Music in Terms of Science

James Q. Feng
To many people, music is a mystery. It is uniquely human, because no other species produces elaborate, well organized sound for no particular reason. It has been part of every known civilization on earth. It has become a very part of man’s need to impose his will upon the universe, to bring order out of chaos and to endow his moments of highest awareness with enduring form and substance. It is a form of art dealing with the organization of tones into patterns.
Despite of cultural differences, music from different civilizations seems to consist of some building blocks that are universal: melody, harmony, rhythm, etc. Almost all musical systems are based on scales spanning an octave—the note that sounds the same as the one you started off with, but at a higher or lower pitch. It was discovered by Pythagoras, a Greek philosopher who lived around 500 BC, that the note an octave higher than another has a frequency twice high. The notes that sound harmonious together have simple rational number ratios between their frequencies.
It is those implicit structures and relationships in apparently mysterious musical experience that I am interested in exploring here. As a scientist by training with a consistent passion for classical guitar playing, I would like make an attempt to explain the musical experience in terms of science and mathematics, hoping to fill some gaps between the knowledge of scientists and artistic intuition of musicians.

Supersonically epic! Riding the Plasma Wave

A cloud forms as this F/A-18 Hornet aircraft speeds up to supersonic speed. Aircraft flying this fast push air up to the very limits of its speed, forming what’s called a bow shock in front of them. Similar bow shocks are also found in a variety of forms in space, and new research suggests they may contribute to heating of the material around them. Credit: Ensign John Gay, USS Constellation, U.S. Navy

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How To Steer Sound Using Light

The ability to create phonons and then steer them using laser beams could lead to a new generation of applications, say physicists

Zap an optical fibre with a couple of laser beams and the resulting interference pattern causes an interesting effect–it squeezes the material, an effect known as electrostriction. This creates a compression wave called a phonon, a packet of sound, which travels along the fibre.

Not to be outdone, phonons also influence light because they change the refractive index of the material. This bends light and alters its frequency, an effect known as Brillouin scattering.

After that, things get complicated. This mechanism sets in train a complex set of feedback effects in which photons generate phonons which influence the photons and so on.

The problem is understanding what’s going on. The ability to influence sound with light, and vice versa, could have interesting applications. But without an accurate model of this phenomenon, it’s hard to exploit.

That looks set to change. Until now, physicists have sought to understand the phenomenon by assuming the phonons have a particular form and working out how this influences the incident light. In other words, they ignore feedback effects.

Today, Jean-Charles Beugnot and Vincent Laude at Université de Franche-Comté in Besançon, France, take a more detailed at look the problem.

For the first time, these guys simulate how light generates phonons inside an optical fibre and how the phonons then interact with the light that generated them. They then test their ideas by measuring the way phonons scatter light in two types of fibre.

Their conclusion has interesting implications. They say that the light ends up guiding the phonons that it creates. In other words, it’s possible to create and then steer sound using light. “The phonon wavepacket generated via [electrostriction] is naturally guided by the light that gave it birth,” say Beugnot and Laude.

These guys aren’t very forthcoming with potential applications but it doesn’t take much imagination to speculate. Engineers already use light to reduce vibrations in mirrors, cooling them close to absolute zero.

Beyond this,the most exciting application is in information processing. In this field, there is  growing interest in controlling phonons because they are essentially noise generated by heat.

Controlling phonons would allow them to be steered away from sensitive areas. More ambitious still is the possibility of computing with phonons, with various groups around the world working on phonon diodes and transistors, the building blocks of logic gates.

Progress so far is tentative but that could soon change. Other suggestions in the comments section please.

Ref: Electrostriction And Guidance Of Sound By Light In Optical Fbers

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