be beyond any foreseeable technical ability to produce it. According to Einstein, the faster the object moves the greater will be the increase of its mass. At a speed approaching that of light a vehicle’s mass would increase tremendously requiring near impossible levels of energy to propel it. Moreover, at speeds approaching that of light, collision with microscopic particles of stellar dust would be catastrophic and the craft would require a massive shield which would also add to its payload. Nevertheless, it has been argued that faster-than-light (FTL) travel is actually compatible with Einstein’s general theory (Crawford, 1995), although there are no practical solutions to the problem of how to manufacture and fuel such a craft.

Proposals for FTL travel often involve appeals to ‘warps’ and ‘wormholes’. These concepts deserve consideration. Warps or ‘warp-drive’ sound like the stuff of science fiction although they are permitted by the laws of physics. Apparently warp-drive involves the manipulation of space-time such that it expands behind the spacecraft and contracts in front of it so that a spacecraft travelling fairly slowly will ‘push’ its departure time back and ‘draw’ its arrival time forward. Now this might be permitted by the laws of physics, but its practical application would require something approaching the energy output of a black hole.

Travelling through wormholes is another proposal for FTL travel. In 1915 Ludwig Flamm saw hints of the possible existence of wormholes in Einstein’s equations. Wormholes can be explained as follows: consider a black hole  so  massive that its powerful gravitational field curves space to such an extent that within it there is a singularity – where the curvature of space-time is so infinitely sharp that the laws of physics break down. Now imagine two singularities which are somehow linked together; for example, one at point A and another at point B, which is thousands of light years away. Presumably, if they are linked, then if  one entered the mouth of the wormhole at point A one could instantly emerge from point B. Apparently wormholes are not disallowed by the laws of physics, although this does not suggest that they actually exist. The problems with wormhole travel are obvious. If they involve faster-than-light travel, there is the objection that it would allow causality violations, such that time travellers might go back and prevent their own birth. So it has been assumed that some law of physics prevents their actual occurrence. This now leaves unresolved the question whether FTL travel is permissible without the causality violations.

Entering a black hole is rather dangerous. Nevertheless, the laws of physics do not prohibit the possibility of joining a black hole with a white hole, one that works on opposite principles to the black hole and expels matter. But then the problem is that a white hole is likely to be unstable and decay rapidly, making it very improbable for a spaceship to emerge. Generally, black holes are regarded as dangerous and unstable places, but some theorists maintain that they can be kept open and stable if they are crammed with ‘exotic matter’ which, however, can be created out of nothing when space and time are curved the right way (Matthews, 2000: 12). So far, speculation regarding wormholes and exotic matter lacks observational support, and is justified only insofar as they are ‘permitted’ by

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the laws of physics. Perhaps the future lies in the actual discovery of a wormhole or the possible manufacture of wormholes in a laboratory.

Marcus Chown (1996) considers travel through wormholes and acknowledges that while Einstein’s general theory of relativity allows for the existence of shortcuts through space and time, it is a far step to proving their existence and constructing them. However, Chown notes that two lines of inquiry are under consideration. The first is the attempt to construct a microwormhole in a laboratory; the second involves a search for wormholes that have either survived after the big bang or have been created by advanced extraterrestrials. The remarkable properties of wormholes would make them detectable from Earth, as light from stars would fluctuate as the wormhole mouth passes between the Earth and the star.

Conventional spacecraft, however, must meet tremendous problems of energy consumption. Even if it is assumed that extraterrestrials are more technologically advanced than us they will still have to cope with a near 100 per cent mass-energy conversion near the speed of light. To appreciate the problem, assume a level of 100 per cent energy conversion efficiency for a 1,000 tonne payload on a round trip from the nearest star to Earth at 70 per cent the speed of light. This would require energy equivalent to 500,000 years worth of the total annual electrical power produced by the USA (Tarter, 1990).

Of course the raw material for the craft’s energy sources might not have to come from terrestrial sources. Gerard K. O’Neill (1975) suggests that a system of satellite solar power stations, manufactured in space and located in a geosyn- chronous orbit from Earth, would not only provide enough energy for Earth but provide enough energy for space flight with no loss of Earth’s resources. These solar power stations could, he says, be initially manufactured out of lunar material. If this were possible for terrestrials, a similar solution to the energy problem could be devised by extraterrestrials.

NASA is currently developing forms of Solar Electric Propulsion (SEP) which could enable a wide range of missions that would be financially prohibitive with the standard chemical propulsion fuel available at present. Instead of burning a chemical propellant, the SEP system operates with a very high-velocity stream of ionized xenon which gently propels a spacecraft over a long interplanetary cruise. The advantage is that SEP requires much less propellant than conventional rockets, although it does take longer for its gentle thrust to build up high velocity. Consequently, it is more appropriate for lengthy voyages or colonies on the move.

If, however, we suspend restrictions on space travel imposed by current technological limits, it is possible to conceive of a craft which is coherent with current scientific theory but well beyond our present skills. There has been speculation about the feasibility of a photon craft, whose  energy would be derived from the annihilation of matter by antimatter – an antiproton plus a positron. This would be the most efficient rocket fuel imaginable, as the mutual annihilation of matter and antimatter leads to a 100 per cent conversion of mass

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