Why can't one build a detector sensitive to the motion of a group wave so
that it would be possible to send a signal faster than c?


On Sat, Sep 2, 2017 at 5:45 AM, Kevin O'Malley <kevmol...@gmail.com> wrote:

> Sound Pulses Exceed Speed of Light
> Live Science ^ | January 12, 2017 | Charles Q. Choi
>    https://www.livescience.com/1212-sound-pulses-exceed-speed-light.html
> 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
> than c.
> PVC science
> 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
> 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.
> "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.
> Common thing?
> 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
> effect."
> 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.

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