A mug of hot chocolate left to sit for a while in a cold room isn't going to warm your belly.
But new research shows that if left in an even colder room, one virtually devoid of heat altogether, your cup of delicious cocoa will be hot again in no time.
Researchers from the Massachusetts Institute of Technology in the US and Swinburne University of Technology in Australia discovered a bizarre thermodynamic phenomenon of rebounding heat waves emerging in materials known as superfluids.
While it's not in any way a practical means of keeping your beverages piping hot, it does tell us a few things about the movements of energy through exotic materials, like the superfluid quantum gas studied in this experiment.
"There are strong connections between our puff of gas, which is a million times thinner than air, and the behavior of electrons in high-temperature superconductors, and even neutrons in ultradense neutron stars," says MIT physicist Martin Zwierlein.
"Now we can probe pristinely the temperature response of our system, which teaches us about things that are very difficult to understand or even reach."
Zwierlein's "puff of gas" consists of an isotope of lithium trapped in a room with walls made of light and cooled to within a whisker of absolutely zero. Under these conditions, nudges and pokes of more subtle atomic forces and quantum behaviors that are typically overwhelmed by the jostle of heat energy come to the fore and create what's known as a Fermi gas.
Atoms in these conditions are capable of some strange activities. They can, for instance, behave as a superfluid – materials that flow with zero viscosity.
One of the physicists behind the theory of superfluidity, Lev Landau, deduced a liquid of helium II cooled into this state would retain a viscous component, and therefore consist of two fluids, one 'super' and one 'normal'.
It's been shown that these two fluids can carry their own waves of energy, producing what is known as a second sound. Just a few years ago physicists from the Universities of Cambridge and Oxford in the UK demonstrated waves of heat and sound can move in distinct fashions through a quantum fog of Bose Einstein condensate, as if neither is aware of the other's journey.
Though the wave-like behavior of heat had been clearly observed in this second sound, it had yet to be studied in any real detail.
"Second sound is the hallmark of superfluidity, but in ultracold gases so far you could only see it in this faint reflection of the density ripples that go along with it," says Zwierlein.
"The character of the heat wave could not be proven before."
To determine the nature of this flow of heat energy, the team devised a method of heat-mapping that captured the frequencies of radiation particular to thermal energy moving through a Fermi gas made of the lithium-6 isotope. This allowed them to develop a dynamic picture of heat as it spread through a two-fluid system.
Under more ambient conditions, particles in fluids like water will jostle with thermal energy, sharing their heat with their neighbors with every collision, which in turn share a portion of their energy with their own neighbors, and so on. This causes heat to dissipate from the warm patch and move through the system until everything is in equilibrium.
Superfluids play by their own rules. Rather than diffusing through a cloud of chaotically buzzing atoms, thermal energy progresses in a more orderly wave-like fashion, 'sloshing' back and forth within the confines of its container.
Understanding the dynamics of heat energy in materials under extreme conditions could lead to better models for improving superconducting technology or theorizing the behavior of particles deep within stars.
It might even help us find a way to keep your cocoa piping hot for a little longer after all.
This research was published in Science.