Rami Arieli: " The Laser Adventure" Chapter 2.4 page 1
2.4 Spontaneous emission of electromagnetic Radiation

One of the basic physical principles (which is the basis of a subject in physics called Thermodynamics) is that:

Every system in nature "prefers" to be in the lowest energy state.

This state is called the Ground state. As an example, we mentioned this principle in the Bohr model of the atom.

When energy is applied to a system, The atoms in the material are excited, and raised to a higher energy level.

(The terms "excited atoms", "excited states", and "excited electrons" are used here with no distinction)

These electrons will remain in the excited state for a certain period of time, and then will return to lower energy states while emitting energy in the exact amount of the difference between the energy levels (delta E).

If this package of energy is transmitted as electromagnetic energy, it is called photon.

The emission of the individual photon is random, being done individually by each excited atom, with no relation to photons emitted by other atoms.

When photons are randomly emitted from different atoms at different times, the process is called Spontaneous Emission. Since this emission is independent of external influence, there is no preferred direction for different photons, and there is no phase relation between photons emitted by different atoms.