State (physics)

In physics, the term state is used in several related senses, each of which expresses something about the way a physical system is.


 * 1) State is sometimes used as a synonym for phase of matter.
 * 2) In classical mechanics state of a system (or body) refers to its condition at a particular moment of time; thus the terms initial state, reference state etc.
 * 3) In thermodynamics/statistical mechanics, a thermodynamic state, or more precisely, a macrostate, is the specification of a particular combination of physical properties (e.g. temperature, volume, pressure, etc). For example,  an equation of state describes the relationship between macrostates of the system.
 * 4) On the other hand, in the same context as above, a microstate is a detailed description of a collection of atoms or other particles. There may be many microstates corresponding to the same macrostate.
 * 5) In the study of dynamical systems, a physical system is typically modeled as a differential equation. The variable in such an equation is often called the state of the system. In some cases, this coincides with the thermodynamic sense, or it may be more abstract.
 * 6) In quantum mechanics, the state vector of an object contains information about the way that object currently is.  For example, the state vector Ψ of a spinless free particle moving on a line, also called particle's wavefunction, is represented by $$e^{i(kx - \omega t)}$$.  In this formalism, the letter t denotes a moment in time, and the letter x denotes the particle's position at moment in time t.  The parameters k, and ω, are related to the speed, and mass of the particle.  Thus, a particle's state vector contains information about the position, mass, and speed of the particle.  In general, one can always describe a quantum mechanical system, which could consist of more than one particle, by its state vector.  It should be noted that the concept of a "state vector" has led to some confusion in physics, as is evidenced by the paradox known as Schrödinger's cat.
 * 7) The term state can also be used as a synonym for a moment in time, a point of time, a single instant.  Since the universe as a whole can be considered a system of particles, we can speak of the state of the universe, using the same meaning used in statistical physics.  In other words when we refer to the "state of the universe," we are referring to the way the entire universe currently is; or to put this still another way, we would be referring to the quantum mechanical state vector of the entire universe.