Initialstate

Initial state is a term used across physics and related fields to denote the condition of a system at the start of a defined process. It is distinguished from the final state, which describes the condition after the process. In classical mechanics, the initial state is specified by initial conditions, such as position and velocity (or momentum) at a chosen start time, and these determine the subsequent trajectory through the equations of motion.

In quantum mechanics, the initial state is represented by a quantum state vector (a wavefunction) or a statistical ensemble described by a density matrix. It encodes all information needed to predict the system's evolution under a given Hamiltonian. The initial state's preparation can be pure or mixed, and uncertainties in its description lead to probabilistic predictions.

In high-energy and nuclear physics, the initial state refers to the configuration of incoming particles before a collision or interaction, including their momenta and spin states. Calculations often use asymptotic free-particle states as the initial condition. Concepts such as initial-state radiation describe photons or gluons radiated by incoming particles before the main interaction, affecting observed outcomes.

In computational simulations and cosmology, initial states or initial conditions specify the starting field values or configurations from which the system's time evolution is computed, using numerical methods or physical laws.

See also final state, boundary conditions, and state preparation.