The famous Alcubierre warp drive, introduced in 1994, has long been considered unphysical because it requires enormous amounts of theoretical negative mass or energy density to function . The authors of this study, Bobrick and Martire, suggest that this negative energy problem may not be a fundamental rule for all warp drives, but rather a specific flaw in Alcubierre’s original design.

The researchers identify key artificial constraints in the Alcubierre model that mathematically force the need for negative energy. The Alcubierre model assumes the warp bubble has no gravitational effect on the spacetime outside of it, effectively “truncating” the gravitational field. The authors show that this assumption is a likely reason it requires negative energy . Furthermore, Alcubierre designed his drive so a passenger’s clock ticks at the same rate as a stationary observer far away . This means the passenger’s time is actually accelerated compared to an observer moving alongside the drive, another feature that requires negative energy .

By removing these artificial constraints, the authors developed a new, general model for a warp drive that can be constructed using purely positive energy, or regular matter. This new, physical warp drive is, however, strictly subluminal, meaning it can only travel slower than the speed of light. It is essentially a massive, hollow, spherically symmetric shell of ordinary matter. Unlike the Alcubierre drive, it would have a normal gravitational field outside of it, just like a planet. A passenger inside this shell would be in “flat spacetime,” experiencing no gravity due to the Shell Theorem, a principle where the gravitational pull from all parts of the shell perfectly cancels out in the interior. The “warp” effect comes from its enormous mass, which causes time to run slower for the passenger compared to an outside observer. The mass requirements are vast; the paper calculates that an Earth-mass shell compressed to a 10-meter radius would only slow time by a tiny 0.04%.

This paper also clarifies a major misconception about how warp drives work. The original Alcubierre paper suggested the drive’s velocity could just be changed as a function of time, but the new study points out that this violates the conservation of energy . The authors stress that a warp drive is an object, specifically a “shell of regular or exotic material moving inertially”. Because it is a massive object, it is not a propulsion system itself. It cannot magically accelerate. To move or change velocity, it would require an external form of propulsion, such as a “propellant exhaust system” (like rockets) attached to it.

While this new positive-energy solution is limited to subluminal speeds, the paper reinforces that faster-than-light travel still appears to require negative energy. The authors also note that even for the unphysical Alcubierre drive, the most energy-efficient shape would be a flattened disc, not a sphere. This research is significant because it moves the warp drive concept from a purely unphysical fantasy to an extremely difficult but not theoretically impossible engineering challenge, suggesting a path to construct such spacetimes based on the laws of physics as we know them.