THE PARTY LINE

By Dennis D. Gagnon

Is Light a Wave?

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 wave as put forward by Thomas Young based upon his Split-Beam experiment.

A passage from my book, The Party Line, discussing Young’s experiment is:  “Young took a beam of light passing through a small hole in a window shade and split it into two parallel beams in close proximity by placing a slim card in the middle of and along the axis of the beam. When he did this, he saw that the beam projected a series of darker and lighter bands on a screen. When he blocked either of the halves of the beam, the image on the screen was a continuous blur of light, just as if he had removed the card and allowed the whole beam to shine upon the screen. The phenomenon of the bands of light appeared only when the beam had been split and both halves were allowed to travel to the screen. This was the physical, observable phenomenon calling out for explanation.”

When discussing the validity of physical theories it is important to distinguish the phenomenon requiring explanation from the actual theoretical explanation itself.  Young’s explanation of this peculiar behavior of light is based upon his findings regarding water and sound waves.  He found that when a (water) wave crest met another crest, then the crest doubled.  Likewise for troughs, when a trough met another trough then they created a deeper trough.  But when a trough met a crest, then they cancelled each other out.  This he called interference.  He reasoned that if light is also a wave, then it should exhibit interference, and this would explain the dark and light bands observed when both sides of the split beam of light are allowed to fall upon the screen.

Young’s reasoning here would look like:

1. If light were a wave, then when performing the split-beam experiment we would see dark and light bands.
2. When we do perform the split-beam experiment we do see dark and light bands.
3. Thus, light is a wave.

But this form of inference 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 wave, is reason to say that that belief has not thereby been justified, then we would have to say that Young’s argument does NOT justify the belief that light is a wave.

In my next blog we will look at Einstein’s argument that light is a particle.

Read more in my book, The Party Line.