Ludwig Boltzmann
Austrian physicist and philosopher of science. His contributions to the development of the kinetic theory of gases and statistical mechanics are very important. He tried to define the thermodynamic concept of entropy in terms of mechanical and probabilistic concepts, and proved the H-theorem which is a mechanical version of the second law of thermodynamics. However, there remained difficult problem of explaining irreversibility. Even in the 21st century, time's arrow is one of the scientific and philosophical enigmas.
As a philosopher, he defended atomistic method in physics against Mach's instrumentalist position. This controversy between realism and anti-realism (there are many versions of each) is another instance of long-standing problems in philosophy.
See my Theory Reduction: the case of the kinetic theory of gases