The study of fluids in motion – now known as hydrodynamics – goes back to the Egyptians, so it is not often that new discoveries are made. However when examining the interaction of an ultra-intense laser with a plasma target, the team observed something unexpected.
Dr. Parsley, who worked with scientists behind the study at the Tata Institute of Fundamental Research in Mumbai and the Technology Facilities Council’s Central Laser Facility in Oxfordshire, was quoted on EurekAlert as having said in a statement that “the stars might be singing – but, since sound cannot propagate through the vacuum of space, no-one can hear them.”
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“One of the few locations in nature where we believe this effect would occur is at the surface of stars. When they are accumulating new material stars could generate sound in a very similar manner to that which we observed in the laboratory – so the stars might be singing – but, since sound cannot propagate through the vacuum of space, no-one can hear them.”
The technique used to observe the sound waves in the lab works very much like a police speed camera. It allows the scientists to accurately measure how fluid is moving at the point that is struck by the laser on timescales of less than a trillionth of a second.
Dr John Pasley of the York Plasma Institute, realized that in the trillionth of a second after the laser strikes, plasma- which is visible in a few things on Earth such as the gas inside of neon signs and lightning strikes- When the researches shot high-intensity lasers at the plasma, they observed the creation of sound as a result of the impacts, as a type of plasma traffic jam was created the instant after each laser strike and the subsequent pressure would create sound pulses – flowed rapidly from areas of high density to more stagnant regions of low density, in such a way that it created something like a traffic jam. Plasma piled up at the interface between the high and low density regions, generating a series of pressure pulses: a sound wave.
However, the sound generated was at such a high frequency that it would have left even bats and dolphins struggling! With a frequency of nearly a trillion hertz, the sound generated was not only unexpected, but was also at close to the highest frequency possible in such a material – six million times higher than that which can be heard by any mammal!
Dr Alex Robinson from Central Laser Facility developed a numerical model to generate acoustic waves for the experiment. He said, “It was initially hard to determine the origin of the acoustic signals, but our model produced results that compared favourably with the wavelength shifts observed in the experiment. This showed that we had discovered a new way of generating sound from fluid flows. Similar situations could occur in plasma flowing around stars.”
