He then went to the University of Michigan, in Ann Arbor, for graduate studies in physics with Walter F. Colby and Oskar Klein.

Klein, already associated with the Kaluza–Klein theory (1921), joined the faculty at Michigan in 1922, after a six-year stay at the Institute for Theoretical Physics, under Niels Bohr, at the University of Copenhagen.

[3] It was through Klein that Dennison heard and leaned much about the current theoretical physics being developed in Europe, which created a yearning in him to go to Copenhagen for further study.

[8] During his three years in Europe, he mostly did postdoctoral research in Copenhagen, where he had associations with other visiting physicists working there, such as Paul Dirac, Samuel Abraham Goudsmit, Werner Heisenberg, Walter Heitler, Ralph H. Fowler, Friedrich Hund, Hendrik Anthony Kramers, Yoshio Nishina, Wolfgang Pauli, and George Eugene Uhlenbeck.

While at Cambridge, Dennison used quantum mechanicals calculations on molecular hydrogen to show that protons, like electrons, were subject to Fermi–Dirac statistics, or had spin-½, and therefore obeyed the Pauli exclusion principle.

[7][12] Dennison was a student of Niels Bohr, and knew Hans Bethe, Wolfgang Pauli, and Enrico Fermi before they became world-famous.

Following the discovery of the spin of the electron in 1925 by George Uhlenbeck and Samuel Goudsmit, the specific heat of hydrogen was a major unsolved problems.