Surfing on spacetime
In 1994, theoretical physicist Miguel Alcubierre published a paper showing how faster-than-light travel was possible within the framework of general relativity. Instead of a ship moving through space at impossible speeds, Alcubierre's solution involves moving space itself. His metric describes a "warp bubble" of flat, undisturbed spacetime where a spacecraft could sit. This bubble would be propelled by spacetime contracting in front of it and expanding behind it. Since spacetime itself can expand or contract at any speed, the bubble could theoretically exceed the speed of light.
A passenger inside this bubble of flat space would feel no acceleration, even as the bubble itself maneuvered and achieved enormous speeds. They would travel on a geodesic, essentially in continuous free-fall. The solution bypasses the light-speed limit imposed by special relativity without technically violating it, as the ship remains locally stationary while the space around it moves. The concept became popular in the scientific community, and research into its feasibility took place at labs including the Advanced Propulsion Physics Laboratory, known as "Eagleworks," at NASA's Johnson Space Center in Houston.
The exotic matter problem
The Alcubierre drive faces several severe and likely insurmountable obstacles. The primary issue is its power source. To create the required spacetime distortion, the drive needs a region of negative energy density. This requires "exotic matter," a theoretical substance with negative mass that would violate several energy conditions of classical physics. Alcubierre's original calculations showed that a warp bubble large enough for a small ship would require an amount of negative energy equivalent to the mass-energy of the entire observable universe.
Later research, including work by Harold "Sonny" White at NASA, proposed modified geometries, such as shaping the warp field into a torus, that reduced the required energy to the mass-energy equivalent of Jupiter. While a significant reduction, this still depends on the existence of exotic matter, which has never been observed. One candidate for creating negative energy density is the Casimir effect, a quantum phenomenon observed between two closely spaced metal plates, but the amount generated is minuscule.
Other problems persist. The front of the warp bubble would create a horizon, preventing the pilot inside from steering, stopping, or sending any signals forward. Quantum effects would cause intense Hawking radiation to build up at this horizon, which could destroy the ship and its occupants. This same radiation, along with any interstellar particles swept up by the bubble, would be released in a cataclysmic burst upon deceleration, potentially vaporizing the destination.