Andrew Masterson
Andrew Masterson is a former editor of Cosmos.
A key science fiction device – a tractor beam powerful enough to lift a human – is now a step closer to reality, following an engineering feat that breaks what was assumed to be a fundamental constraint of acoustic levitating technology.
Acoustic waves exert radiation forces, and these can be controlled in order to create “traps” – points at which these forces converge, and thereby lift objects into the air. The technology for this well enough known for it to be publically available on hobbyist websites.
The drawback, however, is that the only objects that can be lifted are very tiny ones – bits with a diameter smaller than the wavelength used to make the trap.
This is because tractor beams are essentially rotating sound fields. As such, they inevitably transfer some of their energy to the object being held, causing it to rotate as well. If the object is larger than the wavelength the result is uncontrollable and chaotic spinning, causing it to be ejected.
All up, then, acoustic tractor beams have represented little more than a neat physics parlour trick, a long way from the powerful attractors seen in, say, Star Trek: Voyager, fuzzing out from a Galaxy-class starship to hold a Delta Flyer spacecraft in place.
And although it will likely be a long time before NASA starts thinking about installing them as standard kit on missions to Mars, research by a team from the University of Bristol has demonstrated that, set-up correctly, acoustic tractor beams can indeed lift objects larger than their foundation wavelength.
In a paper published in the journal Physical Review Letters, a team led by mechanical engineer Asier Marzo has created a tractor beam capable of lifting a two-centimetre polystyrene sphere – an object double the size of the wavelength used to create it.
The key to the success is the use of rapidly fluctuating acoustic vortices. Marzo and his colleagues discovered that the rate of rotation of such vortices can be controlled, and that the direction of its twisting can be altered. In effect, this combination creates an acoustic mini-tornado, with a loud, twisting outer structure encasing a silent core.
The result, the scientists report, is a very stable beam – and the tantalising prospect that further refinements will lead to ones capable of lifting significantly larger objects, including, potentially, people.
“Acoustic researchers had been frustrated by the size limit for years, so its satisfying to find a way to overcome it,” says Marzo. “I think it opens the door to many new applications.”
The researchers suggest that the development could lead in reasonably short order to acoustic tractor beams that can manipulate pills or miniature surgical devices within the body. Another possible use is the creation of tractor beam factory production lines, allowing small and delicate products to be handled without impact or friction.
The acoustic tractor beam created to cradle the polystyrene ball was done using a wavelength of 40 kilohertz – a sound audible only to some species of bats. The fact that it was able to lift an object larger than its own wavelength suggests that even bigger objects could be lifted by means of scaling up the power input, not the sound itself.
This potentially overcomes another theoretical drawback to large-scale tractor beams. Before Marzo and his colleagues completed their work, it was assumed that in order to create a beam capable of levitating a human, the wavelength required would produce a sound that was both deep and deafening.
