The team was studying hydrodynamics (the motion of fluids). While they were analyzing the interaction of an ultra-intense laser with a plasma target they noticed that in the trillionth of a second after the strike of the laser, plasma flowed quickly from areas of high density to regions of low density. This change caused something similar to a traffic jam. The method used to observe the sound wave in the laboratory resembles a police speed camera. Therefore it enables the investigators to measure exactly how the fluid is moving at the point which it was struck by the laser in less than a trillionth of a second.
What happened was that plasma gathered at the interface between the low and high density areas and it generated several pressure pulses: a sound wave. But the sound was at such a high frequency that it would have hurt even dolphins and bats. The frequency of the sound was almost a trillion hertz, which means six million times higher than the frequency which can be usually heard by any mammal. Co-author of the study, Dr. Alex Robinson of Rutherford Appleton Laboratory’s Central Laser Facility (Oxfordshire), explained that at first it was hard to distinguish the origin of the acoustic signals.
However Dr. Robinson developed a numeric model to produce acoustic waves for the experiment. The model produced results which proved compatible with the wavelength shifts noticed in the experiment. This means that the researchers have found a way of using fluid flows in order to generate sound. Moreover Robinson explained that the same phenomenon could occur in the plasma flowing around stars.
John Pasley of University of York said that one of the few places in nature where this reaction could occur is the surface of the stars. When stars accumulate new material, they can generate sound in a similar way in which the investigators observed in the laboratory. So stars may have their own “melody” but nobody can hear it since the sound cannot propagate though the space vacuum.
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