Roy, W7EL wrote:
"They don`t have to be in phase, but the same frequency requirement
implies that the phase relationship wouldn`t change with time."

Definitions of terms like "coherent" change with time.

Slectromagnetic waves are described as "plane-polarized waves" because
variations in their electric and magnetic fields can be represented by
vectors that lie in a plane. It is proven that light waves and radio
waves are of the same sort but differ in frequency.

The first reference I ever saw of a coherent wave was a description of
light from a laser. It meant the waves started and stopped together
passing through zero at the same time.

Light waves are emitted by molecules or atoms that are excited by
thermal or electrical means. These molecules or atoms are randomly
positioned and so are the waves generated by the energy level changes
within them. Phase and polarization are thus random in light produced by
ordinary light sources. Light is coherent from a laser.

A photon can interact with an atom in a laser if its energy exactly
matches the energy difference (delta E) between two allowed (by Bohr)
energy states for the atom, it can cause a transition. This is called a
stimulated transition.

Best regards, Richard Harrison, KB5WZI