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An Advance in Tractor-Beam Technology

An Advance in Tractor-Beam Technology


The term “tractor beam” is thought to have made its first appearance in “Spacehounds of IPC,” a sci-fi novel by Edward E. Smith published in 1947. Smith, whose work has been cited as an influence by the likes of Arthur C. Clarke, George Lucas, and J. Michael Straczynski, the creator of the show “Babylon 5,” worked as the chief chemist for a Michigan flour mill (his specialty was doughnut mixes). His best-known works, the Lensman and Skylark series, are full of imagined technologies that, like the tractor beam, were far beyond the reaches of contemporary science but nevertheless based on seemingly sound principles.

Scientists first began working on making tractor beams a reality in the nineteen-nineties, after the Russian ceramics engineer Eugene Podkletnov reported that certain small objects, when placed above a superconducting disk supported on a rotating magnetic field, lost up to two per cent of their weight. His experiment—the results of which were met with widespread, albeit somewhat knee-jerk, skepticism in the physics community—seemed to indicate that it was possible to neutralize the force of gravity, at least in part. Further experiments followed; in 2001, Podkletnov and the Italian physicist Giovanni Modanese built what they called an impulse gravity generator, a device that emitted a beam of focussed radiation in a “short repulsive force.”

Until recently, no one had managed to move anything bigger than a particle. (There was brief excitement earlier this year, when researchers from Australia and Spain successfully moved a plastic sphere fifty nanometres across—around a thousand times thinner than a human hair—by splitting a beam of light in two and using it to press in on the sphere from each side, like a pair of tweezers.) Even NASA has tried to get in on the action, although their vision seems somewhat lacking when compared with the many tractor-beam scenarios already laid out in science fiction: the team of scientists tasked with the job are supposed to come up with more efficient ways of clearing “orbital debris,” i.e., space garbage. (And they don’t look happy about it.)

Now scientists from the University of Dundee, in Scotland, have created something with a bit more muscle. While most of the documented experiments with tractor-beam technology so far have involved light waves, the team from Dundee used sound waves to manipulate a half-inch triangular prism made of metal and rubber, successfully pulling the target toward the source of the acoustic beam. Half an inch may not sound like much, but it’s a vast improvement on fifty nanometres. The experiment was part of a larger project across four U.K. universities—Bristol, Southampton, Glasgow, and Dundee—and took nine months to complete. The results have been published in Physical Review Letters.

The Dundee tractor beam is not entirely dissimilar from those in “Star Wars” and “Star Trek,” in that it draws an object toward it without making physical contact. The device works by taking advantage of an acoustic wave’s natural push effect, called radiation pressure. (Photons also exert radiation pressure, which is part of the reason comet tails always point away from the sun.) What the Dundee team was able to demonstrate was an example of negative radiation pressure, otherwise known as pull. According to Christine Démoré, a senior research fellow at the Institute for Medical Science and Technology, at Dundee, and a co-author of the paper, one of the team’s main reasons for staging the experiment was to show how easily it could be done. “It’s a relatively simple concept, but it’s just obscured by complex math,” she told me. “By shaping a beam of energy so that it goes around an object in some way, hitting it in the back, it’s possible to then pull the object instead of push it.”

To do this, the team used a commercial ultrasound-surgery machine to generate two Bessel beams, a type of acoustic radiation that remains focussed as it travels rather than spreading out. They fired these beams from either side of the target; when the beams hit the sloped sides of the prism, they were deflected up, like cue balls bouncing off the side of a billiards table. The sideways momentum of the beams transferred to the target, pushing it down, toward the energy source.

The immediate applications of the Dundee tractor beam are medical. Démoré and her colleagues hope to improve the efficacy of focussed ultrasound surgery, a noninvasive treatment for tumors that works by heating and destroying unwanted tissue. Another potential application is targeted drug delivery, achieved via tiny capsules in the bloodstream. “What we’ve shown in our tractor-beam experiment is that it may be possible to push, drag, or hold the drug capsules at a specific location in the body, improving the targeting of the released drugs,” Démoré told me. And if Dundee’s device could be made to work on larger objects, it could also prove useful for collecting geological samples from parts of the planet currently impossible to reach—volcanic vents, the deep sea, perhaps even space. “Some of this may be a fair way off,” Démoré said. “But we’ve demonstrated the physics that make it conceivable.”


Researchers find tin selenide shows promise for efficiently converting waste heat into electrical energy

( —A team of researchers working at Northwestern University has found that tin selenide (SnSe) has the highest Carnot efficiency for a thermoelectric cycle ever found, making it potentially a possible material for use in generating electricity from waste heat. In their paper published in the journal Nature, the team describes work they’ve conducted on SnSe and how their discovery might lead to even more efficient materials. Joseph Heremans gives a short history of thermoelectric research in a News & Views companion piece and offers some insights into why SnSe might be so efficient and how it might lead the way to the discovery of even better materials.

As the planet continues to experience the impact of global warming, scientists around the world frantically pursue alternate ways to produce electricity—one such possibility is to convert waste heat from industrial process into electricity. To make that happen, a thermoelectric generator must be constructed and used. Such generators operate by taking advantage of differences in temperature experienced by a single material. Two thermoelectric semiconductors are exposed to a temperature gradient and are connected together by conducting plates. Thus far, however, the process has not proved to be efficient enough to warrant the expense of building and using such generators, despite doubling in efficiency over just the past fifteen years—from zT 1 to 2.

The increase in efficiency has been due mostly to research work involving nanotechnology, and the materials used have generally been based on lead telluride. The difficulty in finding better materials has been stymied by the dual properties required: low thermal conductivity and high electrical conduction. SnSe has been used by scientists for a variety of purposes, but due to its stiff bonds and distorted lattice was not really considered as a possibility. But that was because others had not taken into account the compound’s low anharmonicity. When the team at Northwestern tested it as a possible material for use in a thermoelectric generator they found it had the highest zT ever found, 2.6.

The increase in efficiency is clearly welcome, but is still not enough to revolutionize the field—what might would be the discovery of another material with an even higher efficiency—something that might be similar to SnSe.

Explore further: Thermoelectric materials can be much more efficient

More information: Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals, Nature 508, 373–377 (17 April 2014) DOI: 10.1038/nature13184

The thermoelectric effect enables direct and reversible conversion between thermal and electrical energy, and provides a viable route for power generation from waste heat. The efficiency of thermoelectric materials is dictated by the dimensionless figure of merit, ZT (where Z is the figure of merit and T is absolute temperature), which governs the Carnot efficiency for heat conversion. Enhancements above the generally high threshold value of 2.5 have important implications for commercial deployment1, 2, especially for compounds free of Pb and Te. Here we report an unprecedented ZT of 2.6 ± 0.3 at 923 K, realized in SnSe single crystals measured along the b axis of the room-temperature orthorhombic unit cell. This material also shows a high ZT of 2.3 ± 0.3 along the c axis but a significantly reduced ZT of 0.8 ± 0.2 along the a axis. We attribute the remarkably high ZT along the b axis to the intrinsically ultralow lattice thermal conductivity in SnSe. The layered structure of SnSe derives from a distorted rock-salt structure, and features anomalously high Grüneisen parameters, which reflect the anharmonic and anisotropic bonding. We attribute the exceptionally low lattice thermal conductivity (0.23 ± 0.03 W m−1 K−1 at 973 K) in SnSe to the anharmonicity. These findings highlight alternative strategies to nanostructuring for achieving high thermoelectric performance.

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