THE PARTY LINE

By Dennis D. Gagnon

Is Light a Particle?

Some say light is a wave.  Some say it is composed of individual particles.  Some say it is both.  In this post we will look at the logical structure of the historical argument for light being a particle as put forward by Albert Einstein as  based upon the photoelectric effect.  (My last blog, published yesterday, looked at the historical argument from Thomas Young that light is a wave, as based upon his split-beam experiment.)

A passage from my book, The Party Line, discussing Einstein’s interpretation of the photoelectric effect is:  “Einstein’s thinking in this area focused upon the previously observed phenomenon that shining light on the newly cleaned surface of a metal, say zinc, causes the surface of that metal to emit electrons. That phenomenon is what has come to be called the photoelectric effect. The more light shined on the surface, the more electrons emitted. Increasing the intensity of the light of only one frequency (color), say only a very specific frequency of ultraviolet light, increases the number of electrons emitted but not the energy of each electron. Also, even very high intensity light below a particular frequency (color) fails to cause the emission of any electrons at all. It was found that the energy of the emitted electrons is a function of the frequency (or color) of the light and is not a function of the intensity of the incident light beam. This is the phenomenon demanding explanation. This finding was at odds with Newton’s dynamical and Maxwell’s electrical theories, which together erroneously implied that the energy of the emitted electrons should be a function of the intensity of the light beam and not its frequency (color).”

When discussing the validity of physical theories it is important to distinguish the phenomenon requiring explanation from the actual theoretical explanation itself.  Einstein’s explanation of this peculiar behavior of light is based upon his use of the newly developed notion of a quantum of action as developed by Max Planck.  Einstein theorized that if light were composed of particles, where their energy were the quantum of action multiplied by the frequency of that light, then individual particles would impart their energy to individual electrons.  Thus the energy of the emitted electrons would be a function of the frequency (color) of the light, and not the intensity.

Einstein’s reasoning here would look like:

1. If light were a particle, then in producing the photoelectric effect we would see that the energy of the emitted electron is a function of the frequency of light and not the intensity.
2. When we do produce the photoelectric effect we do see that the energy of the emitted electron is a function of the frequency of light and not the intensity.
3. Thus, light is a particle.

But just as was the case for Young’s argument that light is a wave, this form of inference used by Einstein is INVALID!.  Take a look at this argument with the same exact form:

1. If it is someone’s birthday, then we eat cake.
2. We are eating cake.
3. Therefore, it is someone’s birthday.

This is called the Fallacy of Affirming the Consequent (I call it the Birthday Cake Fallacy).  One could be eating cake for any other number of reasons.

So, if using an invalid inference pattern to justify a belief in a theoretical model, such as the picture that light is a particle, is reason to say that that belief has not thereby been justified, then we would have to say that Einstein’s argument does NOT justify the belief that light is a particle.

In my next blog we will look at the argument that light is both a wave and a particle phenomenon.

Read more in my book, The Party Line.