Sound breaks through the speed of light
Using a surprisingly simple experiment, US researchers showed for the first time that sound pulses can travel at speeds beyond the speed of light in a vacuum.
Using a special filter and simple PVC tubing, William Robertson and his colleagues at the University of Middle Tennessee constructed a loop in which the sound signal is split and then travels along two paths of different lengths. The two signals are then combined again. Their superimposition produces a copy of the original sound impulse, which is greatly weakened by absorption effects and which precedes the original. Sound gains speed and even overtook light in the experiment, although the speeds of sound and light waves in air differ by almost six powers of ten.
The phenomenon has already been predicted theoretically and can also be observed in light. However, this does not call into question Einstein's special theory of relativity, which postulates the speed of light as the maximum speed of propagation of matter, energy and information. It is important to distinguish between signal velocity, at which information and energy propagate, and group velocity, the velocity of individual wave pulses. Normally the signal speed corresponds to the group speed. Only with strong absorption can the group speed exceed the signal speed and also the speed of light. In this case, however, the attenuation of the signal ensures that energy and information never propagate faster than light.(vs)
