Can sound move at 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.

physicsweb.org...

This is a very interesting development one that I had not even thought possible. There are many strange and wonderfull things happening in the world of science today. Here is how he accomplished this feat.

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.

[edit on 8-11-2005 by sardion2000]

"YES" sound can move at the speed of light... Once it is converted into bits and bytes and transmitted via fiber optics or laser.

Originally posted by kozmo
"YES" sound can move at the speed of light... Once it is converted into bits and bytes and transmitted via fiber optics or laser.

Did you read the article?

And the research is fascinating, although I admit that I understood less than half of it. My response was a lame attempt at humor. I guess I just don't understand what the implications of such a discovery might be.

What is funny is that I was just reading an article (From Rense so it certainly is questionable) about Einstein's theory of relativity www.rense.com... and how it may not be an absolute in the field of physics. But again, I understand about half of this stuff.

Come on, Koz, you can do better than that. For example,
Whatever speed sound is doing, it is doing the speed of sound!

Get it!!

Ok, I'll shut up, now.

couple questions

1)From the article: "This means that the group velocity would exceed the speed of light in vacuum." i assume that they mean it's actually traveling faster than c and not some sort of mathematic 'trickery', ie some sort of cummulative effect. If i assume correctly isn't this a big deal...i mean a very BIG deal?

2) Also the article states that relativity was not broken because there was no 'transfer' of matter, energy or information. The "pulse" speed is an effect of disspersion which greatly weakens it. Wouldn't this make this "pulse" useless...outside of the theoretical stuff with no practicle use?

Great post, if thats the case light could be accelerated faster than light speed as well I guess....

from the article:

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.

This is true. I have a professor who has actually transmitted pulses at faster than light speed in his research. The same professor also told me that there is a mathematical proof that even though you can produce a pulse with a group velocity faster than "c" you cannot transmit information (energy) at faster than "c".

What has actually been sped up, is the group velocity of a packet of sound waves of different frequencies. The individual sound waves still travel at the speed of sound, yet the group velocity - a mathematical expression - can significantly differ from this. Information cannot be sent faster than light and this fundamental principle is not violated by having group velocities faster than light.

From MathPages.com:
Unfortunately we frequently read in the newspapers about how someone has succeeded in transmitting a wave with a group velocity exceeding c, and we are asked to regard this as an astounding discovery, overturning the principles of relativity, etc. The problem with these stories is that the group velocity corresponds to the actual signal velocity only under conditions of normal dispersion, or, more generally, under conditions when the group velocity is less than the phase velocity. In other circumstances, the group velocity does not necessarily represent the actual propagation speed of any information or energy. For example, in a regime of anomalous dispersion, which means the refractive index decreases with increasing wave number, the preceding formula shows that what we called the group velocity exceeds what we called the phase velocity. In such circumstances the group velocity no longer represents the speed at which information or energy propagates.

To see why the group velocity need not correspond to the speed of information in a wave, notice that in general, by superimposing simple waves with different frequencies and wavelengths, we can easily produce a waveform with a group velocity that is arbitrarily great, even though the propagation speeds of the constituent waves are all low. A snapshot of such a case is shown below. In this figure the sinusoidal wave denoted as "A" has a wave number of kA = 2 rad/meter and an angular frequency of wA = 2 rad/sec, so it's individual phase velocity is vA = 1 meter/sec. The sinusoidal wave denoted as "B" has a wave number of kB = 2.2 rad/meter and an angular frequency of wB = 8 rad/sec, so it's individual phase velocity is vB = 3.63 meters/sec.



The sum of these two signals is denoted as "A+B" and, according to the formulas given above, it follows that this sum can be expressed in the form 2cos(kx-wt)cos(Dkx-Dwt) where k = 5, w = 2.1, Dk = 0.1, and Dw = 3. Consequently, the "envelope wave" represented by the second factor has a phase velocity of 30 meters/sec. Nevertheless, it's clear that no information can be propagating faster than the phase speeds of the constituent waves A and B. Indeed if we follow the midpoint of a "group" of A+B as it proceeds from left to right, we find that when it reaches the right hand side it consists of the sum of peaks of A and B that entered at the left long before the current "group" had even "appeared". This is just one illustration of how simple interfering phase effects can be mis-construed as ultra-high-speed signals. In fact, by simply setting kA to 2.2 and kB to 2.0, we can cause the "groups" of A+B to precess from right to left, which might mistakenly be construed as a signal propagating backwards in time!

Needless to say, we have not succeeded in arranging for a message to be received before it was sent, nor even in transmitting a message superluminally. Examples of this kind merely illustrate that the "group velocity" does not always represent the speed at which real information (or energy) is moving.

See also this thread.