A small perturbation act on a system in equilibrium may produce an effect related to this external field, which can be treat as a linear response up to first order approximation.
1. What is Blackbody Radiation? All normal matter emits electromagnetic radiation when it has a temperature above absolute zero. The radiation represents a conversion of a body’s thermal energy into electromagnetic energy, and is therefore called thermal radiation. It is a spontaneous process of radiative distribution of entropy.
How can we connect the macroscopic thermodynamics laws with the microscopic statistical dynamics? Here is the assumption, that the macroscopic state of a system has the largest possible number of microstates. Or in another way, every microstate of a closed system has the same probability, thus we can get the previous assumption.