Metastability

In physics, metastability denotes the phenomenon when a dynamical system spends an extended time in a configuration other than the system's state of least energy. During a metastable state of finite lifetime, all state-describing parameters reach and hold stationary values. In isolation:

A conceptual analogy may be drawn with a ball resting in a hollow on a slope. Small perturbations will result in the ball settling back into its current hollow, while some larger perturbation may start the ball rolling to a lower level. Isomerisation is another common example of metastability. Higher energy isomers are long lived as they are prevented from rearranging to their preferred ground state by (possibly large) barriers in the potential energy.

The metastability concept originated in the physics of first-order phase transitions. It then acquired new meaning in the study of aggregated subatomic particles (in atomic nuclei or in atoms) or in molecules, macromolecules or clusters of atoms and molecules. Later, it was borrowed for the study of decision-making and information transmission systems.

Many complex natural and man-made systems can demonstrate metastability.

Non-equilibrium thermodynamics is a branch of physics that studies the dynamics of statistical ensembles of molecules via unstable states. Being "stuck" in a thermodynamic trough without being at the lowest energy state is known as having kinetic stability or being kinetically persistent. The particular motion or kinetics of the atoms involved has resulted in getting stuck, despite there being preferable (lower-energy) alternatives.

Metastable states of matter range from melting solids (or freezing liquids), boiling liquids (or condensing gases) and sublimating solids to supercooled liquids or superheated liquid-gas mixtures. Extremely pure, supercooled water stays liquid below 0 °C and remains so until applied vibrations or condensing seed doping initiates crystallization centers. This is a common situation for the droplets of atmospheric clouds.

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