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A team of physicists has discovered that it is possible to build a real, physical warp drive without breaking any known rules of physics. One caveat: the ship doing the distortion can’t exceed the speed of light, so you won’t be getting to any interesting places any time soon. Nevertheless, this research represents an important advance in our understanding of gravity.
Moving without movement
Einstein’s general theory of relativity is a toolbox for solving gravity problems that link mass and energy to deformations in spacetime. These spacetime deformations, in turn, tell the mass and energy how to move. In almost all cases, physicists use the equations of relativity to figure out how a particular combination of objects moves. They have a physical scenario, such as a planet orbiting a star or the collision of two black holes, and they ask how these objects deform spacetime and what the subsequent evolution of the system should look like.
But you can also apply Einstein’s mathematics the other way round, by imagining a desired movement and asking yourself what kind of space-time deformation can make it possible. This is how the Mexican physicist Miguel Alcubierre discovered the physical basis for a warp drive – for a long time a cornerstone of Star Trek Franchise.
The goal of a warp drive is to get from A to B in the time between commercial breaks, which usually involves faster-than-light movement. However, special relativity explicitly prohibits speeds above the speed of light. While this never bothered the authors of Star Trekit irritated Alcubierre. He discovered that it was possible to build a warp drive through clever manipulation of spacetime, setting it up to compress the space in front of a ship and stretch the space behind the ship. This creates movement without actually being movement.
It sounds like a contradiction, but this is just one of the many wonderful aspects of general relativity. Alcubierre’s warp drive avoids violating the speed of light limit because it never moves through The space; instead, space itself is manipulated to essentially move the spacecraft closer to its destination.
Alcubierre’s design, while tempting, has a fatal flaw. To provide the necessary distortions of spacetime, the spacecraft must contain some form of exotic matter, typically thought of as negative mass matter. There are some conceptual problems with negative mass that seem to elude our physical understanding, such as the possibility that if you kick a ball weighing minus 5 kilograms, it will fly backwards, violating the conservation of momentum. Moreover, no one has ever seen an object with negative mass in the real universe.
These problems with negative mass have led physicists to propose various versions of “energy conditions” as complements to general relativity. These are not built into relativity itself, but are necessary additions, since general relativity allows for things like negative mass, which don’t seem to exist in our universe – these energy conditions keep them out of the equations of relativity. They are scientists’ answer to the disturbing fact that simple general relativity allows things like faster-than-light speeds, but the rest of the universe doesn’t seem to agree with it.
Warp factor zero
The energy conditions are not proven experimentally or observationally, but they are statements that are consistent with all observations of the universe, so most physicists take them pretty seriously. And until recently, physicists believed that these energy conditions made it absolutely 100 percent clear that you couldn’t build a warp drive even if you really wanted to.
But there is a way around this. This was discovered by an international team of physicists led by Jared Fuchs at the University of Alabama in Huntsville. (The team is also affiliated with the Applied Propulsion Laboratory of Applied Physics, a virtual think tank dedicated to researching warp drives, among other things.) In an article accepted for publication in the journal Classical and quantum gravityResearchers delved deeply into relativity theory to find out whether some version of a warp drive could work.
The equations of general relativity are notoriously difficult to solve, especially in complex cases like a warp drive, so the team resorted to software algorithms. Instead of trying to solve the equations by hand, they examined their solutions numerically and checked whether they satisfied the energy conditions.
The team did not attempt to design a propulsion device. Instead, they explored various solutions to general relativity that would allow travel from one point to another without accelerating the ship or subjecting the ship’s interior to overwhelming gravitational forces – much to the benefit of all the imaginary passengers. They then checked whether these solutions met the energy conditions that prevent the use of exotic matter.
Researchers have actually found a solution to warp drive: a way to manipulate space so that travelers can move without accelerating. However, nothing comes for free, and the physicality of this warp drive comes with a major caveat: the ship and passengers can never travel faster than light. Also disappointing: the fact that the researchers behind the new work don’t seem to be bothering to figure out what configurations of matter would make the deformation possible.