In 1960s, thanks to the work of Bondi, Sachs, Penrose and their collaborators, people got a better understanding of gravitational radiations. In particular, the total energy of gravitational field is defined. However, the definition of angular momentum suffers from the infinite dimensional supertranslation ambiguity that is inherited in the theory. We discuss how the quasi-local angular momentum defined by Po-Ning Chen, Mu-Tao Wang, and Shing-Tung Yau leads to the first definition that is supertranslation invariant.

Bio: Ye-Kai Wang is a Taiwanese mathematician. Ye-Kai earned his doctorate from Columbia University in 2014, under the supervision of Mu-Tao Wang. He is affiliating to the Department of Applied Mathematics in National Yang Ming Chiao Tung University in Taiwan. His research interest lies in mathematical general relativity.