Human levitation with sound beams closer to reality

Human levitation with Star Trek- style tractor beams may soon be possible, as scientists have shown for the first time that large objects can be suspended in air, using just sound.
Acoustic tractor beams use the power of sound to hold particles in mid-air, and unlike magnetic levitation, they can grab most solids or liquids.
For the first time, researchers from University of Bristol in the UK have shown it is possible to stably trap objects larger than the wavelength of sound in an acoustic tractor beam.
The discovery opens the door to the manipulation of drug capsules or micro-surgical implements within the body.

Container-less transportation of delicate larger samples is now also a possibility and could lead to levitating humans, researchers said.
Scientists had previously thought that acoustic tractor beams were fundamentally limited to levitating small objects as all the previous attempts to trap particles larger than the wavelength had been unstable, with objects spinning uncontrollably.

This is because rotating sound field transfers some of its spinning motion to the objects causing them to orbit faster and faster until they are ejected.
The new approach uses rapidly fluctuating acoustic vortices, which are similar to tornadoes of sound, made of a twister-like structure with loud sound surrounding a silent core.

The researchers discovered that the rate of rotation can be finely controlled by rapidly changing the twisting direction of the vortices, this stabilises the tractor beam.
They were then able to increase the size of the silent core allowing it to hold larger objects.
Working with ultrasonic waves at a pitch of 40 kiloHertz, a similar pitch to that which only bats can hear, the researchers held a two-centimetre polystyrene sphere in the tractor beam.
This sphere measures over two acoustic wavelengths in size and is the largest yet trapped in a tractor beam. The research suggests that, in the future much larger objects could be levitated in this way.

"Acoustic researchers had been frustrated by the size limit for years, so its satisfying to find a way to overcome it. I think it opens the door to many new applications," said Asier Marzo, lead author on the study published in the journal Physical Review Letters.
"In the future, with more acoustic power it will be possible to hold even larger objects. This was only thought to be possible using lower pitches making the experiment audible and dangerous for humans," said Mihai Caleap, Senior Research Associate, who developed the simulations.