He is best known for his work on the Casimir effect, which describes the attractive force between two uncharged plates in a vacuum due to quantum fluctuations of the electromagnetic field.

[4] His Ph.D. thesis dealt with the quantum mechanics of a rigid spinning body and the group theory of the rotations of molecules.

One in particular is still referred to as the “Casimir trick": in particle interaction calculations, it is a familiar procedure of trace formation and projections using products of Dirac matrices.

[7] He remained an active scientist and in 1945 wrote a well-known paper on Lars Onsager's principle of microscopic reversibility.

[9] Although he spent much of his professional life in industry, Hendrik Casimir was one of the great Dutch theoretical physicists.

These contributions include: pure mathematics, Lie groups (1931); hyperfine structure, calculation of nuclear quadrupole moments, (1935); low temperature physics, magnetism, thermodynamics of superconductors, paramagnetic relaxation (1935–1942); applications of Onsager's theory of irreversible phenomena (1942–1950).