[In part I of this post](http://scienceblogs.com/dotphysics/2008/11/interaction-between-light-and-matter-no-room-for-the-photon/), I struggled to show that a particle in an infinite well can only exist at certain energies. If you try to put a particle with more than one energy, the probability oscillates at a frequency (E2 – E1)/h. So, what is next? Well I think I am ready to attack the photon.
According to the ultimate source of truthiness (wikipedia), [the photon is the elementary particle responsible for the electromagnetic interaction](http://en.wikipedia.org/wiki/Photon). In general, the photon is view as the particle manifestation of light where light can exhibit both particle and wave properties.
Before I go too far, I would like to mention a good summary paper of this problem from David Norwood.
- ["The Use and Abuse of the "Photon" in Nanomechanics" (pdf)](http://www.dotphys.net/assets/antiphotonRev1.pdf)
I have said it before, and I will say it again. Science is all about models. In this case, there is a model for light that says it can be particle or wave. This model is not needed. Almost all of the stuff you think is an example of the particle property of light can be explained with the quantum nature of matter. I think the following sums this up well. The photon model for light uses the following relationship:
Where E is the energy of the “photon”, h is Planck’s constant and the greek letter nu (looks like a v) stands for the frequency of the light. There is a relationship like this, but it would be better to write it as:
Where nu is the frequency of the light (or the frequency of the changes in potential for a particle – it actually doesn’t have to be light). Delta E is the change in energy between two energy levels in a quantized system. h is still Planck’s constant.
The fundamental aspect of the problem is to explain the interaction between light and matter. [Here is a great site that has java applets which show the transitions between energy levels for matter.](http://www.falstad.com/mathphysics.html)
I took the liberty of making a movie of one of the applets from this site. I strongly encourage you to run the applet yourself (there are many fine applets on that site).
So, from this you can see what happens when you have an electromagnetic wave incident on the system. If the frequency of the wave (or disturbance) is (E2 – E1)/h, the particle will move from level 1 to level 2 OR from 2 to 1. It will change (either way). This is what happens during absorption and during stimulated emission. It turns out that spontaneous emission is quite a bit more complicated.
What about non-infinite square wells? Well, the same thing holds true. A system will transition from an energy level to another if it is stimulated with a frequency of Delta E/h. Light is not a particle.
But what about….
- Photon momentum: Most introductory texts show how an electromagnetic wave can give momentum to a charged particle.
- The photoelectric effect: This is what people usually claim as evidence for the particle nature of light. Details about how this can be explained with the wave model for light can be found in [Norwood's paper](http://www.dotphys.net/assets/antiphotonRev1.pdf)
- Photo-multiplier tube: Any device that uses the interaction between light and matter will have this Delta E/h = frequency relation. It’s not a photon.
- Shooting light particles: Even people that like the particle nature of light know that they are not actually little balls of light. Too bad you can still find this visual representation in some texts.
So, if there are no photons, why are they in all the textbooks? That is a great question. I am glad I asked it. I really don’t have a great answer here. Maybe someone wrote a book about photons and a student read it. This student eventually wrote his/her own book and included the photon model of light. This new book was then read by a new student and so on.
I know that writing stuff like this might induce many people to think I am wacked out. This may be true, but don’t blame me. I would blame Norwood. I am sure he can give you some names of other people to blame.
**What to do from here?**
Norwood gave a talk to our faculty about this no-photon thing. One of the instructors afterwards was asking about what to do about photons when it comes up in the book. My answer is “why not just call it light?”
**PS** There is a great page [on the history of the development of the photon](http://nobeliefs.com/photon.htm). It is quite interesting story. It is also strange that the idea that light can be both a wave and a particle is what prompted people to think about the wave nature of matter.