In the early 1930s, Sen worked a considerable amount on wave mechanics and Dirac’ relativistic equations. He did some notable work on spherical harmonics and studied the relativistic effects in stellar bodies. Sen was a founding fellow of the Indian National Science Academy and Indian Science News Association in 1935. He was also a founding member of the Indian Statistical Institute (ISI) Council and founding fellow of the National Institute of Sciences of India. In 1936, he established the Computational Laboratory and Fluid Dynamics Laboratory in his department in Calcutta University, the first fluid dynamics laboratory in India. Sen started working on the internal constitution of stars around 1940, after Hans Bethe established the law of energy generation.