Lift this! Tractor beams nearing reality?

Is it possible that tractor beams from the realm of science-fiction movies could become reality?

So far, experiments using a "tornado" of ultrasonic waves has been successful, they say, moving small objects, but a new breakthrough could lead to the possibility of levitation human beings.

Engineers from the University of Bristol have been able to trap (essentially levitate) objects using an acoustic tractor beam that is larger than the wavelengths of sound used by the device.

"Acoustic researchers had been frustrated by the size limit for years, so it's satisfying to find a way to overcome it," said Asier Marzo from the university's department of mechanical engineering. "I think it opens the door to many new applications."

Marzo is lead author on a paper published in the journal Physical Review Letters.

"I'm particularly excited by the idea of contactless production lines where delicate objects are assembled without touching them," said Bristol's Bruce Drinkwater, who oversaw the work.

The breakthrough comes from using rapidly fluctuating acoustic vortices, which the team describes as similar to "tornadoes of sound, made of a twister-like structure with loud sound surrounding a silent core."

By changing the twisting direction of the vortices, the researchers were able to stabilize the tractor beam and increase the size of the silent core, allowing it to hold larger objects. Using this technique and 40kHz ultrasonic waves (simillar to a pitch that only bats can hear), they were able to levitate a two-centimeter polystyrene sphere, the largest object ever trapped in a real-world tractor beam.