Where does equilibrium collide?

A state of an economy in which a set of prices makes the quantity supplied equal the quantity demanded in every market at once, so no market has unmet demand or unsold surplus. Arrow and Debreu proved in Existence of an Equilibrium for a Competitive Economy (1954) that under stated conditions such a price vector exists in a general model of many interacting markets. The referent is a whole economy of prices and quantities, and equilibrium here names a balance of aggregate flows, not a balance of forces and not a meeting of intentions. What is set equal is supply and demand across markets; the object is a market, and the outcome is a clearing price.[1]

Unit

A vector of prices that clears every market at once

Recovery means

Not applicable

Measured by

Existence results and computed price vectors that drive aggregate excess demand to zero

A profile of strategies, one for each player, in which no player can raise their own payoff by changing strategy alone while the others hold theirs fixed. Nash defined this equilibrium point in Equilibrium Points in n-Person Games (1950) and proved that every finite game has at least one, once mixed strategies are allowed. The referent is a set of players choosing among actions, and equilibrium names mutual best response, not a market clearing and not a physical balance. Market equilibrium and Nash equilibrium share the word and both describe a state no one moves away from, but one balances aggregate supply against demand while the other balances each player's choice against the others'.[2]

Unit

A strategy profile from which no player can profitably deviate alone

Recovery means

Not applicable

Measured by

Fixed points of the best-response map, shown to exist by the Brouwer and Kakutani fixed-point theorems

The dynamic state a reversible reaction reaches when the forward and reverse reactions proceed at equal rates, so the concentrations of reactants and products stop changing even though both reactions keep running. The balance is summarised by an equilibrium constant, and Le Chatelier stated in 1884 the principle that a system at equilibrium, when disturbed by a change in concentration, temperature, or pressure, shifts so as to partly offset the disturbance. The referent is a chemical system of molecules, and what is equal are two reaction rates, not prices and not strategies. Nothing here is chosen or cleared; the equality is kinetic, and the equilibrium is a moving balance held by matter.[3]

Unit

The equilibrium constant K, the ratio of product to reactant activities at balance

Recovery means

Not applicable

Measured by

Equilibrium constants read from concentrations at balance, with shifts predicted by Le Chatelier's principle

The condition in which the allele and genotype frequencies of a population stay constant from one generation to the next, in the absence of selection, mutation, migration, drift, and non-random mating. Hardy showed in Mendelian Proportions in a Mixed Population (1908) that under random mating the genotype frequencies settle in a single generation to p², 2pq, and q² for allele frequencies p and q, and then hold there. The referent is a breeding population's gene pool, and equilibrium names the constancy of frequencies across generations, not a rest point in prices, choices, or reaction rates. It is a null model: a population in Hardy-Weinberg equilibrium is one in which none of the forces that change gene frequencies is acting.[4]

Unit

Allele and genotype frequencies that hold steady across generations

Recovery means

Not applicable

Measured by

Chi-square tests comparing observed genotype counts against the p², 2pq, q² expectation