Steady State

∞ generated and posted on 2016.08.22 ∞

A system for which change occurs but no net change occurs, whether or not that lack of net change requires an ongoing input of energy.

A situation in which something is gained as fast as it is lost so that there is, as noted, no net change in its degree of prevalence over time, despite being both lost and gained over that time. In a population that is at steady state there are as many births as there are deaths, so though the individuals making up the population change over time, the size of the population does not change.

Steady state as perturbation away from thermal equilibrium

Figure legend: My thumb, on a cold Winter's day, creates a more or less constant thermal gradient, one that is maintained at the expense of energy used by my body to heat itself. The constancy of this thermal gradient therefore represents a steady state rather than a dynamic equilibrium, i.e., a situation of constancy around my thumb but not constancy over the universe as a whole.

The contents of your refrigerator as a steady state

Figure legend: The contents of your refrigerator as approximating a steady state. Food is constantly flowing through your refrigerator and the food that arrives into the refrigerator, at any given moment, is not identical to whatever food instead is leaving at any given moment. The amount of food is also remaining within fairly narrow bounds, rarely if ever reaching down to zero (though at times it might seem that way) and certainly not continuously increasing such that infinity is approached (again, though at times it might seem that way, too). Thus, the among of food found in your refrigerator approximates a steady state, one with episodic filling (grocery day!) along with episodic, rather than continuous, emptying (meal times).

Steady states are similar but not identical to dynamic equilibria since a system in steady state can be one that is perturbed away from equilibrium such as if a net input of energy is required to maintain the system at steady state (e.g., maintaining a constant speed in your car on the highway is a steady state but not an equilibrium).