Could sound move at the speed of light?
1 November 2005
Can sound waves travel faster than the speed of light? Yes, says Joel Mobley, a physicist at the University of Mississippi in the US. In simulations Mobley has shown that ultrasound pulses could move at "superluminal" speeds when they enter water that contains thousands of tiny plastic beads.
Waves moving in a dispersive medium are described by a phase velocity and a group velocity. The phase velocity is the speed at which a wave of a single wavelength moves, and is typically about 1.5 kilometres per second for sound waves in water. However, pulses of light or sound actually contain a range of wavelengths that all move at different speeds: the group velocity is the speed at which the pulse itself moves.
In recent years, it has been shown experimentally that the group velocity of a laser pulse can exceed the speed of light in vacuum -- 300,000,000 metres per second -- in certain situations. However, special relativity is not violated in these experiments because they do not involve the transfer of information, matter or energy.
Measuring group velocity (pic: physicsweb.org/objects/ne...051101.jpg)
Mobley has now calculated that the group velocity of a pulse of high-frequency sound waves could be increased by five orders of magnitude by sending it through a small chamber that contains about 8 millilitres of water and some 400,000 tiny plastic spheres. This means that the group velocity would exceed the speed of light in vacuum. The spheres have diameters of about 0.1 mm and account for about 5% of the volume of the water-bead mixture.
The increase in speed is caused by dispersion -- the phenomenon that causes different wavelengths to move at different phase velocities. When the pulse enters the mixture it experiences severe dispersion, which causes the different wavelengths that make up the pulse to travel at very different speeds. This changes the shape of the pulse and can result in the pulse itself moving faster than the speed of light. However, the dispersion also significantly reduces the intensity of the pulses.
"It has long been recognised that such velocities should be possible with acoustic waves," Mobley told PhysicsWeb. "My work shows that it can be done in a specific and very simple system and that extreme conditions are not necessary."
Mobley is now planning experiments to observe the superluminal velocities at the National Center for Physical Acoustics at Mississippi. The main challenge will be to increase the signal-to-noise ratio so that it is possible to detect the pulses, which will have been greatly reduced in intensity by the dispersion.
physicsweb.org/articles/news/9/11/1/1
1 November 2005
Can sound waves travel faster than the speed of light? Yes, says Joel Mobley, a physicist at the University of Mississippi in the US. In simulations Mobley has shown that ultrasound pulses could move at "superluminal" speeds when they enter water that contains thousands of tiny plastic beads.
Waves moving in a dispersive medium are described by a phase velocity and a group velocity. The phase velocity is the speed at which a wave of a single wavelength moves, and is typically about 1.5 kilometres per second for sound waves in water. However, pulses of light or sound actually contain a range of wavelengths that all move at different speeds: the group velocity is the speed at which the pulse itself moves.
In recent years, it has been shown experimentally that the group velocity of a laser pulse can exceed the speed of light in vacuum -- 300,000,000 metres per second -- in certain situations. However, special relativity is not violated in these experiments because they do not involve the transfer of information, matter or energy.
Measuring group velocity (pic: physicsweb.org/objects/ne...051101.jpg)
Mobley has now calculated that the group velocity of a pulse of high-frequency sound waves could be increased by five orders of magnitude by sending it through a small chamber that contains about 8 millilitres of water and some 400,000 tiny plastic spheres. This means that the group velocity would exceed the speed of light in vacuum. The spheres have diameters of about 0.1 mm and account for about 5% of the volume of the water-bead mixture.
The increase in speed is caused by dispersion -- the phenomenon that causes different wavelengths to move at different phase velocities. When the pulse enters the mixture it experiences severe dispersion, which causes the different wavelengths that make up the pulse to travel at very different speeds. This changes the shape of the pulse and can result in the pulse itself moving faster than the speed of light. However, the dispersion also significantly reduces the intensity of the pulses.
"It has long been recognised that such velocities should be possible with acoustic waves," Mobley told PhysicsWeb. "My work shows that it can be done in a specific and very simple system and that extreme conditions are not necessary."
Mobley is now planning experiments to observe the superluminal velocities at the National Center for Physical Acoustics at Mississippi. The main challenge will be to increase the signal-to-noise ratio so that it is possible to detect the pulses, which will have been greatly reduced in intensity by the dispersion.
physicsweb.org/articles/news/9/11/1/1
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Re: Sound Past Speed of Light?
Mon, November 7, 2005 - 6:27 PMAh, yeah... no *thing* moves faster than the speed of light.
It sounds like this is a matter of definition:
from www.mathpages.com/home/kmat...ath210.htm
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The velocity of a wave can be defined in many different ways, partly because there many different kinds of waves, and partly because we can focus on different aspects or components of any given wave. The ambiguity in the definition of "wave velocity" often leads to confusion, and we frequently read stories about experiments purporting to demonstrate "superluminal" propagation of electromagnetic waves (for example). Invariably, after looking into the details of these experiments, we find the claims of "superluminal communication" are simply due to a failure to recognize the differences between phase, group, and signal velocities.
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The thing is that sound is a compression-wave made up of moving particles. The particles themselves _cannot_ move faster than the speed of light nor can the fields belonging to the particles affect any neighboring particles at faster than the speed of light. Thus the wavefront cannot move faster than the speed of light.
The *group-velocity* or *phase-velocity* of the wave can though, in fact the page I quoted above says that the phase-velocity is quite often greater than c.
But since this group velocity has nothing to do with information transmission, it sounds like this experiment doesn't really buy us anything. It just proves what is obvious according to the definitions of phase-velocity and group-velocity.
wah-wah.
Would have been cool, but honestly the day we discover some method of transmitting information faster than light in a vaccuum is the day Einstein's General Relativity is discovered to be a special case of something more general, which is unlikely to be true based on our current understanding of physics.
doh! -
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Re: Sound Past Speed of Light?
Sat, November 12, 2005 - 3:01 PMever hear thunder before the lightening flash? I have....it's an example of sensory preception...light must travel through a lens, be processed for scope, filtered for lumen/uv, then pass through the retna, to diliate in accordinance with lens variable....then it can finally be processed...to the optic nerve....which, incedentally is wired to the cerebral cortex...that's a massive chain of synapse....there really is a delay factor to light inception..
then, most sound waveforms, (sinewaves for ex)...travel to the inner ear, not only unfiltered, but amplified...
I think this is either synthesia, or reason to have your vision corrected.
then again, the variables of light vs sound are too numerous to define perception....just remember, about waveform shape ...if amlitude exceeds polarity ratio...and there isn't any carrier offset. as long as there is presice ocilisation, and a balanced harmonic, as well as a saw, or square formation, a frequency willl expand beyond the span of reletave lumen output....so choose your reality...it's just a perception offset, or individual development.
after all, the periodic table of base elements are being changed due to error....in 100 yrs, science will disprove everything the past 100 defined...so ....why not let all possibilities exist?
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