From Dispersion to Matrix Mechanics

Heisenberg's last paper, before submitting his famous paper on matrix mechanics in the summer of 1925, was a paper on dispersion theory, co-authored with Hans Kramers. In my talk, I will fill in the gap between these two papers, building on archival material, in particular Heisenberg's letters. I will show how ideas from the dispersion paper were systematized and how new ideas came in from the theory of spectral intensities, joining to deliver Heisenberg's revolutionary new formulation of quantum theory.