Nothing can travel through space faster than light. But Einstein’s general relativity says nothing about how fast space itself can stretch, and in 1994 the Mexican physicist Miguel Alcubierre used that loophole. In a short paper in Classical and Quantum Gravity he described a region of spacetime that contracts in front of a ship and expands behind it, carrying the ship along in a bubble of flat space.
Inside the bubble the ship would be at rest, with no acceleration to crush its crew and no local motion faster than light. Seen from outside, the bubble could still cover the distance between two stars faster than a light beam. Alcubierre later told William Shatner that the idea was inspired by Star Trek’s warp drive.
The catch: negative energy
The solution has a price. To bend spacetime this way, the wall of the bubble must contain exotic matter, material with negative energy density. Ordinary matter, and everything ever measured, has positive energy. Quantum physics allows tiny amounts of negative energy in special situations, but nothing like what the drive needs.
The amounts are enormous. In 1997 Michael Pfenning and Lawrence Ford found that the bubble wall would have to be only a few hundred Planck lengths thick, and the energy needed was more than the mass of the observable universe. In 1999 Chris Van Den Broeck reshaped the bubble and brought the requirement down to a few times the mass of the Sun, still negative and still vast. In 2012 Harold White of NASA’s Johnson Space Center argued that a rounder, doughnut-shaped ring could reduce it to roughly the mass of the Voyager 1 probe, and set up a laboratory interferometer to look for tiny warps. No warp has been demonstrated.
Other problems
Even with the energy solved, the bubble has troubling features. Signals inside the bubble cannot reach its front edge, so the crew could not steer or stop it, a point made by José Natário in 2002. In 2012 Brendan McMonigal, Geraint Lewis and Philip O’Byrne showed that particles swept up on the journey would gain extreme energies and burst out at the destination when the ship stopped. Other work suggests that quantum effects would destroy the bubble at faster-than-light speeds.
Warp drives without exotic matter?
In 2021 Erik Lentz proposed warp-like solutions made only of positive energy, a claim other physicists dispute. The same year Alexey Bobrick and Gianni Martire described a general class of “physical warp drives”, and in 2024 Jared Fuchs and colleagues published a warp drive that obeys all the usual energy conditions. Lentz claimed his solutions could exceed light speed, but others found errors in them; the other two designs move slower than light. As far as anyone knows, going faster still requires the exotic matter that nobody has found.