For most of us, travelling faster than the cosmic speed limit”the speed of light”is a science-fiction fantasy that breaks the very foundation of modern physics. But in the eyes of an engineering undergrad at the University of Alabama in Huntsville named Joseph Agnew, it’s a theory worthy of study.
The idea first came to Agnew in high school, when he became enamoured of the warp drives he saw in Star Trek. “I thought about some of the technology postulated within,” he says, “and wondered what the scientific backing might be.”
Agnew isn’t just another Trekkie with dreamy eyes. In September 2019, he presented a talk on the subject at an aerospace engineering conference. Reports described Agnew’s audience as “standing room only,” and the event attracted a great deal of media attention.
It might seem remarkable that a piece of science fiction has found itself in an academic conference, but Agnew is far from alone in his interest. He hopes to become one of the dozens of engineers and theoretical physicists who have been studying warp drives in earnest for over two decades. But unlike Agnew, most scientists see the warp drive as little more than a thought experiment, though one that can still teach us a lot.
“It is still rightly perceived as a highly speculative idea which is interesting to illustrate certain points about general relativity but completely impractical,” says Jose NatariÃ³, a mathematician and theoretical physicist at the Instituto Superior Téchnico in the University of Lisbon.
“It is technically sound,” says Gerald Cleaver, a theoretical physicist at Baylor University, “but there have been issues that have come up on almost every level.”
The field of warp drive studies is less than 30 years old. In 1994, a physicist named Miguel Alcubierre, then a doctoral student at Cardiff University in Wales, proposed something remarkable: a way to travel faster than the speed of light”in the eyes of an outside observer”without flaunting the laws of physics.
Alcubierre’s warp drive theory works not by pushing anything faster than light. Instead, his warp drive creates a bubble that literally warps space: compressing it in front and stretching it out behind. If you were in a spaceship travelling inside such a bubble, you’d still be moving under the speed of light, but you’d essentially be travelling through distances that have been squeezed shorter, as if you were riding the crest of a wave through space-time.
“A propulsion mechanism based on such a local distortion of spacetime just begs to be given the familiar name of the “˜warp drive’ of science fiction,” Alcubierre wrote in his original paper. Naturally, among both physicists and the general public, the name caught on.
But for Alcubierre’s mathematical vision to work, the warped space would have negative energy”in other words, less than the zero energy which exists in a perfect vacuum. As bizarre as that sounds, it’s not physically impossible. In order to get that negative energy, Alcubierre’s warp drive theory invokes what physicists call “exotic matter””a sort that has negative mass.
Negative-mass exotic matter would be pretty peculiar if you encountered it on Earth. Both mathematically and literally, it would act as a polar opposite of the positive-mass matter you interact with every moment of your life. If you tried to kick a ball of negative-mass exotic matter away, for instance, it would actually come toward you”and Earth’s gravity, rather than binding that ball to the ground, would push it up into space.
For now, negative-mass exotic matter lives in the realm of hypothesis, but that doesn’t faze Agnew. “There are some theories that indicate exotic matter could exist,” he says.
Carlos BarcelÃ³, a gravity researcher at the Andalusian Institute of Astrophysics in Granada, Spain, agrees. “It is just hypothetical that exotic matter exists, but there are theoretical situations that point to the possibility that it could be manufactured.”
Agnew cites something called the Casimir effect. Imagine two neutral sheets of metal in a vacuum, almost but not quite touching”nanometres apart. Photons whose wavelengths don’t fit into the gap are cut off, reducing the energy of the vacuum; in fact, that gap actually has negative energy. Agnew doesn’t pinpoint how the Casimir effect could be used to get exotic matter, but he does say it’s “a potential indicator of some of these unknown forces at play.”
In any event, Alcubierre’s original paper said that, to support the negative energy of a warp bubble just 100 metres across, you’d need more exotic matter than the mass of the entire Universe. Later calculations whittled that number down to merely the mass of Jupiter, but it’s still a staggering amount of something no one has ever seen”at least, not to our knowledge.
