneous transmissions of visual and audio data, for example, reduce  the  reliance  upon  background agreement.

The major problem with proposals for communication is the limit to background agreement. An ET civilization would have representations of their world which would reflect their systems of knowledge, as do ours. This is why a purely descriptive exchange of symbols cannot work. On Earth, when communities, or even individuals, have apparently lacked common reference points, it has been possible for them to engage in certain shared practices (work, defence, hunting, eating, procreating) which can bring different systems together. Practical activity brings us into contact with the material world and enables us to generate scientific knowledge. This is clearly missing in most schemes of communication with ETI.

The galactic network

One intriguing solution to the communication problem was proposed by Timothy Ferris (1992) who suggested that there might be a galactic network, consisting of probes distributed around the galaxy. According to Ferris, computer-controlled robot engineers can be sent to mineral-rich planets and asteroids where they will construct larger machines which will then in turn construct computer-controlled radio antennae, which can accumulate, receive and broadcast information. Further probes can be manufactured on the site by robots and sent to other mineral-rich planets or asteroids, eventually extending the network to other star systems throughout the galaxy. Communicative intelligences can hook into the network at relatively nearby stations and receive or transmit information which can be disseminated to any other intelligence within reach of the system. Although such a system is beyond the capabilities of present technology on Earth, the idea violates no known laws of physics and information technology. The digital computer is easy to replicate as it is based

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