Editing Bistability (section)

{{Short description|Quality of a system having two stable equilibrium states}}
{{for|electronics|Flip-flop (electronics)|Multivibrator}}
{{Use dmy dates|date=January 2021}}

[[Image:Bistability graph.svg|thumb|upright=1.4|A graph of the [[potential energy]] of a bistable system; it has two local minima <math>x_1</math> and <math>x_2</math>.  A surface shaped like this with two "low points" can act as a bistable system; a ball resting on the surface can only be stable at those two positions, such as balls marked "1" and "2".  Between the two is a local maximum <math>x_3</math>. A ball located at this point, ball 3, is in equilibrium but unstable; the slightest disturbance will cause it to move to one of the stable points.]]
[[Image:Rocker light switch.jpg|thumb|upright=0.8|Light switch, a bistable mechanism]]

In a [[dynamical system]], '''bistability''' means the system has two [[stable equilibrium (disambiguation)|stable equilibrium states]].<ref name="Morris">{{cite book
 | last1  = Morris
 | first1 =  Christopher G.
 | title  = Academic Press Dictionary of Science and Technology
 | publisher = Gulf Professional publishing
 | date   = 1992
 | pages  = 267
 | url    = https://books.google.com/books?id=nauWlPTBcjIC&q=bistable+bistability&pg=PA267
 | isbn   = 978-0122004001
 }}</ref> A '''bistable structure''' can be resting in either of two states.   An example of a mechanical device which is bistable is a [[light switch]]. The switch lever is designed to rest in the "on" or "off" position, but not between the two.  Bistable behavior can occur in mechanical linkages, electronic circuits, nonlinear optical systems, chemical reactions, and physiological and biological systems.

In a [[conservative force]] field, bistability stems from the fact that the [[potential energy]] has two [[local minimum|local minima]], which are the stable equilibrium points.<ref name="Nazarov">{{cite book
 | last1  = Nazarov
 | first1 = Yuli V.
 | last2  = Danon
 | first2 = Jeroen
 | title  = Advanced Quantum Mechanics: A Practical Guide
 | publisher = Cambridge University Press
 | date   = 2013
 | pages  = 291
 | url    = https://books.google.com/books?id=w20gAwAAQBAJ&q=bistability+minimum&pg=PA291
 | isbn   = 978-1139619028
 }}</ref> These rest states need not have equal potential energy.  By mathematical arguments, a [[local maximum]], an unstable equilibrium point, must lie between the two minima.  At rest, a particle will be in one of the minimum equilibrium positions, because that corresponds to the state of lowest energy. The maximum can be visualized as a barrier between them.

A system can transition from one state of minimal energy to the other if it is given enough activation energy to penetrate the barrier (compare [[activation energy]] and [[Arrhenius equation]] for the chemical case). After the barrier has been reached, assuming the system has [[damping]], it will relax into the other minimum state in a time called the [[relaxation time]].

Bistability is widely used in [[digital electronics]] devices to store [[binary number|binary]] data. It is the essential characteristic of the [[flip-flop (electronics)|flip-flop]], a circuit which is a fundamental building block of [[computer]]s and some types of [[semiconductor memory]].  A bistable device can store one [[binary digit|bit]] of binary data, with one state representing a "0" and the other state a "1".  It is also used in [[relaxation oscillator]]s, [[multivibrator]]s, and the [[Schmitt trigger]].
[[Optical bistability]] is an attribute of certain optical devices where two resonant transmissions states are possible and stable, dependent on the input.
Bistability can also arise in biochemical systems, where it creates digital, switch-like outputs from the constituent chemical concentrations and activities. It is often associated with [[Hysteresis#In biology|hysteresis]] in such systems.