by Athena Cai
The idea of levitation has always fascinated science fiction. From Back to the Future to Harry Potter, defying gravity seems to be an imaginative desire common to all of us. The idea of floating objects in real life seems so surreal when in fact, engineers have already made it possible with acoustic levitation. How does acoustic levitation work? It all boils down to the physics of standing waves.
I first heard of standing waves in my physics 11 class and am happy to revisit the concept in the context of real life applications. Although standing waves can occur within any medium, acoustic levitation specifically involves sound. Particles push each other around (or vibrate), producing a wave of pressure known as sound.
When a wave hits a fixed end, it is reflected; the forwards motion is sent in the opposite direction and the wave is turned upside down in an event called inversion. Reflected waves run into and interfere with the upright waves from the source. While this interference usually results in an irregular and non-repeating wave pattern, standing waves have points that are recurrent. The pattern of standing waves creates the illusion that the waves are not moving in any direction. Hence the name ‘standing wave’.
The bold, solid line in the diagram above represents the upright waves; the bold, dashed line represents the reflected waves. Nodes are the points where the two lines meet. Antinodes are the points on the waves exactly in between the nodes.
Acoustic levitation hinges on the application of standing waves. At the nodes of a standing wave, there is little wave movement so the pressure is very low. Being furthest from the nodes, antinodes have the most movement and are therefore areas of greatest pressure. Because objects tend to move to areas of low pressure, the levitating object would sit at a node. The pressure produced by the wave counteracts the force of gravity on the object, producing levitation.
Obviously, researchers developed acoustic levitation technology intending for it to do far more than appease fans of science fiction. Among other things, acoustic levitation is forecasted to play a large role in drug research, nanotech, medicine, and chemical analysis. I think it could also be possible to build hovering vehicles using acoustic levitation. Especially since further development has led to more viable devices capable of moving heavier objects.
A fiction made reality, acoustic levitation is a rising aid for research across all fields of science.
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