Faced with danger, the Doctor, protagonist of the science fiction television series Doctor Who, often availed himself of a tool known as a sonic screwdriver. A bit larger than an ink pen, the sonic screwdriver was, among other things, a lock pick, a remote control, and an alien-detection device. The Doctor’s tool was fictional, of course, but now Michael MacDonald and colleagues at the University of Dundee in the UK, in collaboration with Gabriel Spalding of Illinois Wesleyan University, have developed a real-life sonic screwdriver—an ultrasound device capable of generating high-angular-momentum acoustic vortices. With it, they’ve obtained an elusive measurement of the ratio of orbital angular momentum (OAM) to energy in a vortex beam.1
The notion that propagating waves can possess OAM is just 20 years old. First advanced by Les Allen and coworkers for the specific case of electromagnetic waves, the conclusion follows from the observation that...
