From soaring spacecraft to wily wizards, levitation has been a constant source of material for science fiction and fantasy stories. Science has tried to recreate this fictional phenomenon through a variety of means, but all have come up short of achieving the type of magical floating we typically think of when daydreaming about levitation.
Acoustic levitation - using sound waves to levitate objects - has been the subject of past experiments and has shown promise in achieving true levitation. However, scientists have previously been limited to levitating very small objects or have only levitated objects along one axis, meaning objects had to be semi-restrained.
Now, newly published research out of the University of São Paulo and Heriot-Watt University in Edinburgh, UK shows that the current barriers to achieving acoustic levitation might soon be overcome. Writing in Applied Physics Letters, the researchers claim this new technique surpasses past levitation methods because it allows for levitation of larger objects using what’s known as a standing wave to keep objects afloat:
In contrast to traditional standing wave levitators where small particles are trapped at the pressure nodes, the levitation of a large spherical object occurs because a standing wave is produced between the transducers and the object itself.
Standing waves are acoustic phenomena in which sound waves are reflected back at themselves at precise distances that create a type of feedback loop, causing the alternating high and low points along the wave to become “stuck” in space as opposed to traveling in the direction of the wave.
The researchers in this experiment used multiple transducers (like flat speakers) to aim ultrasonic frequencies at an object, in this case a foam sphere. A standing wave is created between the object and the transducers, and the object floats on this wave. The wave is essentially a field of acoustic energy, and as we all know from physics lessons, when energy causes an object to change motion, it can be called force. Thus, the foam ball in this study is floating on a force field.
One of the researchers in this experiment, Marco Andrade, told Phys.org that this new technique has enabled researchers to levitate objects much larger than in previous experiments:
Acoustic levitation of small particles at the acoustic pressure nodes of a standing wave is well-known, but the maximum particle size that can be levitated at the pressure nodes is around one quarter of the acoustic wavelength. In our paper, we demonstrate that we can combine multiple ultrasonic transducers to levitate an object significantly larger than the acoustic wavelength.
Since the objects are levitating on the standing waves between transducers and a spherical object, why couldn’t transducers couldn’t be mounted on the bottom of say, a hoverboard and create a standing wave between the board and another spherical object - the Earth? A hoverboard sure would look nice under the Christmas tree. Keep it up, scientists.