Sound Pulses Exceed Speed of Light
Live Science ^ | January 12, 2017 | Charles Q. Choi
A group of high school and college teachers and students has
transmitted sound pulses faster than light travels—at least according
to one understanding of the speed of light.
The results conform to Einstein's theory of relativity, so don't
expect this research to lead to sound-propelled spaceships that fly
faster than light. Still, the work could help spur research that
boosts the speed of electrical and other signals higher than before.
The standard metric for the speed of light is that of light traveling
in vacuum. This constant, known as c, is roughly 186,000 miles per
second, or roughly one million times the speed of sound in air.
According to Einstein's work, matter and signals cannot travel faster
However, physicist William Robertson at Middle Tennessee State
University in Murfreesboro, along with a high school teacher, two
college students and two high school students, managed to, depending
on how you look at it, transmit sound pulses faster than c using
little more than a plastic plumbing pipe and a computer's sound card.
"This experiment is truly basement science," Robertson told LiveScience.
The key to understanding their results, reported online Jan. 2 in the
journal Applied Physics Letters, is envisioning every pulse of sound
or light as a group of intermingled waves. This pulse rises and falls
with energy over space, with a peak of strength in the middle.
Messing with Light Speed
In an unrelated previous experiment, Robert Boyd at the University of
Rochester used similar principles to make pulses of light travel
backward and faster than c.
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
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
"I believe that this is the first experimental demonstration of sound
going faster than light," Robertson said. Past research has proven it
possible to transmit electrical and even light pulses with group
velocities exceeding c.
Robertson explained this faster-than-light acoustic effect is likely
commonplace but imperceptible.
"The loop filter that we used splits and then recombines sound along
two unequal length paths," he said. "Such 'split-path' interference
occurs frequently in the everyday world."
For example: "When a sound source is located near a hard wall, some
sound reaches the listener directly from the source whereas some sound
travels the longer path that bounces the sound off the wall. The
sounds recombine at the listener," Robertson said. However, the
weakness of the signals and the fact that any resultant differences in
timing are very slight "mean that we would never be able to hear this
None of the individual waves making up the sound pulses traveled
faster than c. In other words, Einstein's theory of relativity was
preserved. This means one could not, for instance, shout a message
faster than light.
Still, this research might have engineering applications. Robertson
explained that although it is not possible to send information faster
than light, it seems these techniques could make it possible to route
slower-than-light signals in electronic circuits faster than before.