The Fantasy
The “tractor beam” has been a reliable narrative device in science fiction for nearly 100 years, deployed whenever the plot requires seizing a runaway spaceship or manipulating objects at a distance. Author E.E. “Doc” Smith is credited with coining the term in 1931 with his novel Spacehounds of IPC, serialized in the pulp sci-fi magazine Amazing Stories. The language is old-school delicious: “Brandon swung mighty tractor beams upon the severed halves of the Jovian vessel….”
Glorious!
Over the years, the tractor beam concept has been used by Golden Age sci-fi writers like Robert Heinlein and Isaac Asimov; by intergalactic heroes like Buck Rogers and John Carter; by TV shows and games Babylon 5 and Half-Life. But most sci-fi fans probably know it from Star Wars and Star Trek — the sinister Death Star and the mighty USS Enterprise each boasted a frequently convenient tractor beam system.
It’s one of those delightful sci-fi tropes for which we cheerfully suspend our disbelief. It seems like the kind of technology that the future ought to have. Sci-fi wags have even developed a term for this: applied phlebotinum, a nonsense phrase that means these technologies work because the plot needs them to work.
The Reality
We already have tractor beams here on Earth, more or less. Well, emphasis on less. Scientists have been generating small-scale tractor beams for several years now, using tightly focused light and sound waves. These devices can’t move spaceships but they can move tiny things, from microscopic particles to lightweight materials around a half-inch in diameter. It doesn’t seem like much, but these tiny tractor beams could have profound practical applications. More on that in a bit.
The first thing to know about real-life tractor beams is that they work more like another sci-fi concept: force fields.
Bruce Drinkwater, a veteran researcher at University of Sussex in the UK, explains that our current real-world tractor beam technologies don’t actually pull a remote object, as a magnet might. Instead, they use various kinds of waveforms to create movable field of force around the object, trapping it. Then the field itself can be manipulated — moving the trapped object forward or sideways or back toward the emitting device.
“It can be thought of as an acoustic hologram,” says Bruce Drinkwater, whose official title is Professor of Ultrasonics, which is frankly awesome. “It’s a shape that exists in space but is made of sound.”
Drinkwater and his team have developed a kind of sonic tractor beam that uses an array of loudspeakers — 192 at last count — to create a field of sound around a target particle or object. This “acoustic trap” creates high-pressure interference patterns in midair that hold, rotate, and move the object.
“It can be thought of as an acoustic hologram,” says Drinkwater, whose official title is Professor of Ultrasonics, which is frankly awesome. “It’s a shape that exists in space but is made of sound.”
Drinkwater’s team recently demonstrated the system by capturing and moving around a 16-millimeter-wide Styrofoam ball — around four times larger than what was possible with earlier acoustic traps. The acoustic trap uses ultrasound, frequencies above the threshold of the human ear. And because the sound is extremely localized — that’s the whole point, really — there’s no danger to human ears.
“The only danger would be if you were stupid enough to strap this thing on your ear,” Drinkwater says.
Sriram Subramanian, a professor of computer science at University College London, has also worked extensively with tractor beam technologies. He says that theoretically, these kinds of force-field traps can be generated using any kind of waveform — sound, light, water, maybe even gravity in far-future scenarios.
“The trick is to generate wave patterns that intersect and interfere with each other at a distance,” he says. “On that level, it doesn’t matter whether it’s acoustic or optical technology.”
In fact, researchers have had success as far back as the 1990s with light-based tractor beams, in which lasers and photons trap and manipulate on a microscopic scale. (Certain kinds of 3-D hologram systems use exactly this kind of approach.)
Laser traps are limited to extremely small particles measured in micrometers, or one thousandth of a millimeter, in part because of the heat issues involved. Scaling up a light-based tractor beam means you run the risk of incinerating your target.
“At the high end of the optical system, they’re using pretty powerful lasers,” Drinkwater says. “That’s one of the advantages of the acoustics — it’s a very safe setup. You could have these in a hospital or factory.”
The Future
In near-future terms, Drinkwater says the secret to practical applications of tractor beam technology is to keep thinking small. We won’t be using tractor beams to yank around Corellian-class smuggling ships, but we could certainly use them to accomplish touchless manipulation of tiny objects in places where precision is at a premium.
The abdominal cavity, for instance. Subramanian says that in the future, acoustic tractor beams could be used to move tiny nanobots inside the body for targeted drug delivery, when a particular medicine needs to be delivered to a particular area of tissue.
On the industrial front, sonic tractor beams could be used for (very) small-scale manufacturing. Rather than extremely tiny machines moving even tinier components, invisible acoustic holograms could do the lifting.
“And they don’t need repairs, don’t need cleaning,” Drinkwater says.
Drinkwater concedes that it will be “a fair few years” before any of this happens — and many more before we can start scaling up to spaceship-sized tractor beams. “But I’m hopeful that there’ll be some kind of killer application that will lead the way, and everyone will think: Oh, right! Tractor beams! Why didn’t we invest millions in this earlier?”
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