# State (physics)

The state of a physical system at a certain point in time includes, within the framework of a physical sub-area, the entirety of all information that is required for a complete description of the current properties of the system, unless they are already fixed as parameters of the system with its unchangeable properties.

Examples
system Status
1. A body of given mass
in the earth's gravitational field
Location: 1 m above the point of the sword of
the Bremen Roland ,
speed: 2 m / s to the north
2. 1 kg normal dry air in a
stable state of equilibrium
Volume 1 m³,
temperature 300 K
(The pressure is then given by
the equation of state.)
3. Hydrogen atom 2s level
4th A particle with no spin
in the potential${\ displaystyle 1 / r ^ {3}}$
Basic state

The state of a system that is accessible to observation specifies, in particular, all observable properties and thus allows predictions about all measured values ​​of observable physical quantities. If these variables are linked by equations (e.g. equation of state in thermodynamics), it is sufficient to specify a suitable selection of these variables in order to clearly define the state. In the narrower sense, state is therefore understood as a minimum number of physical quantities from which all other observable quantities can be calculated with knowledge of the system properties. (In the 2nd example of the table above, knowledge of volume and temperature results in the pressure from the equation of state for normal dry air.)

If an equation of motion exists for a system (e.g. Newton's 2nd axiom in the 1st example, time-dependent Schrödinger equation in the 4th), it can be used to determine how the state changes from the current properties of the system given with the state. If further details from the previous history of the system influence further development, the definition of the status must include this information.

A physical system can exist in different states. In a mathematical model for a physical system, the set of possible states is referred to as the state space of the system; in classical mechanics also as phase space . If the object assumes a number of different states over time, it goes through a process .

Depending on the branch of physics , different state definitions are used:

• Classical mechanics of the mass point : The state is given by location and momentum.
• Classic mechanics of the rigid body : The state is given by the location and momentum of the center of gravity, the orientation of the body and its angular momentum around the center of gravity.
• Classical statistical mechanics of many mass points: The state (more precisely: the microstate ) is given by the location and momentum of each individual mass point.
• Thermodynamics : The state is given by a selection of macroscopic quantities (such as volume, pressure, temperature, internal energy, magnetization, ...) from which all further macroscopic quantities can be calculated with the aid of the system's equation of state.
• Physical chemistry : The state is given by the state variables of thermodynamics and the mass fractions of various substances.
• Electromagnetic field : The state is given by the electric and magnetic field strength at each location and their time derivatives.
• Quantum mechanics : The state is given by the state vector (e.g. in the form of the wave function). See also state (quantum mechanics) .
• Quantum statistics : The state of the particle system is given by the occupation numbers of all possible single-particle states. See also state (quantum mechanics) .

In contrast to quantum mechanics, in classical physics the current state is already established immediately before the measurement and is only "registered" by the measurement, while in quantum physics the state is generally only "prepared" by the measurement, whereby complicated probability statements apply.