From theoretical physics to biology
Delbrück initially studied astronomy and theoretical physics and interacted with Pauli and Bohr. In Berlin he joined interdisciplinary work on radiation and mutation. He moved to Caltech in 1937 and began studying bacteriophages with Emory Ellis.
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Quantitative viral genetics
With Luria, Hershey and others, he brought quantitative experiments and genetic analysis to viral multiplication, mutation and recombination. Meetings and courses at Cold Spring Harbor encouraged the sharing of strains, methods and findings across laboratories, building the informal phage group.
Sensory physiology
From the 1950s Delbrück increasingly studied sensory physiology, using Phycomyces sporangiophores to investigate responses to stimuli. He also helped establish genetics research at the University of Cologne. The 1969 Nobel Prize recognised his earlier contributions to viral genetics.