How do relaxation phenomena reveal the delay between an external disturbance and a system’s response?

Relaxation phenomena in physics and chemistry reveal the delay between an external disturbance and a system’s response because systems at equilibrium cannot adjust instantaneously to changes in conditions like temperature or pressure.12

When a system is subjected to an abrupt change, it requires time to re-equilibrate. This delay occurs because the system must redistribute energy among its internal states or shift the ratio of its constituents—such as the concentration of reactants and products in a chemical reaction—to reach a new balance.12

This process is quantified by the relaxation time ($\tau$), which is the reciprocal of the constant of proportionality in the rate equation governing the system's return to equilibrium. By measuring this time, scientists can gain insights into atomic and molecular structures, as well as the rates and mechanisms of chemical reactions. For example, if an external condition fluctuates too rapidly, the system's internal adjustment will lag behind the disturbance, a phenomenon that can be observed and measured through techniques like spectroscopy or by analyzing sound wave absorption.312