I'm reading an excellent book by Kauffmann, Discovering The Universe, in which he describes when electron jump to a higher level or a lower level. If they go to a higher level they emit a photon. If they go to a lower level they absorb a photon.
It causes me to ask if photons exist as a seperate entity within all atoms or are they created at certain energy levels for the purpose of absorption, or emission, or do they exist omnipresently in the fabric of the universe? Anyway, where does the photon for absorbtion come from?
Hope this isn't too stupid a question.
It certainly isn't a stupid question, and really cuts to the heart of the question of interpreting quantum mechanics.
The simplest answer is that 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 existance.
So the question is: where does the photon come from?
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.
That's really all we can say about it. There are many interpretations of what this and other phenomena in quantum mechanics mean on a deeper level, and whole libraries worth of books which argue points of view on the matter. But my personal philosophy is that of the famous physicist Richard Feynman, who said: "Shut up and calculate."
Edit by Michael Lam on February 10, 2016: The quote has been misattributed to Richard Feynman, and sometimes Paul Dirac, but evidentally comes from Cornell physicist David Mermin (source).