Bistability is a system with two equilibria, where the system tends towards one of two stable positions. A two-dimensional model can define bistability, which can be seen in a light switch or flip-flop circuit. Bistability can also apply to more complex systems, such as sleep and wakefulness in organisms.
Bistability is a term for any dynamical system with two equilibria. That is, a system that can be at rest in two, and only, two different positions because these are dominant over their immediate neighbors. A bistable system will be in one of its stable equilibria most, though not necessarily all, of the time.
A two-dimensional model of the state space can accurately define bistability. A bistable function has exactly two optima, towards which by definition the system is likely to drift. In a graph, this looks like two troughs with a peak between them, assuming the system tends towards “lower” positions on the y-axis variable. These depressions do not necessarily have the same value; one can be deeper than the other. For the system to exhibit bistability, the troughs need only be sufficiently separated so that the system does not typically slip from the lowest to the highest optimum.
A relatively simple physical example of bistability is a light switch. If the switch is on, it stays on; if it is off, it stays off. Intermediate positions are temporary and tend towards these two optimists. Other balances are rare or non-existent.
In electronics, bistability describes a specific type of circuit called a flip-flop. A flip-flop is constructed for the purpose of holding one of two positions. These circuits change their value only based on a predictable input. The simplicity of this system allows it to serve as a building block for much more complicated apparatus, including computers.
Bistability can also apply to directly more complicated systems that do not conform precisely to this model. Other variables can affect the state of the system, and a high enough degree of chaos can keep the system out of balance. Modeling these systems as bistable can still be useful.
The difference between sleep and wakefulness in an organism, for example, can be understood as bistable. Awake and asleep are the two best; the intermediate conditions tend towards these positions. Physical activity related to these two states is extremely complicated, and there are many different ways to be awake, asleep, and somewhere in between. However, the system has bistable tendencies. Once an organism begins to wake up, this process usually triggers a large number of secondary signals that help it maintain wakefulness. Humans have found a variety of ways to complicate this bistability with drugs and internal enlightenment, but bistability remains powerful for them as well.
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