The universe has no shortage of bizarre materials. Superfluids are liquids that can flow straight up walls, Bose-Einstein condensates are gasses that will vibrate eternally, and neutron stars are essentially city-sized subatomic particles.
Physicists have now developed a mathematical theory that describes how collective quantum mechanical weirdness leads to the strange properties of these materials. While previous work has focused on each individual system, the new theory unites the behavior for many materials, including magnets, superfluids, and neutron star matter.
“It’s like shooting many, many birds with one stone,” said particle physicist Hitoshi Murayama of UC Berkeley, co-author of a paper on the work that appeared in Physical Review Letters June 15.
Murayama and his graduate student, Haruki Watanabe, showed that the behavior of these materials hinges on a phenomenon known as spontaneous symmetry breaking. Symmetry breaking happens when a group of particles that once had no preferred alignment or direction suddenly does, creating a collective behavior.
One of the best-known occurrences of symmetry breaking happens when certain metals -- such as iron -- cool down and form a magnet. Each atom in the metal contains an electron that forms a microscopic magnetic field. When the metal is hot, the atoms have their individual magnets pointing willy-nilly in random directions.
But as they cool down, the atoms start to point their magnets in the same direction as their neighbors. If enough of the atomic magnetic fields align, their collective action will be strong enough to attract and repel other magnetic materials.
In the 1960s, physicists Yoichiro Nambu and Jeffrey Goldstone worked out how spontaneous symmetry breaking gives materials such as superfluids their bizarre properties. If you stir a glass of ordinary liquid like water, it will eventually succumb to friction and come to rest. But when cooled to extremely low temperatures, superfluids can flow forever, even climbing straight up the wall of a container and dripping out onto the floor.
But Nambu and Goldstone’s equations only worked to explain subatomic particles in a vacuum, at zero temperature and density. They had to be recalculated for different real-world materials and sometimes turned up the wrong answer.
Murayama and Watanabe refined the work so that physicists don’t have to look at the details of each specific system, and instead can tell whether or not weird behavior will arise just based on the number of symmetries broken.
“It is a neat tie-up of things that we know about individually,” said condensed-matter physicist Anthony Leggett of the University of Illinois at Urbana-Champaign, who was not involved in the work. “With this theory, it may be possible to predict or classify new materials.”
Image: Individual rubidium atoms form into a single super-atom in a Bose-Einstein condensate. NIST/JILA/CU-Boulder
