There aren’t many visual effects in a film more stunning than seeing something levitate. We’re all so used to gravity that seeing an item float around (without being powered by an engine) defies expectation and, seemingly, logic. While using sound to levitate objects isn’t exactly a new concept, I bet you haven’t seen it put into effect quite like this team from the University of Tokyo does.
You know the kind of sound waves that you can feel in your stomach? That’s like what’s going on here, except instead of your stomach bouncing around on a wave, minute objects such as drops of soap and little plastic balls float around on three-dimensional sound waves. In addition to sound waves, when music comes out of a speaker it also emits compressed air that matches the sound’s wavelength. When the waves overlap, they create new waves, such as in the image below where the blue and green waves create the red one.
But the waves are different, as you can see. Unlike the blue and green waves, the red one has fixed points along the axis. Those points are called nodes, and the wave that generates them is called a standing wave. Nodes are, by nature, neutral, pushed from opposite sides by air pressure. Any object smaller than the node and light enough will not only levitate but remain stable. In the video above, you can see the standing waves when they bring out the dry ice.
The team from the University of Tokyo took this concept and made it practical—in three dimensions, which is what allows them to levitate items bigger than the plastic balls. Using this methodology, one can apparently levitate objects smaller than 8mm with a density of less than 1,000kg/m3. So while we might not be floating humans just yet (unless that light as a feather, stiff as a board game I played as a kid turns out to work after all), it could have practical applications for medicine, nanotechnology, and space travel. And while it’s not done using sound, this project reminds me of the awesome quantum levitation work being done on larger objects. No flying DeLoreans or hoverboards yet, though, but we’re well on our way.