Tractor beams make intuitive sense. Matter and energy interact with each other in countless ways throughout the Universe. Magnetism and gravity are both natural forces that can draw objects together, so there's sort of a precedent.
But engineering an actual tractor beam is something different.
A tractor beam is a device that can move an object from a distance. The idea comes from a 1931 sci-fi story called SpaceHounds of IPC:
If science fiction had anything to say about it, tractor beams would already be commonplace, and we could thank Star Trek and Star Wars for their proliferation.
But tractor beams do already exist, though their reach is only microscopic.
Microscopic tractor beams are employed in devices called optical tweezers. Optical tweezers use lasers to move microscopic objects like atoms and nanoparticles. They're used in biology, nanotechnology, and in medicine.
These tractor beams work on microscopic objects but aren't strong enough to pull larger macroscopic objects.
Now a team of researchers has successfully demonstrated a macroscopic tractor beam. They published the paper explaining their work in the journal Optics Express. Its title is "Macroscopic laser pulling based on the Knudsen force in rarefied gas," and the lead author is Lei Wang from QingDao University of Science and Technology in China.
"In previous studies, the light-pulling force was too small to pull a macroscopical object," said Wang.
"With our new approach, the light pulling force has a much larger amplitude. In fact, it is more than three orders of magnitudes larger than the light pressure used to drive a solar sail, which uses the momentum of photons to exert a small pushing force."
This macroscopic tractor beam only works under particular laboratory conditions, so it's a demonstration, not a practical development. At least not yet.
First of all, it works on purpose-built things: macroscopic graphene-SiO2 composite objects that the researchers built for the experiments.
Secondly, it works in a rarefied gaseous environment, which has a much lower pressure than Earth's atmosphere. While that limits their effectiveness here on Earth, not every world has as much atmospheric pressure as our planet.
"Our technique provides a non-contact and long-distance pulling approach, which may be useful for various scientific experiments," said Wang.
"The rarefied gas environment we used to demonstrate the technique is similar to what is found on Mars. Therefore, it might have the potential for one day manipulating vehicles or aircraft on Mars."
Their device works on the principle of gas heating. A laser heats the composite objects, but one side is hotter than the other. Gas molecules on the back side receive more energy, which pulls the object. Combined with the lower pressure in the rarefied gas environment, the object moves.
The researchers built a torsional – or turning – pendulum device made from their graphene-SiO2 composite structure to demonstrate the laser-pulling phenomenon. That demonstration made it visible to the naked eye. They used another device to measure the effect.
"We found that the pulling force was more than three orders of magnitudes larger than the light pressure," said Wang. "In addition, the laser pulling is repeatable, and the force can be tuned by changing the laser power."
Other researchers have tackled tractor beams in recent years with mixed results. NASA was interested in pursuing the idea of using tractor beams to gather samples with the MSL Curiosity surface rover. One of Curiosity's instruments is the ChemCam.
It includes a laser that vaporizes rock or regolith and then a micro imager to measure its components spectroscopically. But NASA wondered if a tractor beam could draw tiny particles from the vaporized sample into the rover for a more complete study.
A NASA NIAC presentation from 2010 said: "If Tractor Beam Technology was included in a 'ChemCam2' to pull in dust and plasma particles, tractor beams could add a suite of additional science capabilities:
- laser desorptive ion spectroscopy
- mass spectrometry
- RAMAN spectroscopy
- X-Ray Fluorescence"
The same presentation said that tractor beams could be used to collect particles from comet tails, ice plumes on Enceladus, and even clouds in Earth's atmosphere or other atmospheres.
That never materialized, but it illustrates how compelling the idea is.
This new research produced interesting results, though it's nowhere near an actual practical implementation. There's a lot of work and engineering needed before it even approaches practicality.
For one thing, there needs to be a well-understood theoretical underpinning that describes how the effect works on objects with different sizes and shapes and with lasers of different powers in different atmospheres.
The researchers know this, of course, but point out that it's still an effective demonstration of feasibility.
"Our work demonstrates that flexible light manipulation of a macroscopical object is feasible when the interactions between the light, object, and medium are carefully controlled," said Wang.
"It also shows the complexity of laser-matter interactions and that many phenomena are far from being understood on both macro and micro scales."
The critical part is that this study moves tractor beams from the microscopic to the macroscopic. That's a significant threshold that's difficult to cross.
"This work expands the scope of optical pulling from microscale to macroscale, which has great potential in macroscale optical manipulations," the authors write in their conclusion.
Spacecraft may very well use tractor beams one day, but they're unlikely to look anything like they do in science fiction. Star Wars, Star Trek, and Spacehounds of IPC all feature tractor beams in combat and conflict.
But in reality, they could turn out to be valuable scientific tools.