Editing Metastability (section)

{{Short description|Intermediate energetic state within a dynamical system}}
{{For|metastability in digital electronics|Metastability (electronics)}}
{{More citations needed|date=April 2011}}
[[File:Meta-stability.svg|250px|thumb|A metastable state of weaker bond (1), a transitional "saddle" configuration (2) and a stable state of stronger bond (3).]]

In [[chemistry]] and [[physics]], '''metastability''' is an intermediate [[Energy level|energetic state]] within a [[dynamical system]] other than the system's [[ground state|state of least energy]].
A ball resting in a hollow on a slope is a simple example of metastability. If the ball is only slightly pushed, it will settle back into its hollow, but a stronger push may start the ball rolling down the slope. [[Bowling pin]]s show similar metastability by either merely wobbling for a moment or tipping over completely. A common example of metastability in science is [[isomer]]isation. Higher energy isomers are long lived because they are prevented from rearranging to their preferred ground state by (possibly large) barriers in the [[potential energy]].

During a metastable state of finite lifetime, all state-describing parameters reach and hold stationary values. 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.

The metastability concept originated in the physics of [[phase transition#Modern classifications|first-order phase transitions]]. It then acquired new meaning in the study of aggregated [[subatomic particle]]s (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.

Metastability is common in physics and chemistry – from an [[atom]] (many-body assembly) to statistical ensembles of [[molecule]]s ([[viscous fluid]]s, [[amorphous solid]]s, [[liquid crystal]]s, [[mineral]]s, etc.) at molecular levels or as a whole (see [[#States of matter|Metastable states of matter]] and [[#Condensed matter and macromolecules|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 [[systems dynamics|dynamic systems]] (with [[feedback]]) like electronic circuits, signal trafficking, decisional, neural and immune systems, the [[Time-invariant system|time-invariance]] of the active or reactive patterns with respect to the external influences defines stability and metastability (see [[#Computational neuroscience|brain metastability]] below). In these systems, the equivalent of [[thermal fluctuations]] in molecular systems is the "[[white noise]]" that affects signal propagation and the decision-making.