Published on Apr 13, 2012
Sonic Screwdrivers Serve up Fundamental Physics
A real "Sonic Screwdriver" has been used to lift and spin a free-floating 10 cm rubber puck. Energy beams, such as ultrasound or laser beams, carry momentum that can be used to push objects in the path of the beam, or even rotate objects if it is a vortex beam. The helical wavefronts of vortex beams mean there is a rotating, or angular component of momentum that can exert a torque on an object.
In the video, we generate acoustic vortex beams with many intertwined helices using a 1000-element ultrasound transducer array. These beams are strong enough to both levitate and spin the 90 g rubber puck in water.
We use the screwdriving effect to test the theory that the ratio of 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. This theory is used in topics from quantum physics to biophotonics but it has not previously been proved in a single experiment. For the first time, our experimental results confirm directly the validity of this fundamental theory.
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