A University of Dundee research team led by Prof. Mike MacDonald has demonstrated that both levitation and twisting forces can be applied to an object by application of ultrasonic beams. This latest breakthrough is part of a wide-ranging U.K. research effort to develop a device not unlike the "sonic screwdriver" made famous by the TV series Doctor Who.

The sonic screwdriver has been a favored tool of ten Doctors at this point in time (it was introduced by the second Doctor). It has evolved from a simple screwdriver and lock-pick to include such functions as repairing electronics, remotely controlling the Tardis, amplifying X-rays, and even converting a cellphone to truly universal roaming. Without the screwdriver, Earth would have fallen countless times to Daleks, Cybermen or Silurians.

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Back here on Earth, ultrasonic beams have primarily been used for imaging within opaque objects and to shake objects – the ultrasonic medical scanner and ultrasonic cleaner are the prime examples. Now the team of physicists at the University of Dundee in Scotland (with associates at Bristol University in England) have succeeded in generating an ultrasonic vortex beam strong enough to lift and rotate a rubber disk submerged in water. The water allows for efficient coupling between the sound source and the disk.

This impressive video begs the question – what is an ultrasonic vortex beam, and why is it so clever at manipulating objects? Briefly, an ultrasonic vortex beam is rather like a laser beam with an axial phase singularity. This means that the phase of the sound wave varies as you orbit a given spot on the optical axis. As the orbit is drawn smaller and smaller, the same amount of phase change is found. As a result, on the axis of the beam it has all possible phases at once. As this is not allowed by conservation and continuity laws, the on-axis ultrasonic intensity has to be zero. The cross-section of an ultrasonic vortex beam thus appears a bit like a donut – no signal on axis (the donut hole), a large signal away from the axis (the thick part of the donut), and a signal that falls off as you look further from the axis (you run out of donut).

Consistency also requires that the phase change around the axis of an ultrasonic vortex beam must be a multiple of a full vibrational cycle – 2 pi, 4 pi, etc. If this is not the case, self-interference effects will destroy the vortex after a very small propagation distance.

One might think of a vortex beam with a 2 pi phase singularity as being a single helix, one with 4 pi singularity a double helix, and one with 16 pi singularity as an octuple helix. The size of the phase change determines what the orbital angular momentum of the beam is, and this orbital angular momentum can be coupled to an object so that it will experience a torque, and will spin if free to move.

Theory predicts that the ratio of orbital angular momentum to energy in a vortex beam is equal to the ratio of the number of intertwined helices to the frequency of the beam. The Dundee experiment was designed to measure both radiation pressure and the torque of an ultrasonic vortex beam, so that the results would allow the theoretical prediction to be tested. The scientists' direct experimental observation that the ratio of the torque to power does convincingly match the expected value (given by the topological charge to angular frequency ratio of the beam) is a fundamental result.

Beyond the amusing comparison to the Doctor's sonic screwdriver, this ability to produce complex motions of an object at a distance has many potential near-term applications.

"This experiment not only confirms a fundamental physics theory but also demonstrates a new level of control over ultrasound beams which can also be applied to non-invasive ultrasound surgery, targeted drug delivery and ultrasonic manipulation of cells," said Dr MacDonald.

Should we expect ever to be able to mount cabinets, pick locks, and fight Daleks with an ultrasonic vortex beam? The power requirements and environmental safeguards required for an actual "screwdriver" will make this a very difficult goal. Then again, that is what we thought about tricorders a few years back, and reasonable facsimiles are beginning to appear in research labs. The tenth Doctor said humanity was "indomitable." Who knows, we might prove him right.

The results of the Dundee "sonic screwdriver" experiment are published in the American Physical Society's journal Physical Review Letters. The work is part of a broader "Sonotweezers" project run by the Engineering and Physical Sciences Research Council (EPSRC).

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