Bistability is a fundamental phenomenon in nature. Something that is bistable can be resting in two states. These rest states need not be symmetric with respect to stored energy. The defining characteristic of bistability is simply that two stable states (minima) are separated by a peak (maximum).
In physics, for an ensemble of particles, the bistability comes from the fact that its free energy has three critical points. Two of them are minima and the last is a maximum. By mathematical arguments, the maximum must lie between the two minima. By default, the system state will be in either of the minima states, because that corresponds to the state of lowest energy. The maximum can be visualised as a barrier.
A transition from one state of minimal free energy requires some form of activation energy to penetrate the barrier (compare activation energy and Arrhenius equation for the chemical case.) After the barrier has been reached, the system will relax into the next state of lowest energy again. The time it takes is usually attributed the relaxation time. (There might be uncertainty as to which state will be the new one, but it is often well defined in the situation.)
Optical bistability is an attribute of certain optical devices where two resonant transmissions states are possible and stable, dependent on the input.
In biological and chemical systems
Bistability is key for understanding basic phenomena of cellular functioning, such as decision-making processes in cell cycle progression, cellular differentiation, and apoptosis. It is also involved in loss of cellular homeostasis associated with early events in cancer onset and in prion diseases as well as in the origin of new species (speciation).
Bistability can only arise in biological and chemical systems if three necessary conditions are fulfilled: positive feedback, a mechanism to filter out small stimuli and a mechanism to prevent explosions.
Bistable chemical systems have been studied extensively to analyse relaxation kinetics, non-equilibrium thermodynamics, stochastic resonance, as well as climate change.
Bistability is often accompanied by hysteresis. On a population level, if many realisations of a bistable system are considered (e.g. many bistable cells (speciation).
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