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Could Sound Move At The Speed Of Light?

ramonmercado

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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
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.

About the author
Belle Dumé is science writer at PhysicsWeb

http://physicsweb.org/articles/news/9/11/1/1
 
Interesting - am currently a bit worse for the booze, but my knowledge of physics (right now) doesn't include the effects of dispersion on sound waves. I'm not sure why dispersion would increase the rate of something travelling as slowly as sound to the projected velocities. I mean the article sounds good and all, but I'd like to some maths and some references (and have a really good science dictionary handy too :? )
 
You pipped me to the post Gadaffi, though I was about to ask for an English translation myself. AFAIK all wavelengths travel at the same speed.
 
No, I think there are different velocities - the speed of light (as we normally think of it) is an average across the spectrum - sorry this is from memory. However, I take your point. Lets see what comes of this 8)
 
I should have said, AFAIK all wavelengths of sound travel at the same frequency.

If there's anything in this, it's way over my head.
 
This report contains all sorts of errors, mis-spellings and what appears to be totally shonky science....but interesting nonetheless.

I note;
"...a type of explosion that sends waves faster than the speed of light."
"...uses the dentation"
(wow - it's got teeth)
"...stopping the powerful shock" (I thought the shockwave was caused by the build up of the air pressure in front of the vehicles leading edges, nothing to do with the engines?)

https://www.dailymail.co.uk/science...pulsion-enable-jets-reach-speeds-Mach-17.html
 
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