For the first time, a quantum gas has been found to transmit sound waves at two quite different speeds simultaneously. Any event that makes a sound produces two waves that diverge so that an observer, if they could survive the epic cold, would hear everything on repeat.
Liquid helium is classed as a superfluid, meaning it can flow infinitely with no loss of energy or viscosity. Just as superconductivity of electricity leads to extraordinary behaviors such as magnetic levitation, superfluids display numerous behaviors so unlike familiar liquids they seem magical. One of these that doesn't make it to party tricks videos, but is familiar to quantum physicists as an identifying feature, is the duplication of sound waves traveling at different speeds.
In Physical Review Letters, a team led by Dr Timon Hilker of the University of Cambridge describe making a quantum or Bose gas out of potassium atoms cooled to a millionth of a degree above zero and trapped in laser beams. Sound waves produced in this gas doubled up, with different resonances and standing waves.
However, when the gas is warmed above its critical temperature only one of these resonances survives.
Key to the behavior is that the gas partly forms a Bose-Einstein Condensate (BEC) – a state of matter where substantial numbers of atoms collectively show the strange mixture of particle and wave-like properties seen in photons and electrons. BEC's are normally thought more similar to liquids than solids or gases, but in this case, the substance was as compressible as air and therefore definitively a gas.
The behavior observed is best described by a two-fluid model with thermal and condensed atoms co-existing. The intense cold makes references to thermal atoms a little ironic, and they are certainly very slow moving, but continue to behave like a gas. The first sound involves a pressure wave of density oscillations in these thermal atoms, similar to the sound we experience in air. The second wave is transmitted through the condensate. This matches the model used since the 1940s to describe superfluid helium.
At temperatures barely above absolute zero, the speed of the two sound waves is similar, the authors found. However, as temperatures become a large fraction of the critical temperature, the first sound speeds up, while second sound slows down, opening up a large gap between them.
Having conducted experiments at a millionth of a degree above absolute zero might be enough for most people, but the paper expresses a desire to “explore lower temperatures” where they think even stranger things may happen.
It is of course impossible for humans to survive, let alone interact, under conditions this cold. However, teachers might dream of conducting a class in a Bose gas, where every message bears repeating and you don't need to tell people twice, because they'll get to hear it that way anyway.