We often think that when we have completed our study of one we know all about two, because "two" is "one and one." We forget that we still have to make a study of "and."

He began by extending Karl Schwarzschild's earlier work on radiation pressure in Emden polytropic models. He developed a new statistical method based on the apparent drift of two background stars winning him the Smith's Prize in 1907. He was also a philosopher of science and a popularizer of science.

World War I severed many lines of scientific communication and new developments in German science were not well known in England and vice versa. He also conducted an expedition to observe the Solar eclipse of 29 May 1919 that provided one of the earliest confirmations of relativity and he became known for his popular expositions and interpretations of the theory. He is famous for his work regarding the theory of relativity.