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FW: wha?







'Mach c'? Scientists observe sound traveling faster than the speed of light




In this schematic of the acoustical test system, the scientists could 
create 
superluminal group velocity of sound waves, as well as negative group 
velocity. In the latter case, the peak of the output pulse traveling 
through 
the loop filter exited the filter before the peak of the input pulse had 
reached the beginning of the filter. Image credit: Bill Robertson, et al.
For the first time, scientists have experimentally demonstrated that sound 
pulses can travel at velocities faster than the speed of light, c. William 
Robertson’s team from Middle Tennessee State University also showed that 
the 
group velocity of sound waves can become infinite, and even negative.

Past experiments have demonstrated that the group velocities of other 
materials’ components—such as optical, microwave, and electrical 
pulses—can 
exceed the speed of light. But while the individual spectral components of 
these pulses have velocities very close to c, the components of sound 
waves 
are almost six orders of magnitude slower than light (compare 340 m/s to 
300,000,000 m/s).

“All of the interest in fast (and slow) wave velocity for all types of 
waves 
(optical, electrical, and acoustic) was initially to gain a fundamental 
understanding of the characteristics of wave propagation,” Robertson told 
PhysOrg.com. “Phase manipulation can change the phase relationship between 
these materials’ components. Using sound to create a group velocity that 
exceeds the speed of light is significant here because it dramatically 
illustrates this point, due to the large difference between the speeds of 
sound and light.”

The experiment was conducted by two undergrads, an area high school 
teacher 
and two high school students, who received funding by an NSF STEP 
(Science, 
technology, engineering, math Talent Enhancement Program) grant. The grant 
aims to increase recruitment and retention of students to these subjects.

In their experiment, the researchers achieved superluminal sound velocity 
by 
rephasing the spectral components of the sound pulses, which later 
recombine 
to form an identical-looking part of the pulse much further along within 
the 
pulse. So it’s not the actual sound waves that exceed c, but the waves’ 
“group velocity,” or the “length of the sample divided by the time taken 
for 
the peak of a pulse to traverse the sample.”

“The sound-faster-than-light result will not be a surprise to the folks 
who 
work closely in this area because they recognize that the group velocity 
(the velocity that the peak of a pulse moves) is not merely connected to 
the 
velocity of all of the frequencies that superpose to create that pulse,” 
explained Robertson, “but rather to the manner in which a material or a 
filter changes the phase relationship between these components. By 
appropriate phase manipulation (rephasing) the group velocity can be 
increased or decreased.”

To rephase the spectral components, the sound waves were sent through an 
asymmetric loop filter on a waveguide of PVC pipe, about 8 m long. The 
0.65-meter loop split the sound waves into two unequal path lengths, 
resulting in destructive interference and standing wave resonances that 
together created transmission dips at regular frequencies.

Due to anomalous dispersion (which changes the wave speed), sound pulses 
traveling through the loop filter arrived at the exit sooner than pulses 
traveling straight through the PVC. With this experiment, the group 
velocity 
could actually reach an infinitely small amount of time, although the 
individual spectral components still travel at the speed of sound.

“We also achieved what is known as a ‘negative group velocity,’ a 
situation 
in which the peak of the output pulse exits the filter before the peak of 
the input pulse has reached the beginning of the filter,” explained 
Robertson. “Using the definition for speed as being equal to distance 
divided by time, we measured a negative time and thus realized a negative 
velocity.”

It might not seem that a negative velocity would exceed the speed of 
light, 
but in this case, Robertson said, the speed of the pulse is actually much 
faster than c.

“Consider the pulse speed in a slightly less dramatic case,” Robertson 
said. 
“Say the peak of the output pulse exits the filter at exactly the same 
time 
as the input pulse reaches the beginning. In this less dramatic case, the 
transit time is zero and the speed (distance divided by zero) is infinite. 
So we were beyond infinite! (‘To infinity and beyond,’ to steal a line 
from 
Toy Story.) In our experiment, we measured a negative transit time 
corresponding to a negative group velocity of -52 m/s.”

Although such results may at first appear to violate special relativity 
(Einstein’s law that no material object can exceed the speed of light), 
the 
actual significance of these experiments is a little different. These 
types 
of superluminal phenomena, Robertson et al. explain, violate neither 
causality nor special relativity, nor do they enable information to travel 
faster than c. In fact, theoretical work had predicted that the 
superluminal 
speed of the group velocity of sound waves should exist.

“The key to understanding this seeming paradox is that no wave energy 
exceeded the speed of light,” said Robertson. “Because we were passing the 
pulse through a filter, the sped-up pulse was much smaller (by more than a 
factor of 10) than the input pulse. Essentially, the pulse that made it 
through the filter was an exact (but smaller) replica of the input pulse. 
This replica is carved from the leading edge of the input pulse. At all 
times, the net energy of the wave crossing the filter region was equal to, 
or less than, the energy that would have arrived if the input pulse had 
been 
traveling in a straight pipe instead of through the filter.”

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.

Citation: Robertson, W., Pappafotis, J., Flannigan, P., Cathey, J., 
Cathey, 
B., and Klaus, C. “Sound beyond the speed of light: Measurement of 
negative 
group velocity in an acoustic loop filter.” Applied Physics Letters 90, 
014102 (2007).

By Lisa Zyga, Copyright 2006 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, 
rewritten or redistributed in whole or part without the express written 
permission of PhysOrg.com.




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