In quantum mechanics, <answer>energy</answer> is defined in terms of the <answer>energy</answer> operator as a time derivative of the wave function. The Schrödinger equation equates the <answer>energy</answer> operator to the full <answer>energy</answer> of a particle or a system. Its results can be considered as a definition of measurement of <answer>energy</answer> in quantum mechanics. The Schrödinger equation describes the space- and time-dependence of a slowly changing (non-relativistic) wave function of quantum systems. The solution of this equation for a bound system is discrete (a set of permitted states, each characterized by an <answer>energy</answer> level) which results in the concept of quanta. In the solution of the Schrödinger equation for any oscillator (vibrator) and for electromagnetic waves in a vacuum, the resulting <answer>energy</answer> states are related to the frequency by Planck's relation:  (where  is Planck's constant and  the frequency). In the case of an electromagnetic wave these <answer>energy</answer> states are called quanta of light or photons.
In quantum mechanics, what is defined in terms of the energy operator as a time derivative of the wave function?
energy