A group of international researchers that included Gennevieve Macam, PhD, an associate professor of physics at the National Institute of Physics of the UP Diliman College of Science (CS) recently discovered that the behavior of Weyl fermions can be understood through the use of Albert Einstein’s ideas on causality.

Weyl fermions are exotic subatomic particles that are similar to electrons but have no mass. These were first proposed by mathematician and physicist Hermann Weyl in 1929 but their existence was only discovered in 2015 by an international team led by Princeton University scientists.

In the study, Causal Structure of Interacting Weyl Fermions in Condensed Matter Systems, Macam and other researchers found that the concepts of causality that normally apply to space and time could also be used to describe the behavior of Weyl fermions in terms of energy and momentum.

Causality refers to how one event can directly lead to another event in a cause-and-effect relationship. Einstein realized that nothing can travel faster than light, which led to the concept of “light cones.” Light cones 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.”

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

The study is a collaboration among UP’s Macam; Wei-Chi Chiu from Northeastern University in Boston, Massachusetts; Guoqing Chang, PhD, an assistant professor at the School of Physical and Mathematical Sciences of Nanyang Technological University in Singapore; Ilya Belopolski of Princeton University in New Jersey, USA; Shin–Ming Huang, PhD of the National Sun Yat-sen University; Robert S. Markiewicz, a professor of physics at Northeastern University; Jia-Xin Yin of the Southern University of Science and Technology in Shenzhen, Guangdong, China; Zi-Jia Cheng, PhD of Princeton University in New Jersey, USA; Chi-Cheng Lee, DSc of Tamkang University in Taiwan; Tay-Rong Chang of National Cheng Kung University in Taiwan; Feng-Chuan Chang, PhD, a professor at the National Sun Yat-sen University in Taiwan; Su-Yang Xu, PhD, an assistant professor at Harvard University in Cambridge, Massachusetts, USA; Hsin Lin of Institute of Physics, Academia Sinica in Taiwan; M. Zahid Hasan, PhD of Princeton University; and Arun Bansil of Northeastern University in Boston, Massachusetts. —With a report from the CS