Q & A: quantized light

Since my days of physics class I have never been satisfied with theory treating light as both a wave and a particle. But that was years ago, and I didn’t have the Physics Van then! I know that light from a single source passing through two narrow slits will create an interference pattern characteristic of a wave. I also know that a photon can strike an electron and there will be a conservation of momentum characteristic of particles (Compton Effect). However, it would seem to me we humans have devised mathematics that (accurately) describe & predict phenomena we experience, but have not gotten to the core of what is going on. I myself can’t envision light being both a particle and a wave. I’d say I’ve never seen "a" photon and might propose electro-magnetism doesn’t really come in particles but instead only acts like particles (although you might argue I’ve never seen anything BUT photons.) In any case, do you know of any theory that marries the two descriptions seamlessly, or could you postulate what a combined particle/wave photon really is? My sincere thanks for your answer. P.S. When scientists use "a photon" in an experiment, do they assume it is a single photon simply because they can measure the characteristic quantum of energy? How are single photons generated in experiments?
- Scott
Denver

Wow, this is an extraordinarily sophisticated set of questions. i'll just try to get you started on the answers.

First, as background, there's nothing special about light in this regard. All 'particles' are represented by fields which obey wave equations, yet under some circumstances show particle-like behavior. Both electrons and neutrons have been shown to exhibit wave-like properties in interference experiments.

As for your final questions, we routinely measure individual photon blips from ordinary light sources using devices such as photomultiplier tubes. For a change, by 'we' I really mean including myself, not just the collective scientific 'we'. So it really is easy and routine. As for preparing single photons, that too can be done, less routinely, with solid-state devices driven by single electrons. Of course you could also think of gamma rays from a radioactive source as single photons.

Now for the core of your question, concerning the interpretation of quantum mechanics. This won't be so easy.  There really is no point in any of these processes where we need to represent the physical state with anything like a classical particle. At every point we can represent the whole thing with a quantum wave. Under circumstances where some large thing (a needle, your brain...) ends up in a different state depending on where that wave goes, we only see one of the possible outcomes- as if the other parts of the wave disappeared. So if you set up some sort of detector which is sensitive to where the wave hits, the outcome we see looks like the wave got localized in one little almost-particle-like region.

How that happens, and whether the rest of the wave disappears or joins other versions of us in other versions of reality remain hotly discussed issues among physicists and philosophers. Meanwhile, the old 'particle-wave' issue as such has pretty much gone away.

Mike W

(published on 10/22/2007)

Is light a wave, or is it made of particles? Is it true that light is both particle and wave? (Wave-particle duality) What is a photon? Is light made of photons? Or is it a wave? If light is both wave and particle,does an electron exhibit wave-particle duality too? [because if it is true...then the wave produced by the electron could by the energy level/shell of an atom - Bohr's model] What is the latest model of an atom - currently accepted?
- Reshma (age 14)
TVM, Kerala, India

These are pretty deep questions to which it's traditional to give sloppy verbal answers. Since it's so easy to find empty words about "wave/particle duality" in all sorts of sources, I'll try to write something a bit more careful and modern, at the risk of sounding technical.

Let' start with the simpler question. The Bohr model has almost nothing to do with modern physics. The Schroedinger picture of an atom captures a lot of the modern picture, especially when confined to the single-electron hydrogen atom. However, the full modern picture of what the electrons are doing in multi-electron atoms is given by quantum field theory, including quantum electrodynamics.

Now for the next question. The whole "wave-particle duality" has little to do with a modern picture of quantum mechanics. In the modern picture, everything (electrons, photons, etc.) is always represented by space-filling fields. Under some circumstances (called "measurements") large scale things (meters, brains,...) end up in different states depending on which way some little thing went. Our experience is always of a single state of large-scale things, for reasons I'll defer discussing for now. Therefore, instead of a whole spread-out wave (say of an electron approaching a CRT screen) your experience captures only one part, say the flash of light that would come from an electron hitting a small part of the screen. That's what motivated people at one time to say that the electron (or light, or whatever) sometimes behaved like a particle. However, there's no known way to consistently describe events like that except as the wave behaving as if it more more or less localized.

So is there any part of the current content of physics that's descended from the "particle" picture? Yes, these waves have a measurable physical variable which takes on possible values of 0,1,2,.... That's very much like counting classical particles, so we call that variable "particle number". However, frequently the waves do not have any single value of the "particle number" but are a superposition of different possible values. That's certainly true for an ordinary electromagnetic wave. That's not the sort of behavior you could imagine for things made up of little particle parts.

Mike W.

(published on 01/06/12)