  A theme that has come to the fore in advanced planning for long-range space
exploration is the concept of "propellantless propulsion" or "field
propulsion." One version of this concept involves the projected possibility
that empty space itself (the quantum vacuum, or space-time metric) might be
manipulated so as to provide energy/thrust for future space vehicles. Although
such a proposal has a certain science-fiction quality about it, modern theory
describes the vacuum as a polarizable medium that sustains energetic quantum
fluctuations. Thus the possibility that matter/vacuum interactions might be
engineered for space-flight applications is not a priori ruled out, although
certain constraints need to be acknowledged. The structure and implications of
such a far-reaching hypothesis are considered herein.
