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Scientists made sound waves twist backwards – it’s “analogous to turning back time”

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Posted March 28, 2018

Sound is just waves in the air. It has a particular frequency, some of which can be sensed and interpreted by our auditory system. But can the sound be of a negative frequency? Scientists say such idea sounds almost as ‘real’ as turning back time, so surely it shouldn‘t be possible, right?

Not exactly. Scientists from the UK and the US have demonstrated that sound waves from a rotating source can produce negative frequencies.

Rotational Doppler effect is a difficult phenomenon to understand, but it allows creating negative frequency at subsonic speed. Image credit: Ulrich Rahm via Wikimedia

An international team of researchers from the Universities of Glasgow, Exeter and Illinois Wesleyan built a simple device, which is capable of reversing the angular momentum of a sound wave without the need for supersonic speeds. While it does sound like a complex matter, it actually builds upon the Doppler effect. When a car is coming, you hear the noise of its engine getting compressed ahead of it, producing a high-pitched sound. And when it is going away from you, the sound is stretched and thus sounds deeper. But let’s say that the car or an ambulance if getting away from you extremely fast, although not at the speed of sound. And you are chasing it as supersonic velocity. What would happen? A negative frequency would happen.

A chemist Bruce Garetz demonstrated the rotational Doppler effect back in 1981. The rotational Doppler effect is when the frequency shifts occur when electromagnetic waves (such as, light waves) move in a circle around a single fixed point. Scientists have explored how the orbital angular momentum of light waves affects the Doppler effect and now they decided to take it a step further and explore it with sound waves. They arranged 16 speakers on a disc and put two microphones on a rotating ring. Scientists did notice that they can reverse the orbital angular momentum of sound waves. Essentially, they could reverse the twist of the acoustic waves.

While this effect is unlikely to be used in any practical devices, scientists say that it is going to make scientific research a bit more interesting. In fact, this effect could be very useful in the research of the quantum field theory. Dr Graham Gibson, primary author of the paper, said: “we could generate those negative frequencies while our microphone ring span at very low, sub-sonic speeds, with a rotation rate of around 25Hz, something that is impossible in linear Doppler shifts”. This makes such equipment easier to make and maintain.

It is difficult to say how important this discovery is. Scientists compare reversing sound waves to going back in time, but we will have to wait and see if anything good will come out of it.

Source: University of Glasgow

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