# Sound goes faster than light!

According to Physics Web in 'Sound breaks the light barrier,' a professor of physics in Tennessee has designed an experiment which proves that sound can move faster than light. This looks like impossible -- and it is. In fact, the physicist has tweaked some scientific definitions. No sound can go faster than light. But a sound pulse, or more precisely, all the wavelengths associated to a sound, have a "group velocity" that far exceeds the real physical limits. Have I lost you? Read more...

According to Physics Web in Sound breaks the light barrier (Free reg. required), a professor of physics in Tennessee has designed an experiment which proves that sound can move faster than light. This looks like impossible -- and it is. In fact, the physicist has tweaked some scientific definitions. No sound can go faster than light. But a sound pulse, or more precisely, all the wavelengths associated to a sound, have a "group velocity" that far exceeds the real physical limits. Have I lost you? Read more for some explanations that even a lawyer couldn't have invented.

Here are a couple of paragraphs from the Physics Web article.

In a normal dispersive medium, the velocity of a wave is proportional to its wavelength, resulting in a group velocity that is slower than the average velocity of its constituent waves. But in an "anomalously" dispersive medium -- one that becomes highly absorbing or attenuating at certain frequencies -- velocity is inversely proportional to wavelength, meaning that the group velocity can become much faster.
Indeed, the group velocity of light has already been shown to travel faster than the speed of light in a vacuum. But until now, superluminal acoustic waves have existed only in theory, and would require the group velocity to increase almost a million times over.

But what exactly is a superluminal phenomenon? Here is a short answer from Eric Weisstein's World of Physics.

A superluminal phenomenon is a frame of reference traveling with a speed greater than the speed of light c. There is a putative class of particles dubbed tachyons which are able to travel faster than light. Faster-than-light phenomena violate the usual understanding of the "flow" of time, a state of affairs which is known as the causality problem (and also called the "Shalimar Treaty").

Anyway, this was the purpose of the experiment designed by William Robertson from Middle Tennessee State University with the help of some colleagues and students. And their research work was recently published by Applied Physics Letters under the name "Sound beyond the speed of light: measurement of negative group velocity in an acoustic loop filter" (Volume 90, Issue 1, Article 014102, January 1, 2007). Here is a link to the abstract.

The results confirm recent theoretical predictions that faster-than-light group velocity propagation of sound is possible. Further, the results show that the spectral rephasing achieved in a loop filter is sufficient to produce negative group velocities independent of the phase velocity of the spectral components themselves. Thus, superluminal propagation is realized despite almost six orders of magnitude difference between the speeds of sound and light.

Here is another link to the full paper (PDF format, 3 pages, 214 KB), which shows the test system.

In Sound pulses exceed speed of light, Charles Q. Choi, from LiveScience, gave additional details. And it's quite funny to discover that this experiment used only a plastic plumbing pipe and a computer's sound card. "This experiment is truly basement science," Robertson told LiveScience.

Robertson and his colleagues transmitted sound pulses from the sound card through a loop made from PVC plumbing pipe and connectors from a hardware store. This loop split up and then recombined the tiny waves making up each pulse.
This led to a curious result. When looking at a pulse that entered and then exited the pipe, before the peak of the entering pulse even got into the pipe, the peak of the exiting pulse had already left the pipe. If the velocities of each of the waves making up a sound pulse in this setup are taken together, the "group velocity" of the pulse exceeded c.

For even more information, you can read 'Mach c'? Scientists observe sound traveling faster than the speed of light, an unusual attempt by PhysOrg.com to provide original contents. Here is the conclusion of this article.

Is this phenomenon simply the result of a clever set-up, or can it actually occur in the real world? According to the scientists, the interference that occurs in the loop filter is directly analogous to the "comb filtering" effect in architectural acoustics, where sound interference occurs between sound directly from a source and that reflected by a hard surface. "The superluminal acoustic effect we have described is likely a ubiquitous but imperceptible phenomenon in the everyday world," the scientists conclude.

So does sound goes faster than light? No, except if you're a physicist...

Sources: Jon Cartwright, Physics Web, January 12, 2007; and various other websites

You'll find related stories by following the links below.