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A team of physicists has discovered that it is possible to build a real, actual, physical warp drive without breaking the known rules of physics. One caveat: the ship doing the warping cannot exceed the speed of light, so you won’t encounter anything interesting anytime soon. But this research still represents an important advance in our understanding of gravity.
Movement without movement
Einstein’s theory of general relativity is a toolbox for solving problems with gravity, which links mass and energy to distortions in spacetime. In turn, these spacetime distortions instruct the mass and energy how to move. In almost all cases, physicists use the equations of relativity to figure out how a certain combination of objects will move. They have a physical scenario, such as a planet orbiting a star or two black holes colliding, and they wonder how those objects warp spacetime and what the subsequent evolution of the system should be.
But it is also possible to do Einstein’s mathematics in reverse by imagining a desired motion and asking what kind of spacetime distortion could make it possible. This is how Mexican physicist Miguel Alcubierre discovered the physical basis for a warp drive – long a staple of the Star Trek franchise.
The goal of a warp drive is to get from A to B in the time between commercial breaks, which typically involves faster-than-light movement. But special relativity expressly prohibits speeds faster than light. Although this never bothered the authors of this book Star Trek, it did irritate Alcubierre. He discovered that it was possible to build a warp drive through a clever manipulation of spacetime, arranging it so that the space in front of a ship contracts and the space behind the ship expands. This generates movement without, strictly speaking, movement.
It sounds like a contradiction, but that is just one of the many wonderful aspects of general relativity. Alcubierre’s warp drive avoids violations of the light speed limit because it never moves Through room; instead, space itself is manipulated to essentially bring the spacecraft’s destination closer.
Although tempting, Alcubierre’s design has a fatal flaw. To achieve the necessary distortions of spacetime, the spacecraft must contain some form of exotic matter, usually considered to be negative-mass matter. Negative mass has a number of conceptual problems that seem to defy our understanding of physics, such as the possibility that if you kick a ball weighing minus 5 kilograms, it will fly backwards, violating the conservation of momentum. Furthermore, no one has ever seen a negative mass object in the real universe.
These problems with negative mass have led physicists to propose various versions of ‘energy conditions’ to complement general relativity. These are not baked into the theory of relativity itself, but add-ons are needed because general relativity allows for things like negative mass that don’t seem to exist in our universe; these energy conditions keep them out of the equations of relativity. It’s scientists’ reactions to the disturbing fact that vanilla GR makes things like superluminal motion possible, but the rest of the universe doesn’t seem to agree.
Warp factor zero
The energy conditions have not been proven experimentally or observationally, but they are statements that agree with all observations of the universe, so most physicists take them quite seriously. And until recently, physicists viewed these energy conditions as making it absolutely 100 percent clear that you can’t build a warp drive even if you really wanted to.
But there’s a way around this, discovered by an international team of physicists led by Jared Fuchs of the University of Alabama in Huntsville. (The team is also affiliated with the Applied Propulsion Laboratory of Applied Physics, a virtual think tank focused on research into warp drives, among other things.) In a paper accepted for publication in the journal Classical and quantum gravitythe researchers delved deep into the theory of relativity to investigate whether a version of a warp drive could work.
The equations of general relativity are notoriously difficult to solve, especially in complex cases such as a warp drive. So the team turned to software algorithms; instead of trying to solve the equations by hand, they examined their solutions numerically and checked whether they met the energy conditions.
The team did not actually attempt to build a propulsion device. Instead, they explored various solutions to general relativity that would make it possible to travel from point to point without a ship undergoing any acceleration or experiencing overwhelming tidal forces within the ship, to the great comfort of all conceivable passengers. They then checked whether these solutions met the energy conditions that prevent the use of exotic matter.
The researchers indeed discovered a warp drive solution: a method to manipulate space so that travelers can move without accelerating. There’s no such thing as a free lunch, though, and the physicality of this warp drive does come with a major caveat: the ship and its passengers can never travel faster than the speed of light. Also disappointing: the fact that the researchers behind the new work don’t seem to bother figuring out which configurations of matter would make the warping possible.