Free The Physics of Star Trek by Lawrence M. Krauss

perspicacity of the Star Trek writers. I have
     described how no rocket
    propulsion mechanism can ever get around the three roadblocks to interstellar travel set
     up by special relativity: First, nothing can travel faster than the speed of light in
     empty space. Second, objects that travel near the speed of light will have clocks that are
     slowed down. Third, even if a rocket could accelerate a spacecraft to near the speed of
     light, the fuel requirements would be prohibitive.
    The idea is not to use any sort of rocket at all for propulsion, but instead to use
     spacetime itselfby warping it. General relativity requires us to be a little more precise
     in our statements about motion. Instead of saying that nothing can travel faster than the
     speed of light, we must state that nothing can travel
    
    
     locally
    
    
     any faster than the speed of light. This means that nothing can travel faster than the
     speed of light
    
    
     with respect to local distance markers.
    
    
     However, if spacetime is curved, local distance markers need not be global ones.
    Let me use the universe itself as an example. Special relativity tells me that all
     observers who are at rest with respect to their local surroundings will have clocks that
     tick at the same rate. Thus, as I move throughout the universe, I can periodically stop
     and place clocks at regular intervals in space and expect that they will all keep the same
     time. General relativity does not change this result. Clocks that are locally at rest will
     all keep the same time. However, general relativity allows spacetime itself to expand.
     Objects on opposite sides of the observable universe are flying apart at almost the speed
     of light, yet they remain at rest relative to their local surroundings. In fact, if the
     universe is expanding uniformly and if it is large enoughboth of which appear to be the
     casethere exist objects we cannot yet see which are at this very moment moving away from
     us far faster than the speed of light, even though any civilizations in these far reaches
     of the universe can be locally at rest with respect to their surroundings.
    The curvature of space therefore produces a loophole in special relativistic argumentsa
     loophole large enough to drive a Federation starship through. If spacetime itself can be
     manipulated, objects can travel locally at very slow velocities, yet an accompanying
     expansion or contraction of space could allow huge distances to be traversed in short time
     intervals. We have already seen how an extreme manipulationnamely, cutting and pasting
     distant parts of the universe together with a wormholemight create shortcuts through
     space-time. What is argued here is that even if we do not resort to this surgery,
     faster-than-light travel might globally be possible, even if it is not locally possible.
    A proof in principle of this idea was recently developed by a physicist in Wales, Miguel
     Alcubierre, who for fun decided to explore whether a consistent solution in general
     relativity could be derived which would correspond to “warp travel.” He was able to
     demonstrate that it was possible to tailor a spacetime configuration wherein a spacecraft
     could travel between two points in an arbitrarily short time. Moreover, throughout the
     journey the spacecraft could be moving with respect to its local surroundings at speeds
     much less than the speed of light, so that clocks aboard the spacecraft would remain
     synchronized with those at its place of origin and at its destination. General relativity
     appears to allow us to have our cake and eat it too.
    The idea is straightforward. If spacetime can locally be warped so that it expands behind
     a starship and contracts in front of it, then the craft will be propelled along with the
     space it is in, like a surfboard on a wave. The craft will never travel locally faster
     than the speed of light, because the light, too, will