Researchers have now built a working tractor beam that uses high-amplitude sound waves to generate an acoustic hologram which can pick up and move small objects.

"In our device we manipulate objects in mid-air and seemingly defy gravity,” said one of the researchers Sriram Subramanian, professor of informatics at University of Sussex in England.

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"Here we individually control dozens of loudspeakers to tell us an optimal solution to generate an acoustic hologram that can manipulate multiple objects in real-time without contact," Subramanian explained.

The technique, could be developed for a wide range of applications, for example a sonic production line could transport delicate objects and assemble them, all without physical contact.

On the other hand, a miniature version could grip and transport drug capsules or microsurgical instruments through living tissue.

The researchers used an array of 64 miniature loudspeakers to create high-pitch and high-intensity sound waves.

The tractor beam works by surrounding the object with high-intensity sound and this creates a force field that keeps the objects in place.

By carefully controlling the output of the loudspeakers the object can be either held in place, moved or rotated, the researchers explained.

"We all know that sound waves can have a physical effect. But here we have managed to control the sound to a degree never previously achieved," Bruce Drinkwater, professor of ultrasonics at University of Bristol in Britain, pointed out.

The findings were detailed in the journal Nature Communications.