centuries they have observed us and seen no sign of technology. Yet during the past fifty years it has grown at an unprecedented rate. Their problem is whether to crush us or help us but, according to Papagiannis, for the moment they have postponed any decision as they are aware that, for the present, we cannot find them.

The Zoo Hypothesis

Another explanation of the silence is the ‘Zoo Hypothesis’, proposed by J.A. Ball (1980: 242), who said that, ‘The perfect zoo … would be one in which the fauna inside do not interact with, and are unaware of, their zookeepers.’ This hypothesis suggests that benevolent super-beings look on us as a species in quarantine, at present too dirty and dangerous for contact, or that the Inter- galactic Council has designated Earth as a nature reserve. Perhaps, also, if they are very advanced they do not consider us worth contacting. The Zoo Hypothe-sis has a number of versions, most of which are falsifiable but not confirmable. As Ball put it: ‘We shall never find them because they do not want to be found and they have the technological ability to ensure this. Thus this hypothesis is falsifiable, but not, in principle, confirmable by future observations’ (ibid.: 243). Intelligent ETs may have penetrated the solar system with their probes and be keeping a low profile because of an embargo on contact with any inhabitants who are still planet-bound. This is to avoid lending any encouragement to plans to quit their planets prematurely, as first they must somehow demonstrate their fitness to mix with other beings. In such a case, silence would not mean absence, but an unwillingness to speak. According to Papagiannis (1988), even if no message is heard after the next million hours of search, we should not conclude that we are alone. Communicating civilizations may have an ethical rule whereby newcomers are obliged to pass an entrance test; for example, avoidance of overpopulation, disease, global war or environmental disaster. He concludes that a negative result of a long and comprehensive search need not simply mean that we are alone in the galaxy, but that we are not ready to join a sophisticated ‘Galactic Club’ which has rules for membership, and the headquarters of our galactic region has not yet issued an instruction for us to be admitted. But if we can solve our problems then we may, in time, receive an invitation. Papagiannis concludes that we need not waste efforts on expensive searches but should just concentrate on our own problems and meanwhile try to eavesdrop on them while we wait.

Critics of the Zoo Hypothesis draw attention to its inherent untestability and its failure to explain why the Earth and its neighbourhood were quarantined before life appeared on it. But most critics point out that it requires a total and an almost incredible degree of unanimity throughout the entire galaxy. Given the problem of travel and information transfer – even at the speed of light – it would seem that a well co-ordinated policy regarding Earth’s quarantine would be practically impossible to implement and maintain. It is, say critics, impossible

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to maintain social cohesion over vast distances when the message, sent out at the speed of light, would take 100,000 years to cross the galaxy.  Thus  an  edict,  ‘Leave Earth alone!’, would take 100,000 years; a reply, ‘Why?’, would take another 100,000 years; and ‘Because … ’ would be another 100,000.

An answer to this problem of social cohesion through communication has been proposed by I.A. Crawford (1995) who argues that faster-than-light (FTL) travel is not impossible. Against the widespread view that Einstein’s general theory of relativity prohibits FTL travel, Crawford cites numerous sources which suggest that it is not incompatible with current interpretations of general relativity. Crawford’s point is that no theory actually prohibits FTL travel; we simply do not know whether it is possible or not. Crawford, however, acknowl-edges the paradoxes involved with time travel – for example, travelling back in time and assassinating one’s grandparents – but he insists that providing that the causality violation (associated with time travel) does not occur, there are no theoretical objections. Faster-than-light travel, he argues, need not involve time travel and there may well be limits to time travel which permits FTL travel, such as tachyon drive (the conversion of ordinary matter into tachyons and back again), wormholes (short cuts through space and time), and a form of warp drive involving a distortion of space-time ahead and behind a space vehicle.

One must be wary of arguments which appeal to theoretical solutions which have no more substance than the fact that they are compatible with extensions of prevailing theories. However, Crawford does acknowledge that no one has any idea how to achieve FTL travel in practice: ‘No one has any idea how to build a warp drive, or construct a wormhole or turn a space-ship and its crew into tachyons’ (1995: 211). But it is theoretically possible, that is, permitted by the laws of physics as currently understood. The problems occur when the issue is classified as a technical problem which, of course, can be solved by any  sufficiently advanced civilization. For these societies can do anything that does not contradict established theory. This is a convenient way around any problem.

Merely because relativity theory does not forbid something does not make it possible or remotely likely. Without some inkling as to how FTL travel is to be achieved, appeals to what is possible ‘in principle’ carry little weight. Permission does not make things possible. But as a piece of reasoning in the context of discovery, such an argument indicates where further investigation could be undertaken by opening up the scope of permissible research.

But let us stay with this argument. If FTL travel and communication are possible, then galactic colonization is also a possibility. This would seem to support Tipler and Hart’s view of our uniqueness: if ETs can travel or disperse information faster than light and are not here now, then it would suggest that we are unique. However, FTL travel and communication offer another possibility. They would facilitate more social coherence over a larger area, giving galaxy-wide political controls which could maintain a policy of non-interference, thus removing one of the main objections to the Zoo Hypothesis. Thus Crawford concludes, that ‘the Zoo Hypothesis can only be a valid explanation of the

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