Which scientific fields did Max Planck contribute to after his quantum discovery?

After his discovery of the quantum of action in 1900, Max Planck continued to make significant contributions to several branches of theoretical physics. His work remained focused on high-level research in the following fields:

• Optics
• Thermodynamics
• Statistical mechanics
• Physical chemistry1

In addition to these scientific pursuits, Planck was the first prominent physicist to champion Albert Einstein’s special theory of relativity, recognizing its fundamental importance to physics. In his later years, he also devoted considerable attention to philosophical, aesthetic, and religious questions, while maintaining a firm stance against the indeterministic, statistical worldview that emerged in quantum mechanics during the 1920s.21