Experimenters Observe Strange Magnetic Phenomenon First Predicted In The 1930s

Scientists have experimentally observed a physical concept that was first theorised in 1931 for the first time - one that could result in important applications in quantum computing and even the study of string theory. Maybe.

A quasiparticle (Image: Eva Rinaldi/Wikimedia Commons)

A team of researchers from around the world worked together to discover these so-called Bethe strings - essentially, a special behaviour that can travel between particles like the way a sports crowd creates "the wave". But a deeper understanding of the mathematics behind these these things could be a key to unlocking even deeper questions about our universe.

"This is important, fundamental physics," lead author Zhe Wang from Helmholtz Center Dresden-Rossendorf in Germany told Gizmodo.

Bethe strings themselves have nothing to do with "string theory", a proposed theory meant to understand the very deepest nature of physics. They're a behaviour based on an innate property of particles called spin, which is described by equations that look a lot like spinning. Magnetism is the result of electrons' spins, which can either have the value of "up" or "down", and how they align.

The researchers studied a basic model of these spins - molecules whose spins could take on one of two values when placed in a straight line, in which their spins can either point in the same direction or opposite directions along the line. Scientists can track the behaviour of magnons - places where one spin is pointed differently from the rest and behave like their own little magnet particle.

Think of it as a row of fans at a sports game when "the wave" passes by. Let's say that each person is a particle, and hands up means one spin state, and hands down means another spin state. A magnon is a single unit of "the wave", just one person lifting their arms and putting them down at a time, travelling down the aisle. A Bethe string would essentially be the state of two or three people lifting and lowering their arms that always travels together.

German physicist Hans Bethe's equations predicted these collective behaviours in electrons while trying to solve some of the earliest problems of quantum mechanics. Other theorists such as Minoru Takahashi and Michel Gaudin have since refined the theory. Zhe Wang has been working on these "spin chain systems" for a while - essentially, crystals in a magnetic field where there are chains of magnetic particles (the aisles at the sports game) where "the wave" can be induced by pulsing an infrared light and studied by changing the strength of the external magnetic field.

Wang had an experimental result that looked interesting, but it was one he didn't quite understand. After meeting a theoretical physicist at a conference, he soon realised that he had discovered this phenomenon. "It turned out that it was a big surprise," he told me, "these were the string states." Wang and his team published their results today in Nature.

Finding some sense of order in one of these many-particle chains is important. Perhaps these Bethe strings can be manipulated to store data, for example. And there's a seemingly one-to-one correspondence between these particles and actual string theory, a proposed theory of everything that tries to link gravity and quantum physics together. "I'm still working on it, said Wang, "but we hope there's some way to understand the string state in order to understand the string theory." It's similar to the way some physicists hope to study Weyl semimetals to better understand gravity.

Ultimately, this is an experimental observation but not an earth-shaking result. It's a weird physical phenomenon that's only just been observed. Wang told me that there's still a whole lot of work left. "This is the first observation in an experiment. I would say there are more steps to go in order to reach final applications."

[Nature]