An international team of scientists headed by Dr G. Ravindra Kumar of the Tata Institute of Fundamental Research in Mumbai, India, has provided experimental evidence that stars may produce sound.
This image shows the famous Trifid Nebula to the right of center; it appears as faint and ghostly at these infrared wavelengths when compared to the familiar view at visible wavelengths. Image credit: ESO / VVV consortium / D. Minniti.
When examining the interaction of an ultra-intense laser with a plasma target, Dr Kumar and his colleagues from the Rutherford Appleton Laboratory and the University of York, UK, observed something unexpected.
They realized that in the trillionth of a second after the laser strikes, plasma 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.
The results appear in a paper published online March 17 in the journal Physical Review Letters.
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 – 6 million times higher than that which can be heard by any mammal!
“One of the few locations in nature where we believe this effect would occur is at the surface of stars,” said co-author Dr John Pasley of the University of York and the Rutherford Appleton Laboratory’s Central Laser Facility.
“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 physicists 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.
“It was initially hard to determine the origin of the acoustic signals, but our model produced results that compared favorably with the wavelength shifts observed in the experiment,” said co-author Dr Alex Robinson of the Rutherford Appleton Laboratory’s Central Laser Facility in Oxfordshire, UK.
“This showed that we had discovered a new way of generating sound from fluid flows. Similar situations could occur in plasma flowing around stars.”
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Amitava Adak et al. Terahertz Acoustics in Hot Dense Laser Plasmas. Phys. Rev. Lett. 114, 115001, published online March 17, 2015; doi: 10.1103/PhysRevLett.114.115001
