Dot Physics

Models, photons, and more

Quantum mechanics is not my area of expertise. Really, I have no area of expertise. However, I think it is time to bring the whole photon thing back up. Yes, I know I was a little harsh before. Maybe I should start over.

First, models. Yes models. I think science is all about models. Scientists build models that attempt to agree with observations. These models could be mathematical, physical, conceptual or numerical (like a computer program). For example, take Newton’s Law of gravity (which isn’t really a law). It says that the gravitational force between two objects has the magnitude of:

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This mathematical model relates the gravitational force to the masses of the two objects and the distance between them. It is experimentally determined and not the absolute truth. If it is truth you are looking for, Dr. Tyree’s philosophy class is down the hall. (I am pretty sure I have used that quote before). I can think of several situations where that model does not agree with observations. It is still useful.

So back to the photon. In my original post I made the claim that the photoelectric effect is not a great experiment to show photons. Maybe that is not how it came off, but that is what I meant. The photoelectric effect can be explained quite well with the classical electromagnetic waves model and a quantum nature of matter. Of course there is a quantum nature to light as well.

I think the biggest problem with the photon is that the manner it is introduced encourages students to think of it as an actual particle. One thing about particles is that they are localized. I am pretty sure that even quantized light (the real photon) is not confined to a set space. Again, let me remind you that this is not my area of expertise. Let grab the first intro physics book I see and look up photons.

The most classically-looking book I found was Serway and Jewitt’s Physics for scientists and engineers. After spending about a bajillion chapters on optics, the authors start talking about the photoelectric effect. To their credit, they do not draw photons as little balls (but you know older texts that do have that).

Ok, the point I am trying to make:

  • What is the purpose of introducing the photo electric effect?
  • We don’t want students to get the wrong idea that photons are particles
  • There IS quantum effects of electromagnetic radiation.
  • Science is about models and Maxwell’s equations are a pretty good model for light.

This finally brings me to the reason I started talking about photons. At the AAPT conference, I met someone from some institution in Germany (can’t correctly read the institution in german). They have some great online materials that shows data for the photon. http://www.didaktik.physik.uni-erlangen.de/quantumlab/english/index.html. That is a link to the english version of the site. The site has some flash applets that simulate some optical experiments showing evidence for photons and stuff. I like it.

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Also – Neil you CAN keep using the term “photon torpedo”.

Update: Here is a paper by my colleague David Norwood (The Use and Abuse of the “photon” in Nanomechanics ) regarding the whole photon thingy. Well worth a read with many excellent references.

Comments

  1. #1 Anonymous Coward
    February 24, 2009

    What is the purpose of introducing the photo electric effect?

    It is one of the experiments that got folks starting thinking light was quantized, so it’s very important from a historical perspective. Attempts to explain it classically strike me as foolish: not only is the photon real, but it is also the simplest and most elegant explanation of the effect. Just because the PE doesn’t prove that light is quantized doesn’t mean that you shouldn’t explain the PE effect using the simplest correct explanation.

    We don’t want students to get the wrong idea that photons are particles

    They are quanta of the electromagnetic field. If you believe in field theory, all the other things we normally think of as particles are also quanta of some field. I’m not sure what you’re getting at here. There are certainly a huge number of errors that students can make if they treat photons as classical particles, but that’s true for a lot of quantum mechanics.

    There IS quantum effects of electromagnetic radiation.

    You bet!

    Science is about models and Maxwell’s equations are a pretty good model for light.

    It depends what you’re doing. Maxwell’s equations are an incomplete description of the world and fail to describe certain phenomena (the same way that Newtonian mechanics fails as v -> c). If you’d like to restrict yourself to pre-1900 physics, that’s fine, but there’s no reason to player-hate on those that don’t.

    Let grab the first intro physics book I see and look up photons. The most classically-looking book I found was Serway and Jewitt’s Physics for scientists and engineers.

    Are you kidding? I’ve taught out of S&J, and it’s a OK book for a survey class of “modern physics”, but that’s really not where a physicist should be looking if he wants to understand the photon.

    Sakurai’s Advanced QM does a nice job of quantizing the EM field, and Mandel & Wolf and Scully & Zubairy are some of the “standard” modern texts on quantum optics. If you want to write seriously about the photon, you should probably have a look-see. If you’d like a colloquial understanding of quantum mechanics, Chad Orzel’s blog is quite good, both in accuracy and readability.

  2. #2 Neil
    February 24, 2009

    Awesome! Photon torpedoes are really all I care about.

  3. #3 Commenter
    February 27, 2009

    It is very unfortunate that you are a professor… What kind of place hired you?!