Why do nuclear, atomic, and molecular systems need time to reach equilibrium after a sudden change?

Nuclear, atomic, and molecular systems require time to reach equilibrium after a sudden change because the redistribution of energy or the adjustment of chemical species is not instantaneous. When a system is subjected to an abrupt physical change—such as a shift in temperature, pressure, or an external magnetic field—it must undergo a time-dependent process to re-equilibrate under the new conditions.1

This delay, known as the relaxation phenomenon, occurs for several reasons:

• Energy Redistribution: Systems must redistribute energy among various nuclear, electronic, vibrational, and rotational states. For example, transferring energy between translational and vibrational modes can require thousands of molecular collisions, making the process time-dependent.12
• Chemical Transformation: In chemical systems, relaxation may result from a shift in the ratio of reactant to product molecules, which takes time to reach a new equilibrium state.1
• Spin-Lattice Relaxation: In magnetic fields, excited nuclei must dissipate excess energy to their surroundings (the "lattice") to return to a low-energy orientation.34

Measuring these relaxation times allows scientists to study rapid energy transfers and the mechanisms of chemical reactions.14