Nonetheless, for theoretical physicists, Alcubierre’s paper became like a lamp in the dark. In the quarter-century since then, numerous mathematicians, engineers, and physicists have fluttered around his theory, plucking it apart and then hammering it back together.
One such physicist was Jose NatariÃ³. “There seemed to be a great deal of arbitrariness in the choices [Alcubierre] made,” he says. “I wanted to see if there were better choices.”
Then, in 2001, NatariÃ³ showed that Alcubierre’s warp drive was only one possible warp drive”and that Alcubierre’s stretching and squeezing space wasn’t actually necessary. He devised an alternative: a warp bubble that moves forward through space, as he says, “much like a fish slides through water without compressing or expanding it.”
But NatariÃ³’s warp drive failed to eliminate Alcubierre’s dependence on negative-mass exotic matter. Moreover, his warp drive faced other hurdles that prevented both Alcubierre’s and his drives from becoming reality.
One issue is that photons exiting the front of a warp bubble, where space is compressed, are blueshifted; they have shorter wavelengths. That means the photons gain energy, but in order to preserve conservation of energy, they’d siphon it from space that already has negative energy, destabilising the whole bubble.
“In my paper,” says NatariÃ³, “I prove that this problem occurs for all warp drives.”
“Unless one finds a way to tame those instabilities,” says BarcelÃ³, “the warp drive would get destroyed in a very short time period.”
NatariÃ³ highlights another issue. Even if you did successfully create a warp bubble with negative energy, there’s no known way to move it forward in space, in much the same way you can’t hear a supersonic aeroplane before it arrives. “You cannot tell space to deform so that the warp bubble can move forward,” he says.
Those issues make warp drive travel, as we imagine it, utterly impractical. Agnew, an engineering student, seems to think that these obstacles are surmountable and that a warp drive is within reach. “I’m confident there is more to explore,” he says. “There is certainly reason to be hopeful!”
Theoretical physicists don’t quite agree.
Creating a warp drive today, says Cleaver, is “impossible, to be honest.”
There’s no way to build a warp drive, says NatariÃ³, “not with current technology and possibly not with any future technology.”
“The chances are very low that a configuration like that is ever going to be built,” says BarcelÃ³.
“I don’t think it is feasible at all unless we discover something new about fundamental physics in the future,” Alcubierre himself says. Now director of the Nuclear Sciences Institute at the National Autonomous University of Mexico, Alcubierre hasn’t done any work on warp drives since that first paper in 1994.
Joseph Agnew believes that it will take something experimental, “some quantitative, physically representative progress in research,” in order to change how physicists view warp drives.
For instance, NASA engineers have proposed experiments that would measure the warping of space with lasers, but so far, no experiment has proved useful, and Cleaver has argued that such experiments are flawed.
Still, even if the warp drive remains beyond engineers’ reach, that doesn’t mean they’re a cosmic dead-end. In studying theoretical warp drives, physicists have been able to learn a lot about relativity and about how the universe works in extreme circumstances.
“I have always [been interested in] the reasons why the speed of light cannot be surpassed,” says BarcelÃ³. To him, studying warp drives is a way of exploring the horizons of physics, where the cosmic speed limit starts to break down.
Additionally, Cleaver says that “there’s a lot of connections between [warp drives] and wormholes,” theoretical “tunnels” linking different places in space-time. Like Alcubierre’s warp drive, wormholes could offer a way to traverse vast distances faster than the speed of light. Furthermore, some wormhole theories depend on negative energy to stay open. Those links are a big part of why Cleaver calls warp drives “a very interesting idea.”
And even if warp drives never become anything more than a thought experiment, many warp drive physicists draw their inspiration from Agnew’s same well.
As Cleaver says, “I think we’re all Star Trek fans.”
Rahul Rao is a science writer and Doctor Who fan.