The future is here! Geek has the story on these so called “tractor beams” and how they work.
“Tractor beams” are mostly mentioned in reference to starships; the phrase has probably been uttered more by science fiction characters than by everyone else combined. As a result, a new acoustic tractor beam recently unveiled in the journal Physical Review of Letters might not be quite what you’re expecting; since there’s no air between two starships, and thus no possibility of sound, an acoustic tractor beam could never work in space. Still, the breakthrough technology could find dozens of useful applications here on Earth, and pave the way for other, more space-friendly solutions.
The physics at work here is both simple and complex, a dizzying confluence of wave mathematics that ultimately sum to allow the team to create a simple pulling force with sound. Sound, by the way, is simply vibrations in a medium (usually air), and those vibrations manifest as regions of greater or lesser air pressure; the peak of a sound wave diagram corresponds to an area of high pressure, while the trough represents an area of low pressure. These expansions and contractions in a physical medium can do work when they interact with an object — just hold your hand up to a powerful subwoofer to assure yourself of that…
…Now, by shaping the object to be moved so it interacts specifically with the waves doing the moving, this team has managed to move a centimeter-scale object with sound. That’s millions of times larger than the objects we’ve managed to move with light waves, which have never exceeded the nanometer scale. By setting the angle and frequency of vibrations just right, this experiment created a low pressure zone in front of the object, pulling it forward, while simultaneously bouncing waves off the back end to push from the other side. The object used in this experiment was triangular, so the angled back end could transfer up as much kinetic energy forward as possible.
This study used a tank of water for its experiment, rather than of air, largely because water is denser than air and thus allows better propagation of waves. This means their waves will lose less power as they travel and, far more importantly, they will carry more kinetic energy. In theory the technique should work just as well in air, but would result in a great reduction in overall pulling power.
This breakthrough could be of use to a wide array of sectors in research and industry. Moving small objects in a liquid medium could be useful to extremely fine surgical procedures, like small-scale modifications to the cornea. Sending specialized waves through a complex medium should only affect the objects specifically designed to interact with those waves, and as such should not both the rest of, say, your eye. On the other hand, complex environments also have more structures for waves to bounce off, thus making the whole process more difficult.