Sound Traveling Faster Than C?

I am trying to follow this, but having trouble. In normal materials V=density and elasticity of the material. How can a exiting pulse occur before an input pulse. Give me a break , i am an antique Texan.

"How can an exiting pulse occur before an input pulse"

Read more carefully - it's not about the pulse but about the peak of the pulse. If you have a pulse with a shape like this:

You can alter the shape (by using an appropriate filter) to look like this:

As you can see, the peak of the output puls shape occurs before the peak of the input puls shape - no big deal in doing that. Depending on the time difference, they can still have a "negative time" if the propagation delay (in their experiment) is shorter than the time difference - no big deal in doing that too.

So, I have a question: what's the big deal after all?

I just glanced through the article. They make the point that the sound waves themselves do not move faster than c but that a mathematical extrapolation they call the group velocity.

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."

Read more at: http://phys.org/news88249076.html#jCp

While Wikipedia should not be considered the final word on any topic, it is easy to site and usually very informative for laymen. Here is the Wikipedia article on group velocity calculations.As my fuzzy memory of solid state physics tells me, this is the math that explains why the drift velocity of the electrons and holes of an electric current can be surprisingly many orders of magnitude less than the speed of light. Yet the speed that an electric current signal propagates down a wire is close to the speed of light. The reason this signal doesn't exceed the speed of light (here's my memory fuzz) is that as complicated as these calculations are, they do not incorporate time and space dilation effects that happen at these higher velocities.

Excuse me, come again. I did not get that.;)

Nor is information transferring faster than C.

Again, reporting gone amiss.

For one thing that article is over 8 years old, there's been very little since except for those who repeat the article. A quick look at their setup probably helps to explain that this is a vestige of the instrumentation. My guess is that the PVC pipe transmits the sound from the source "faster" than the air in it so that when a pulse emanates from the speaker it travels through both media and the mike doesn't discriminate whether the source is the pipe or the air.

If I were "Myth Busters" I would use a variety of materials for the pipe, especially something with a high damping coefficient like lead, and see how well the experiment repeats. As a further check I would also instrument the outer surface of the pipe to detect any acoustic surface waves traveling on it.

Personally I thought it was an April Fool's PR piece.

It's not a new concept, nor was it new back in 2006 when this article was first published. I remember from years ago my quantum mechanics prof discussing the concepts of group velocity and wave velocity, and how the group velocity can exceed 'c' - though he was discussing light waves.

It's important to realize that the 'trick' they've done with sound waves is a matter of superposition of the waves, and that no actual information can be transmitted using this process. It remains true that 'information' cannot be transmitted faster than the speed of light in a vacuum. (I believe that's true even using quantum entanglement.)

Must be gossip.

Really ? the vector of the tip of my index finger travels 7.5 billion light years in less 2 seconds.

I've never dared to do the full turn, I don't know what might happen.

Here is an animation showing the relationship between group and phase velocities and showing how the group velocity can be faster than the propagation velocity.

https://www.youtube.com/watch?v=tlM9vq-bepA

Makes as much sense as:

1. Promiscuous Quail: The National Institute of Health gave the University of Kentucky $176,000 to determine if Japanese quail are more likely to have sex when high on cocaine. The study is scheduled to last through 2015.

Here is a (hopefully) simple way to picture the effect of group and phase velocity in a dispersive medium..

1. First, you can think of a wave as moving in the x-t plane (distance versus time). Its velocity will be the ratio of the wavelength in the x direction divided by the wavelength in the time direction (or it's period). Its direction in the x-t plane corresponds to its velocity.

2. If you add two waves together, the sum is maximum where the peaks correspond and minimum where the troughs correspond. If the frequencies are different, the result will be a complex waveform that will be the product of the difference frequency and the sum frequency (beat pattern). The difference frequency will be the envelope as shown below. This is also called the group velocity.

3. Imagine two waves of slightly different frequencies propagating in the x-t plane, adding together and generating beat pattern on the x axis and t axis. If their velocities are the same, the ratio of the wavelength difference on the x axis to the period difference on the time axis (the group velocity) will be the same as the ratio for each individual wave. It will be the velocity of propagation, V.

4. If the medium is dispersive, the velocity varies as a function of frequency. If the velocity increases with wavelength, the wavelength difference on the x axis divided by the period difference on the time axis (the group velocity) will be greater than V.