Exotic Forces: Do Tractor Beams Break the Laws of Physics?
Warp drive. Site-to-site transporter technology. A vast network of interstellar wormholes that take us to bountiful alien worlds. Beyond a hefty holiday wish-list, the ideas presented to us in sci-fi franchises like Gene Roddenberry's "Star Trek" have inspired countless millions to dream of a time when humans have used technology to rise above the everyday limits of nature, and explore the universe.
But to guarantee the shortest path to turning at least some of these ideas into genuine scientific breakthroughs, we need to push ideas like general relativity to the breaking point. Tractor beams, one of the most exotic ideas proposed by the genre that involves manipulating space-time to pull or push objects at a distance, take us beyond the everyday paradigm of science, to the very edge of theoretical physics. And, a team of scientists examined how they might work in a recent study shared on a preprint server.
"In researching sci-fi ideas like tractor beams, the goal is to push and try to find a demarcation point where something more is needed, like quantum gravity," said Sebastian Schuster, a scientist with a doctorate in mathematical physics from the Charles University of Prague, in an interview with IE. And, in finding out if tractor beams can work, we might also uncover even more exotic forces, like quantum gravity. So strap in.
Bending warp drive theory into tractor beam studies
"Warp drives, tractor beams, wormholes — all of these more exotic ideas we often find in sci-fi," said Schuster. Along with his colleagues, he examined how tractor beams might work under a framework of general relativity, where the fabric of space-time bends and contorts from the presence of mass, in a process we know as gravity. In 2021, Schuster and his colleagues, Matt Visser and Jessica Santiago, both of the Victoria University of Wellington, analyzed a series of studies supporting the possibility of a viable warp drive engine. Their conclusions weren't swayed by the hype that has surrounded new theories about faster-than-light travel, but they saw promise in testing other sci-fi ideas under general relativity.
Ideas like tractor beams.
"The basic idea is to significantly modify and adapt the 'warp drive' space-times in a suitable manner, giving them a 'beam-like' profile, and analyzing the induced stresses and forces," wrote the researchers in the preprint study. "Instead of a spaceship riding inside a warp bubble, we will assume that the warp field is in the form of a 'beam' generated to pull/repel a target." Of course, we lack the technological prowess to bend space-time with machines, which is why Schuster and his colleagues "assume that some arbitrarily advanced civilization might have developed the appropriate beam generation technology."
Violating the null energy condition under general relativity
The appeal may at first feel too obtuse, but the implications of an ability to control the force of gravitational attraction and "pull" massive objects toward us would change everything. Imagine pushing a dangerous, apocalypse-triggering asteroid away from Earth by bending the fabric of space-time around it, like a ship turned away by a giant wave. Or, better yet, why not mine that asteroid? At least one asteroid has been found worth an estimated $11 trillion — and if we pulled it into a comfortable orbit, unspeakable riches would be parked only minutes away by rocket. Maybe we could even set it slowly down on the surface of Earth, and dig in.
"For tractor beams, if we consider the 'pulling' force, that's both what a tractor beam does and what two planets would do if they're set close and released (before colliding)," said Schuster. "But with a beam, you have something bounded in size — it's directional, and doesn't go beyond the distance you want." In simply bounding (or restricting) the area where we want to create a tractor beam, "that's already enough to require weird exotic materials, even though we have several physical examples of these exotic materials that violate the local versions of energy conditions (so they're not all that exotic)."
Of course, there are other, non-gravitational forms of tractor beams. "The most famous example uses some kind of sound device," said Schuster. "In such cases, you typically have an array of small speakers, and use the sound to pull or push the object toward or away from you — effectively creating an acoustic tractor beam." However, as sound waves, this attractive harmony relies on a volume of matter to propagate the wave of sound, or something in which an object to be "tractored" closer or farther can move around. "That's usually air," he said. But in space, "while there is always the odd particle flying around, there's nothing to make a sound wave with." This is why Schuster turned to the idea of gravitational fields as a means of moving objects in space.
To test a tractor beam that manipulates the fabric of space-time (gravity), Schuster and his colleagues invoked several energy conditions, one of which is called the null energy condition (NEC). "Basically, the NEC is a statement for particles that move at the speed of light — no matter which direction or trajectory a particle at lightspeed has, it will see certain physical quantities, and the NEC is a statement about whether or not these will be positive or negative," explained Schuster to IE. "If a particular combination of these quantities is always positive, then the NEC is fulfilled."
Schuster's team used a simple kind of light to build a theoretical model for tractor beams, beginning with the NEC, and then added implications. Until they hit a theoretical snag. Under a framework of general relativity, the implications for tractor beams tacked on top of the NEC didn't hold true.
A theory of quantum gravity could reveal the secret to warp drive, tractor beams, and more
"Since we started with the NEC in a chain of implications, we knew that the NEC couldn't hold," said Schuster, summarizing why tractor beams ran into trouble under general relativity. "We had a list of implications that were violated in our case, and therefore we know that the NEC is violated in our kind of space-time." And since the NEC is the weakest of all energy conditions, the researchers could deduce that all the other ones would be violated, too. But this doesn't mean all hope for a feasible tractor beam is lost.
"The Casimir effect is one real and measured effect where we know these energy conditions are violated," he added, referencing a quantum effect that took decades to successfully measure. If violating energy conditions like the NEC doesn't necessarily rule out tractor beams, maybe we're on the cusp of a new kind of physics. On its own, the Casimir effect is far too tiny and sensitive to lend direct credence to something like the exotic space-times [like a tractor beam] we see depicted in sci-fi shows like "Star Trek."
However, the Casimir effect still shows that a crucial threshold — creating a real, exotic force that also violates the NEC — is already satisfied. "This proves that energy conditions can be violated, but also raises the question: Can it be violated in a controllable way, on a scale more accessible to us?" asked Schuster, referencing something of a more practical size, like a spacecraft. In other words, a new body of theories about gravity might change the rules. "A theory of quantum gravity might answer definitively whether you can build a viable tractor beam or not, and it might also answer some questions about exotic matter," Schuster told IE. But, alas, there remains no firm consensus on how quantum gravity would work. "Many people like to say it's string theory, others say it's causal set theory," he added.
"Suffice to say there exist many different theories of quantum gravity, which means there's much room for other physics to arise between now and an experimentally functional version of quantum gravity," said Schuster. And we have a long way to go before we can do that. It's, "a frontier we know is out there, but we don't know all the land we have to traverse to get there. In some ways, it's worse than Frodo's journey in 'Lord of the Rings'." So while the work goes on, to master exotic gravitational concepts like warp drives or tractor beams, we might need one theory to rule them all.