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FW: wha?
- To: undisclosed-recipients: ;
- From: "samba -" <sambacomet@hotmail.com>
- Subject: FW: wha?
- Date: Fri, 19 Jan 2007 22:28:27 -0800
'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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