By Don Lincoln, Fermilab
Quantum mechanics claims that, at the subatomic level, objects like electrons and photons are both particles and waves. This isn’t in any way a new debate. Historians can trace it back to the classical Greek period, with early progressive thinkers like Democritus weighing in on the subject. What did he propose? What was his line of thinking and those of others that followed him? When did the wave idea gain momentum?
Democritus
Democritus believed that every known substance was composed of smallest elements, like how grains of sand make up a beach. Democritus coined the word atomos, Greek for uncuttable, from which we get our modern word atom.
Democritus’s ideas about atoms differed greatly from our modern understanding and were quite wrong in detail. Yet, he used his overall guiding principle to infer that light was made of individual, discrete, particles. He didn’t know about electrons, of course, but, if he had, he’d have claimed that they were particles too.
Renée Descartes
A more modern conversation spanned the 17th century, when French polymath Renée Descartes published his book The World, or Le Monde. This book, written in 1630, devised a theory of light where light was a wave that propagated through a substance called the luminiferous aether, or just the aether for short. Aether was thought to be a substance that permeated the entire universe. Its purpose was to conduct light, like water conducts water waves, or things like air conduct sound. Since then, we have learned that Descartes’s aether isn’t real, but that the wave idea has considerable merit.
A few decades later after Descartes’ heyday, Sir Isaac Newton proposed a fairly well-developed theory of light, which was more along the Democritus line of thinking, with particles of light that he called, corpuscles.
No Definitive Data
Newton was one of the greatest scientists of all times and his reputation carried considerable weight. He was known to dabble in alchemy, numerology, and mysticism. His work on motion, light, gravitation, and calculus were all extraordinary and we accept them all even today. However, for all of his well-deserved reputation, not all of Newton’s contemporaries accepted his ideas. He was right about so many things but nobody is right all the time.
Robert Hooke, Newton’s competitor, developed the wave idea, along with Dutch scientist Christiaan Huygens and French physicist Augustin-Jean Fresnel. This debate went on for years, decades, really as there wasn’t definitive data. Researchers of the day could use mirrors and lenses and even prisms to break light into its constituent colors and then combine them again. Thus, although it wasn’t like they knew nothing, none of these experiments were definitive.
In fact, the key point, when it comes to learning how we know what we know about modern physics is always, always, data. Models and theories and conjectures are just ideas. Ideas are powerful things to be sure, and they help us understand the world around us. But ideas are easy. Walk into the fiction section of any library and one can see that. All of those books are ideas and none of them are true. The thing that distinguishes between a flight of fancy and a solid explanation is data. Data is what tells us if a hypothesis is true.
This article comes directly from content in the video series The Evidence for Modern Physics: How We Know What We Know. Watch it now, on Wondrium.
Wave
In order to understand the nuances of this debate, we need to first need to talk about waves. What are they? Well, waves are not discrete objects. They exist over a large area and oscillate up and down. The most familiar form of waves is, of course, water waves, where the height of water raises and lowers in a rhythmic pattern, over and over again. The distance between two adjacent peaks in the pattern is called the wavelength. And, if the wave is passing by us, the amount of time it takes for consecutive peaks to pass, is called the period. The height of the wave is called the amplitude.
There is another, more subtle, feature of waves that is only apparent when we compare two of them and that’s their offset, what physicists call the phase. If two waves are otherwise identical but shifted from one another so that the peaks and troughs don’t line up exactly, the two waves have a relative phase.
Thus, those are the terms that define a wave: wavelength, period, frequency, amplitude, and phase.
Oscillating Electric and Magnetic Fields
Now exactly what is oscillating depends on the specific wave. Obviously, in water waves, it’s the height of water. In sound waves, it’s the air pressure that gets higher or lower, which, of course, means louder or quieter. Light, on the other hand, is made of oscillating electric and magnetic fields and what is changing is the strength of the fields and the direction they are pointing.
It’s much less obvious than a water wave, but when we see light falling on a picturesque meadow, what’s going on at a deep physical level is the whole field is bathed in oscillating electromagnetic fields. That image isn’t as easy for an artist to paint, but it’s the deeper reality.
Common Questions about Quantum Mechanics and the Wave Idea
Democritus used his overall guiding principle to infer that light was made of individual, discrete, particles.
Robert Hooke, Newton’s competitor, developed the wave idea, along with Dutch scientist Christiaan Huygens and French physicist Augustin-Jean Fresnel.
What is oscillating depends on the specific wave. Obviously, in water waves, it’s the height of water. In sound waves, it’s the air pressure that gets higher or lower, which, of course, means louder or quieter. Light, on the other hand, is made of oscillating electric and magnetic fields and what is changing is the strength of the fields and the direction they are pointing.
