“This study changes the conversation about warp drives,” pronounced Dr. Jared Fuchs, as a transition between science-fiction fuel and a model that remains within the ordinary bookkeeping of the weak force of gravity, general relativity.
The most graphic mathematical account of the “warp” Miguel Alcubierre solution of 1994, was that a spacecraft could be safe in a bubble of the flat spacetime, as space shrank in front of it, and expanded behind it. The ship does not locally overtake light, rather the geometry does. That is the conceptual jump that preserved relativity, but at a cost that seemed like a showstopper: the spacetime structure seemed to require negative energy density–a component not found in normal physics.
The more recent suggestion, linked to the scientists of the University of Alabama in Huntsville, restates what an operable warp drive needs to cost. Rather than depending on exotic negative energy, it constructs “classic warp drive spacetime” composed of positive ADM mass, in the form of a stable shell of matter, with a properly chosen shift vector to form the surrounding metric. The bubble is still there in more humble language, however, the bill is being presented in familiar currency: mass-energy which acts like the matter and radiation already familiar in existence. The practical limit was summarized by co-author Dr. Christopher Helmerich: “Although such a design would still require a considerable amount of energy, it demonstrates that warp effects can be achieved without exotic forms of matter.”
The fact that we say “without” is important because the objections of scale have not been the sole concerns expressed in the past, but objections of consistency. Relativity Physicists frequently use “energy conditions” which are constraints posed on stress-energy resembling sanity checks. These are not axioms, but they reflect a century of experience: the energies densities are positive, the energy flows do not locally exceed the speed of light and gravity is normally attractive. Geometries of the type of Alcubierre were seen to fail a variety of such tests, in particular the weak energy condition, which prohibits negative local energy density. The Casimir effect provides a small loophole in the door of quantum theory where tiny areas could measure negative energy between tightly spaced plates, however that loophole can hardly be scaled to a large size engine. Something bigger, the behavior of quantum fields in extreme, engineered curvature, lies in the undiscovered space between relativity and quantum gravity.
The UAH proposal circumvents that stalemate by defining a warp geometry which operates at fixed subluminal velocities. This limitation transforms the promise: it does not provide a Star Trek jump to the immediate stars, but it conserves a less grandiose purpose detailed in the debates of the team describing the “warp bubble momentum flux” a conveyor-belt-like transport of fast moving energy-momentum around an inner passenger volume. It is important to note in this framing that warp is not a magic stretch of space upon command; it is a meticulously designed structure of traveling stress-energy that creates a ride mediated by gravitation. A more human-scale implication, the most interesting to contemplate, is not superluminal travel, but the prospect of transporting a payload without exposing occupants to crushing acceleration- a result, which the authors relate to the ability of the interiors of the bubble to be relatively inertial.
Computation is one of the facilitating tools. The group tested and confirmed spacetime metrics, finding out which matches the specified constraints, using a software package known as Warp Factory, making of what had previously existed as a celebrated equation something more like a design space, with the capability of search, testing, and revision.
Gianni Martire, the CEO of Applied Physics, did not minimize the distance between paper and hardware, but announced a new dawn of opportunities: “While we’re not yet preparing for interstellar voyages, this research heralds a new era of possibilities.” It is no longer so hard to name the bubble as it is to learn how matter and energy and stability can act when they are called upon to take the shape of a geometry which nature does not normally suggest.
