It looks like you're using an Ad Blocker.
Please white-list or disable AboveTopSecret.com in your ad-blocking tool.
Thank you.
Some features of ATS will be disabled while you continue to use an ad-blocker.
At this point, it is worth remembering that sound waves -- like all waves -- are made of two main parts: (1) the underlying wave oscillations, in this case pressure oscillations in a medium such as air or water, which travel at the normal speeds of sound, and (2) the "envelope" that gives the wave its shape. In Mobley's setup, the envelope has the shape of a bell curve. The speed at which the envelope moves is called the group velocity. One measures the group velocity by following the envelope's peak (its maximum height, or amplitude).
In a mixture of water and beads, an ultrasound pulse experiences severe dispersion, meaning that different frequencies in the pulse travel at very different speeds. The components of the wave add up so that the peak of the wave can move faster than c. With even greater degrees of dispersion, the peak can actually start traveling backwards, so that a detector deeper in the mixture detects the peak earlier than a shallower detector. This would result in a negative group velocity. None of this violates the principles of causality, since the leading edge of the ultrasound wave still arrives at the shallower detector first and the deeper detector next. It's just the peak of the envelope, which determines the group velocity, which would move around in weird ways.
www.aip.org...
layperson paper
www.acoustics.org...
www.abovetopsecret.com...
While Mobley has not yet demonstrated this feat experimentally, his preliminary experiments on ultrasound in a water-sphere mixture have shown close agreement with theory and indicate that very large group velocities are possible. If experimentally confirmed, superluminal group velocity in sound waves could potentially be exploited for useful applications, such as making electronic filters and high-frequency ultrasound oscillators.
www.abovetopsecret.com...