A chance discovery by a team of researchers has provided experimental evidence that stars may generate sound.

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 between an ultra-intense laser and a plasma target, the team, which included scientists from the CLF, the York Plasma Institute at the University of York, and the Tata Institute of Fundamental Research in Mumbai, India, noticed something unusual.

They realised 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.

Dr Alex Robinson from the Plasma Physics Group at STFC’s 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”

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 John Pasley from the York Plasma Institute said: “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 very 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.

The research was funded by the Engineering and Physical Sciences Research Council and the Tata Institute of Fundamental Research. It is published in Physical Review Letters.

Reference

Terahertz Acoustics in Hot Dense Laser Plasmas, Amitava Adak, A. P. L. Robinson, Prashant Kumar Singh, Gourab Chatterjee, Amit D. Lad, John Pasley, and G. Ravindra KumarPhys. Rev. Lett.114, 115001 (2015). DOI: http://dx.doi.org/10.1103/PhysRevLett.114.115001 (link opens in a new window)