Researchers have captured the sound of a "perfect fluid"

Scientists at MIT have announced that they have recorded the sound of a "perfect fluid." The sound is important because what a so-called "perfect flow" sounds like could help researchers study the viscosity in neutron stars and other strongly interacting fluids. In physics, a perfect flow refers to a fluid that flows with the smallest amount of friction, or viscosity, allowed by the laws of quantum mechanics.Scientists admit that perfectly fluid behavior is rare in nature, but they believe it occurs in the interiors of neutron stars and inside the plasma of the early universe. MIT scientists were able to record the sound noting that it is the product of a glissando of soundwaves the team sent through a carefully controlled gas of elementary particles known as fermions.

The pitches heard in the recording are particular frequencies at which the gas resonates like a plucked string. Thousands of soundwaves traveling through the gas were used to measure the "sound diffusion," which is how quickly sound dissipates in the gas. How quickly sound dissipates is directly related to the material's viscosity or internal friction. The team discovered the fluid's sound diffusion was so low as to be described by a "quantum" amount of friction, given by a constant of nature known as Planck's constant, and the mass of individual fermions in the fluid.

The value confirmed strongly interacting fermion gas behaves as a perfect fluid and is universal in nature. Scientists say they can now use the fluid as a model of other, more complicated perfect flows to estimate the viscosity of plasma in the early universe. The model could also be used to estimate quantum friction within neutron stars. Both are properties that would otherwise be impossible to calculate.

MIT physics Prof. Thomas A. Frank says that while it's difficult to listen to a neutron star, it could be mimicked in the lab using atoms to let researchers know how a neutron star would sound. The gas used by the researchers and that actually found inside a neutron star is different in terms of size and speed at which sound travels. However, rough calculations have shown that the star's resonant frequencies would be similar to those of the gas used in the lab and would be audible.