Researchers pull back the quantum curtain on 'Weyl fermions'

Northeastern researchers have made what they describe as a groundbreaking discovery in the field of quantum mechanics.

Wei-Chi Chiu, a postdoctoral researcher at Northeastern reporting to Arun Bansil, university distinguished professor of physics at Northeastern, tells Northeastern Global News that his team has published a novel study examining the nature of a specific class of subatomic particles, whose very existence has eluded quantum physicists for nearly a century.

Chiu and his colleagues propose a new theoretical framework to explain how these particles, called Weyl fermions, interact with each other in certain materials. The findings, published in Nature Communications earlier this month, look beyond the framework of Albert Einstein's theory of relativity to probe these mysterious particles, Chiu says.

Weyl fermions were first discovered in 2015 by a group of physicists at Northeastern and Princeton universities. The discovery capped off an 85-year search for the massless particle, considered a basic building block of other subatomic particles, since its existence was first theorized by physicist Hermann Weyl in 1929.

In June of 2015, Bansil and the team of researchers predicted that a specific crystalline material, called Tantalum arsenide (TaAs), would host Weyl fermions. Soon after, the researchers demonstrated in an experimental paper the presence of the particles in TaAs through photoemission spectroscopy.

The discovery prompted a surge of experimental and theoretical explorations of Weyl fermions in different materials for want of a mechanical explanation of how they actually behave in real-time.

"Weyl fermions are relativistic particles that had actually never been seen, or observed, until 2015," Chiu says. "Our main focus in this work is understanding how these kinds of quasiparticles are interacting, and what the mechanisms are behind these interactions."

The most significant part of the researchers' findings, he says, is that the new framework for these quantum-level interactions challenges the longstanding view of "causality" as existing in spacetime. In Einstein's theory of relativity, the traditional view of causality—or mere cause-and-effect—is that it is "time-ordered," meaning it is established in relation to chronological time. Under this view, causality refers to the principle that an event can only be influenced by other events that occur within its past light cone, Chiu says.

"This means that if 'A' causes event 'B,' then event 'A' must occur before event 'B' in both space and time," he says. "The light cone forms the event horizon or the boundary in spacetime which distinguishes between events that are and are not causally connected."

In other words, no object or signal can travel faster than the speed of light. This principle is sometimes referred to as the causality principle in relativity. The concept of causality in relativity is important, Chiu says, because it sets fundamental limits on what can be observed and measured in the universe.

Instead of thinking about the behavior of Weyl fermions in terms of spacetime, Chiu and his associates say these causal interactions are better understood as a result of measurable changes in the "energy-momentum" space.

"Our study, for the first time, shows how key concepts of causality and the associated event horizon in spacetime can be carried over into the field of correlated Weyl materials, and thus unveils fundamental connections between condensed matter and high-energy physics," Chiu says.

The work, he says, "opens up opportunities for exploring new connections between the world of particles and the larger world we experience every day."

"This study reveals for the first time how the ideas of causality that are enshrined in Einstein's theory of relativity—and lead to the concepts of 'light cones' and 'event horizons' that have become the stuff of movies and common sci-fi works—can be generalized and expanded into the world of quantum materials," Bansil says,