A small stream of liquid flows quietly in midair, horizontally, as if inside an invisible tube. Nothing touches it. About 5 centimeters (2 inches) away, a cylinder lined with hundreds of tiny speakers surrounds the stream, yet there is no sound. The liquid simply keeps flowing.

“Of all the experimental setups I have seen, I would say this is the closest thing to magic,” says Ari Salmi, professor of electronics at the University of Helsinki, who led the development of the massless tube.

It is not magic, however, but ultrasound. The University of Helsinki’s Electronics Research Laboratory has a long tradition of ultrasound research. The stream of liquid is held in place by ultrasonic waves at a frequency of about 40 kilohertz. The waves are emitted by 256 small speakers surrounding the stream.