state the way an object or system basically is; the fundamental, intrinsic properties of an object or system, and the basis of its other properties. An instantaneous state is a state at a given time. State variables are constituents of a state whose values may vary with time. In classical or Newtonian mechanics the instantaneous state of an n-particle system consists of the positions and momenta (masses multiplied by velocities) of the n particles at a given time. Other mechanical properties are functions of those in states. Fundamental and derived properties are often, though possibly misleadingly, called observables. The set of a system’s possible states can be represented as an abstract phase space or state space, with dimensions or coordinates for (the components of) each state variable.
In quantum theory, states do not fix the particular values of observables, only the probabilities of observables assuming particular values in particular measurement situations. For positivism or instrumentalism, specifying a quantum state does nothing more than provide a means for calculating such probabilities. For realism, it does more – e.g., it refers to the basis of a quantum system’s probabilistic dispositions or propensities. Vectors in Hilbert spaces represent possible states, and Hermitian operators on vectors represent observables.
See also DISPOSITION , INSTRUMENTALISM, PROBABILITY , PROPENSITY , QUANTUM ME — CHANICS , STATE OF AFFAIR. D.S.