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Could sound move at the speed of light? 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.
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