Superluminal interstellar travel using space-time warp drives

Physics of Warp Drive

If one is more or less familiar with special relativity and the effects it has, it becomes apparent that the Alcubierre drive has some strange aspects. In particular, Miguel Alcubierre showed that a ship with the aforementioned drive will travel on a free-fall geodesic even, while the warp bubble is acceleration. The ship's crew would be in free-fall when accelerating but without having to experience accelerational g-forces. Though, huge tidal forces would appear around the edges of the flat-space volume, due to the large space curvature there. Suitable specification of the Alcubierre metric would keep them very small, within the volume occupied by the spaceship.

The original warp drive metric and those similar have the ADM form. This is commonly used when discussing the initial-value formulation of general relativity. Therefore, this could explain the common misconception that this particular space-time is a solution of the field equation of general relativity. Metrics in the ADM form are altered to a certain family of inertial observers. Still, these certain observers are not physically distinguished from other families. Miguel Alcubierre interpreted the 'warp bubble' in terms of a contraction of space happening in front of the bubble and the expansion of it behind. However, this could be deceptive because the contraction and expansion is truly to do with the relative motion of close by members of the ADM family observers.

A realistic distribution of matter and energy is often specified in general relativity, along with the geometry of the space-time associated with it. But, it is also feasible that the Einstein field equations could operate in the opposite direction. This would involve, first specifying a metric and then locating the energy-momentum tensor associated with it. When constructing his own metric, they were the steps that Alcubierre took. Therefore, many energy conditions could be violated because of this practice. Moreover, exotic matter will be needed. This specific requirement for exotic matter causes the question to arise as to whether one will be able to distribute the matter in an initial space-time that will lack a warp bubble in a way so that the generation of the bubble will occur at a later time.

On the other hand, a few scientists think that models of dynamical warp-drive space-times when a warp bubble is formed in space that was previously flat. Furthermore, generating a bubble in previously flat space for a one-way faster than light trip may need exotic matter to move at local FTL speeds. This, in turn, will require the existence of tachyons. This theory is mainly postulated by Serguei Krasnikov. However, Krasnikov also said that when the space-time is not flat at the starts, a result that could be very much the same could be achieved without the need for tachyons. This could be done by placing some 'devices', in advance, along the proposed travel path, which would then be programmed to come into operation at pre-decided instances and will operate in a pre-decided manner.

Other suggested approaches evade the concern of the tachyonic motion. However, a naked singularity will almost certainly be generated at the front of the bubble. Additional comments suggest that the findings of Krasnikov are not completely suggestive of the fact that the Alcubierre drive could be used for FTL travel. It may only mean that the actions needed to alter the metric and to create the bubble must be taken earlier by an observer whose forward light cone contains the entire trajectory of the warp bubble. For instance, if one wished to travel to the star Deneb (2,600 ly away) and arrive less than 2,600 years in the future according to external time-pieces, then it would be required for the work on the warping of the space in between Earth and Deneb to begin 2,600 years in the past. Therefore, a spaceship properly placed with respect to the trajectory of the bubble may then choose to enter the bubble. This will make the superluminal journey. Causality considerations do not prevent the spaceship's crew from formulating, themselves, a round trip from Earth to another star in a short time period (as measured by clocks on Earth) by changing the metric.