UP Diliman College of Science National Institute of Physics (UPD-CS NIP) associate professor Dr. Gennevieve Macam and her colleagues were investigating Weyl fermions, exotic subatomic particles that are similar to electrons but have no mass. They found that the behavior of these particles can be understood by adapting Einstein’s ideas on causality.

Causality refers to how one event can directly lead to another event in a cause-and-effect relationship. Einstein took this idea further when he realized that nothing can travel faster than light. This led to the concept of “light cones,” which represent all the possible paths that light—or any signal moving at the speed of light—can take from a given event in space and time. Anything inside the light cone of an event could potentially be influenced by that event, while anything outside the light cone cannot be affected by it due to the limitation imposed by the speed of light. The outer boundary of this cone is called the “event horizon.”

Dr. Macam collaborated with Prof. Guoqing Chang of Nanyang Technological University and his team. They found that these concepts, which normally apply to space and time, could also be used to describe the behavior of Weyl fermions in terms of energy and momentum.

“Our work shows how Einstein’s equations can be adapted to describe quantum materials,” Dr. Macam said. “This paves the way to a better understanding of how the strange quantum world and our everyday reality are intertwined.”

Weyl fermions were first theorized by German physicist Hermann Weyl in 1929 but their existence was only proven almost a century later, in 2015. Due to their charged but massless nature, Weyl fermions may have future applications in electronics and computers.

References: 

Chiu, WC., Chang, G., Macam, G. et al. Causal structure of interacting Weyl fermions in condensed matter systems. Nat Commun 14, 2228 (2023). https://doi.org/10.1038/s41467-023-37931-w