Rudolf Haag (17 August 1922 – 5 January 2016) was a German theoretical physicist, who mainly dealt with fundamental questions of quantum field theory.

In April 1953, he joined the CERN theoretical study group in Copenhagen[note 1] directed by Niels Bohr.

Without making assumptions about non-observable charge-changing fields, Haag, in collaboration with Sergio Doplicher and John E. Roberts, elucidated the possible structure of the superselection sectors of the observables in theories with short-range forces.

[23] In quantum statistical mechanics, Haag, together with Nicolaas M. Hugenholtz and Marinus Winnink, succeeded in generalizing the Gibbs–von Neumann characterization of thermal equilibrium states using the KMS condition (named after Ryogo Kubo, Paul C. Martin, and Julian Schwinger) in such a way that it extends to infinite systems in the thermodynamic limit.

This theory has proven to be a central element in structural analysis and recently[note 6] also in the construction of concrete quantum field theoretical models.

[note 7] Together with Daniel Kastler and Ewa Trych-Pohlmeyer, Haag also succeeded in deriving the KMS condition from the stability properties of thermal equilibrium states.

[26] Together with Huzihiro Araki, Daniel Kastler, and Masamichi Takesaki, he also developed a theory of chemical potential in this context.

By working with Klaus Fredenhagen, Heide Narnhofer, and Ulrich Stein, Haag made important contributions to the understanding of the Unruh effect and Hawking radiation.

[28] Haag had a certain mistrust towards what he viewed as speculative developments in theoretical physics[note 8] but occasionally dealt with such questions.

[note 9][30] In 1970 Haag received the Max Planck Medal for outstanding achievements in theoretical physics[31] and in 1997 the Henri Poincaré Prize[32] for his fundamental contributions to quantum field theory as one of the founders of the modern formulation.