The tractor beam is a staple of science fiction. Aliens use them to haul up unwilling earthlings into flying saucers for probing, and spacecraft use them to seize enemy ships or tow captured objects around in space. Now a group of researchers working at the University of Dundee actually claim to have built one. But instead of lasers, it uses ultrasonic waves to pull macroscopic objects in.
Working in cooperation with colleagues from Southampton University and Illinois Wesleyan University, the team used an ultrasound array to direct energy behind an object to exert force upon it and drag it towards the energy source. Using an ultrasound device clinically approved for use in MRI-guided focused ultrasound surgery, the team was able to move surprising large objects of approximately 1 cm in size.
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"We were able to show that you could exert sufficient force on an object around one centimeter in size to hold or move it, by directing twin beams of energy from the ultrasound array towards the back of the object," said Dr Christine Demore of the Institute for Medical Science and Technology (IMSAT) at Dundee. "This is the first time anyone has demonstrated a working acoustic tractor beam and the first time such a beam has been used to move anything bigger than microscopic targets."
The object targeted by the acoustic tractor beam was triangular in shape, which allowed the researchers to use the effects of "nonconservative" forces (in this case, friction), to be overcome by directing ultrasonic beams of equal power and angle of incidence at the sides of the object. As they were deflected, they exerted a pressure behind the object, thereby pushing it toward the ultrasonic energy source.
And it isn’t just tractor beams the team has been working on either. In collaboration with Dr Gabe Spalding from Illinois Wesleyan University, the Dundee researchers have previously demonstrated that Dr Who’s “sonic screwdriver” could also be created using a comparable ultrasonic array pointed at an object to push it away as well as make it spin.
Lasers have previously been used by other researchers to create tractor beams, but their influence has so far been limited to the microscopic level. NASA is also conducting experiments in this field, but the University of Dundee acoustic tractor beam seems to be the first one so far to move objects of a significant size.
The work was carried out as part of the "Electronic Sonotweezers: Particle Manipulation with Ultrasonic Arrays" program initiated by the Engineering and Physical Sciences Research Council (EPSRC).
The results of their research have been published in the journal Physical Review Letters.
Source: The University of Dundee