When a photon is absorbed by an electron, it is completely destroyed, all its energy is imparted to the electron, which instantly jumps to a new energy level. The photon itself ceases to be. In the equations which govern this interaction, one side of the equation (for the initial state) has terms for both the electron and the photon, while the other side (representing the final state) has only one term: for the electron.
The opposite happens when an electron emits a photon. The photon is not selected from a "well" of photons living in the atom; it is created instantaneously out of the vacuum.
The electron in the high energy level is instantly converted into a lower energy-level electron and a photon, there is no in-between state where the photon is being constructed. It instantly pops into existence.
So the question is: where does the photon come from and why is it instantly travelling at 186,000 miles per second?
Strangely, it doesn't seem to come from anywhere, the universe must put the extra energy somewhere, and because electrons in atoms are electromagnetic phenomena, a photon is born with the required energy. In a weak-force interaction, say the decay of a neutron, that energy goes into a neutrino particle which is also instantaneously created.
Each force has its own carrier particles, and knows how to make them.
So that is why I asked the question, where does a photon get it's motive force from....?