Death of a photon

A dear friend asked me some questions about photons recently, (photons being, at least by analogy, 'particles' of light).  I might have misunderstood the deepness (or otherwise) of her real question.  She is a good christian, and to give her credit she is definitely up for a hard hitting debate about the existence of (her particular) god.  I think she was seeking more of a metaphysical reply with significance to an analogy about the nature of god, but having failed in that respect it seemed a shame to waste a friendly lesson in sensational physics.

But before that, I will mention another related question.  Working, as I do, on a rather successful nuclear fusion device, I often take groups of people around the site on tours.  Interestingly, some of the most interesting questions come from young people.

Now that is not to say that you get good questions from school groups, who's main objective in life is to look cool in front of members of the opposite sex.  But when young people come to visit with parents in tow they often want to find out about the mysteries of physics and engineering.  Naturally, the fact that they are there with their parents limits their age to a maximum of about 12 or 13, but already some of them have a real insight into the world.  One day I was asked "where do all the photons go?  A good question.  See the end for a considered answer, but at the time it took me a minute or two think of a good explanation.

Let's return now to the original topic, which was about the 'life' of a photon and whether it ever dies, and what would happen to a photon created at the beginning of time.  One might speculate what it is like to be an photon, and whether time seems to pass in its frame of reference.  But it is of course not like anything at all and there is no way that a photon could be aware of anything that it might be like, or to sense that it ages or changes.  So it is a meaningless question but it is interesting to examine the creation and ultimate doom of an individual photon.

Photons are produced inside atoms when one of their electrons 'jumps' from a higher energy state to a lower energy (as they do spontaneously e.g. perhaps when their host atom gets energy from bumping into another atom).  Quantum mechanics tells us that these jumps can only be of certain discreet sizes and that for a particular type of atom the number of options is limited, immutable and predictable.

The difference in energy has to go somewhere and that somewhere is a photon.  It is just a packet of energy, and a very small packet at that, but with a very specific and fixed amount of energy.  We see the energy as colour and specific atomic transitions produce specific colours.

The photon is not is produced from nothing.  There is no creation of mass involved and conservation of energy is all that matters.  The photon will then 'live' and travel at the speed of light until it bumps into another atom.  At that point it might bounce off the atom (called scattering).  However, if the atom happens to be receptive to a photon of that particular and exact energy, the photon is absorbed, thus raising the energy level of an electron in that atom.  (In astronomy these are also observable as specific 'absorption bands' and their existence helps us to know what other stars are made of).

Photons no more 'live forever' in space than they live for ever if you put on the light in a dark room.  The photons are created at the light and absorbed at the walls, maybe after bouncing around for a while to find an atom that can absorb them.  Indeed, those photons that were produced near the beginning of time (whatever that means) may well have been reabsorbed, by interstellar dust long ago, and then re-emitted at lower energies.

So that seems to me to explain the physics in terms that I can understand, with just a little reference to the incomprehensibility of quantum mechanics. 

Going back to that intuitive student question, 'what happens to all the photons' . . . it seemed that the best way to answer it was to ask another question.  "Do you know where photons come from?"  Fortunately the young man gave me quite a nice description about energy levels of electrons, and he seemed happy with the explanation that

"That is where the photons go back to".  

At least that was one satisfied customer!