"change its position" in a counter-intuitive manner, without violating conservation of momentum. The Alcubierre drive is a hypothetical method of apparent faster-than-light propulsion for interstellar travel postulated from the theory of general relativity. Although this concept may be allowed by the currently accepted laws of physics, it remains unproven; implementation would require a negative energy density, and possibly a better understanding of quantum gravity. It is not clear how (or whether) this effect could provide a useful means of accelerating an actual space vehicle and no practical designs have been proposed. "Swimming in spacetime" is a general relativistic effect, where an extended body can change its position by using cyclic deformations in shape to exploit the curvature of space, such as due to a gravitational field. In weak gravitational fields, like that of Earth, the change in position per deformation cycle would be far too small to detect. See also Beam-powered propulsion Bernard Haisch Field propulsion Harold E. Puthoff Inertialess drive Perpetual motion Spacecraft propulsion Stochastic electrodynamics RF resonant cavity thruster (EmDrive) References External links "Breakthroughs" commonly submitted to NASA Inertial Propulsion Engine Reactionless Propulsion (Not) at MathPages Spacecraft propulsion Pseudoscience Perpetual motion Hypothetical technology Propulsion Discovery and invention controversies Fringe physics Pathological science