Imagination has never been the most intractable problem in the physics of warp drive; it has been bookkeeping. The Alcubierre concept of 1994, the celebrated one, fits in the general relativity theory, allowing a craft to ride in a sort of “bubble,” as the space continues to contract in front and continues to expand behind but the accounting line that causes the model to go wrong is the demand that negative mass or negative energy must exist in sufficient amount that it is physically unavailable.
More recent theoretical suggestions by scientists at the University of Alabama in Huntsville and Applied Physics resolves that issue as a problem of metric design: to specify a spacetime geometry such that it still exhibits a warp effect, but according to which the distributions are constructed out of standard, positive-energy matter. It is a change of focus on whether a warp bubble is possible at all to whether what sort of matter structure would necessarily provide one without breaching the established conditions of energy.
In the standard image, the ship is not locally overtaken by light; rather, it is the geometry of the spacetime that pushes the ship around its locality. Its description by Alcubierre explains it: it is possible to have faster than the speed of light motion by a purely local expansion of the spacetime behind the spaceship and a contraction in front of it, whereas the outside observers will see it as faster than the speed of light. The inner part is a sort of inertial cavity, with the maximum curvature being placed on the boundary of the bubble. It is also along that boundary that the steepest possible theoretical costs have been long computed, the energy cost, instabilities at the bubble edge, and the incompatibilities between quantum-field behavior and classical geometry.
The new model retains the bubble with alterations of sourcing. It postulates a stationary shell of matter with positive ADM mass instead of a negative-energy shell, accompanied by a well-designed “shift vector,” a conventional element of the ADM formulation of general relativity that determines the relation between spatial coordinates of one time slice to the next. A computation design tool named Warp Factory was used by the team to find spacetime metrics which produce warp-like behavior within those constraints. According to co-author Dr. Christopher Helmerich, such a design, even though still using a significant amount of energy, is capable of showing that warp effects can be realized without the use of exotic matter.
The point of that assertion is significant in that it attempts to address the main gripe against the Alcubierre drive, namely that the energy required is enormous, but the signature of the density of the energy required is incompatible with known matter. The wider scientific community on warp measures has found negative energy fascinating, both as something tantalizing as quantum theory permits small bubbles of it, e.g. the Casimir effect and gravely constraining, as it seems impossible to scale the effect of such bubbles to any visible scale. Other pathologies have also been revealed by space-time analyses, as in situations when the quantum fields on the bubble boundary move apart, and in situations when the stress-energy required to keep the geometry together does not coherently remain “attached” to a moving bubble.
Another thread of the warp-drive theory has considered geometry variants that ease requirements by varying the shape of the bubble, decreasing the area of the region that does the compressing and expanding and expanding the habitable interior. In other studies, this is referred to as reducing the “neck” of the bubble in order to reduce the burden of bookkeeping. Despite such refinements, published estimations of traditional Alcubierre-style bubbles can still be fantastically large, e.g. a macroscopic bubble might need negative energy exceeding the amount in the universe when making some assumptions.
The newer positive energy building does not eliminate the engineering challenge it moves it. Even a warp metric without exotic matter would be an extraordinary control of the distributions of dense energy, and the curvature gradients defining the bubble wall. The model also fails to evade open questions at the interface point between general relativity and quantum physics where a variety of researchers believe that the ultimate limitations to superluminal geometries will become manifest.
Nonetheless, the work alters the meaning of “impossible” as it does in this subsector of physics, displaying a mathematically consistent march towards the warp-like spacetime effects of positive-energy matter. Dr. The repositioning was made simple by Jared Fuchs: “This research shifts the discussion on warp drives.”
