In 1921, his thesis “Potentials of Uniform” was approved by G.T. Walker, D.N. Mallik, and Asutosh Mukherjee to be worthy of being awarded the doctorate (D.Sc.) degre from the university. With an extra allowance of five hundred taka per month, he went to Germany for research work at the universities of Berlin, Munich, and Paris. His work with Professor Arnold Sommerfeld in Munich from 1921 to 1922 was published in the Physikalische Zeitschrift. In the summer of 1922, he went to Berlin and studied under Max von Laue in the newly established Institute for Physics. Under von Laue, Sen received a Ph.D. from the University of Berlin for a general relativity dissertation on the boundary conditions for the gravitational field equations on surfaces of discontinuity. In his dissertation, Sen found that Einstein's equations imply that gravitational forces hold together the parts of a particle, then he calculated the equilibrium of a charged particle with a defnite spherical boundary. Sen also worked with von Laue on de Sitter universe and changes in potential of ions and emitted electrons of glowing metals during his stint in Germany.