The quantum mechanical time evolution operator governs how observables and/or states evolve during finite time steps, and is always unitary. Use this tag for questions about the time evolution operator, or the different equations of motion in the Schrödinger/Heisenberg/Dirac pictures. For time-independent Hamiltonians, the time evolution operator is simply exp(-iHt).

Time-evolution of an observer means how it changes over time; it's derivative over time. In quantum mechanics, it is described by the Hamiltonian. I.e.

$$H|\psi\rangle=i\hbar\frac{\partial|\psi\rangle}{\partial t}$$

This is also related to noether's theoremp, which proves that time-invariance is equivalent to energy-conservation.

DO NOT use this tag for evolving laws of physics.

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