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 hold things together, so there is a kind of precedent.
But the actual tractor beam geometry is something different.
A tractor beam is a device that can move an object from a distance. The idea comes from a 1931 science fiction story called SpaceHounds of IPC:
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If sci-fi has anything to say about it, tractor beams are already familiar, and we can thank Star Trek and Star Wars for their pervasiveness.
But tractor rays do exist, although their reach is only microscopic.
Microscopic tractor beams are used in devices called optical tweezers. Optical tweezers use lasers to move microscopic objects such as atoms and nanoparticles. They are used in biology, nanotechnology, and medicine.
These tractor packs work on microscopic organisms but are not strong enough to pull large objects.
Now, a team of researchers has successfully demonstrated a microscopic tractor beam. They have published the paper describing their work in the journal Optics Express. Its title is “Macroscopic Laser Clouds Based on the Knudsen Force in Rare Gas,” and the lead author is Li Wang from Qingdao University of Science and Technology in China.
“In previous studies, the drag force of light was too small to pull a microscopic object,” Wang said. “With our new approach, the drag force of light has a much greater amplitude. In fact, it is three times greater than the pressure of light used to drive a solar sail, which uses the momentum of photons to exert a small thrust.”
Macroscopic tractor beam only works under certain lab conditions, so it’s a demonstration rather than a practical development. At least not yet.
First of all, it works on the stuff designed for this purpose: macroscopic graphene-SiO2 Compound objects made by researchers for experiments. Second, it operates in a rarefied gaseous environment, which has a much lower pressure than Earth’s atmosphere. While this limits their effectiveness here on Earth, not every world has an atmospheric pressure like ours.
“Our technology provides a non-contact, long-range traction method, which could be useful in many scientific experiments,” said Wang. “The rarefied gas environment we used to demonstrate this technique is similar to what was found on Mars. Therefore, it may have the potential to manipulate vehicles or aircraft on Mars one day.”
Their device works on the principle of gas heating. The laser heats up the composite objects, but one side is hotter than the other. The gas molecules on the back side receive more energy, which pulls on the body. The body moves with low pressure in a rarefied gas environment.
The researchers built a device to twist – or rotate – a pendulum made of graphene-SiO2 Composite structure to illustrate the phenomenon of laser drag. This display made it visible to the naked eye. They used another device to measure impact.
“We found that the pulling force was three times greater than the light pressure,” Wang said. “In addition, the laser towing 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 collect samples from the MSL Curiosity surface vehicle. ChemCam is one of Curiosity’s tools. It involves a laser vaporizing the rock or regolith and then a microscope to spectrophotometrically measure its components. But NASA wondered if the tractor beam could pull small particles from the vaporized sample into the rover for a more complete study.
A NASA NIAC presentation from 2010 said: “If tractor beam technology is included in ChemCam2 for clouds of dust particles and plasma, tractor beams could add a host of additional science capabilities:
- Laser ion spectroscopy
- mass spectrometry
- Raman spectroscopy
- X-ray fluorescence”
The same presentation said 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.
It never materialized, but it shows how attractive the idea can be.
This new research has produced interesting results, although it doesn’t come close to actual practical implementation. There is a lot of work and engineering required before you can get any closer to practicality. For one thing, there has to be a well-understood theoretical basis describing how the effect works on objects of different sizes and shapes and blazers of different powers in different atmospheres.
The researchers know this, of course, but point out that it’s still an effective proof-of-concept.
“Our work demonstrates that the flexible manipulation of light to a microscopic object is possible when the interactions between light, object, and medium are carefully controlled,” said Wang. “It also shows how complex the laser-matter interactions are and that many phenomena are far from being understood at the macro and micro levels.”
The critical part is that this study transfers turbine beams from microscopic to macroscopic. This is a huge threshold that is difficult to cross. In their conclusion, the authors write: “This work extends optical clouding from the microscopic scale to the macroscale, which has great potential for macroscopic optical manipulation.”
Spacecraft may put tractor beams to good use someday, but they’re unlikely to look like they do in science fiction. IPC’s Star Wars, Star Trek, and Spacehounds all feature tractor beams in combat and conflict.
But in fact, they can turn into valuable scientific tools.
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