What is "metastability"

n. An unstable but potentially long-lived state of a system; for example, a supersaturated solution or an excited atom.

n. the quality of a physical system that persists in its existing equilibrium when undisturbed (or only slightly disturbed) but able to pass to a more stable equilibrium when sufficiently disturbed

Metastability denotes the phenomenon when a 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. While in isolation:

• the state of least energy is the only one the system will inhabit for an indefinite length of time, until more external energy is added to the system (unique "absolutely stable" state);
• the system will spontaneously leave any other state (of higher energy) to eventually return (after a sequence of transitions) to the least energetic state.

A single particle analogy may be drawn with a ball resting in a hollow on a slope. With some perturbation the ball may start rolling again to lower levels. Isomerisation is another common example, where higher energy isomers are long lived as they are prevented from rearranging to the ground state by (possibly large) barriers in the potential energy.

The metastability concept originates in the physics of first-order phase transitions later to acquire new meanings in the study of aggregated subatomic particles (in atomic nuclei or in atoms) or in molecules, macromolecules or clusters of atoms and molecules. Later on it was borrowed for the study of decision-making and information transmitting systems.

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

• Metastability is common in physics and chemistry - from an atom (many-body assembly) to statistical ensembles of molecules (viscous fluids, amorphous solids, liquid crystals etc.) at molecular levels or as a whole (see metastable phases of matter and grain piles below). The abundance of states is more prevalent as the systems grow larger and/or if the forces of their mutual interaction are spatially less uniform or more diverse.
• In dynamic systems (with feedback) like electronic circuits, signal trafficking, decisional systems and neuroscience - it is the time-invariance of the active or reactive patterns with respect to the external influences that defines stability and metastability (see brain metastability below). Here the equivalent of the thermal fluctuations is the "white noise" affecting the signal propagation and the decision-making.