This is the first in a series of articles exploring the birth of quantum physics.
We now live in the digital age. We owe the panorama of technological marvels that surrounds us to a hundred physicists who, at the dawn of the 1920sth century, they were trying to understand how atoms worked. Little did they know what their courageous and creative thinking would become a few decades later.
The quantum revolution was a very hard process of letting go of the old ways of thinking, ways that had framed science since the days of Galileo and Newton. These habits were firmly rooted in the notion of determinism: simply put, scientists argued that physical causes have predictable effects, or that nature follows a simple order. The ideal underlying this world view was that nature had a meaning, that it obeyed rational rules, as clocks do. To abandon this way of thinking required enormous intellectual courage and imagination. It is a story that needs to be told many times.
The quantum age was the result of a series of laboratory discoveries during the second half of the 19th centuryth century that refused to be explained by the prevailing classical worldview, a view based on Newtonian mechanics, electromagnetism, and thermodynamics (the physics of heat). The first problem seems simple enough: heated objects emit radiation of a certain type. For example, you emit radiation in the infrared spectrum, because your body temperature hovers around 98° F. A candle glows in the visible spectrum because it’s hotter. The question then is to understand the relationship between the temperature of an object and its glow. To do this in a simplified way, physicists have not studied hot objects in general, but what happens to a cavity when it is heated. And that’s when things got weird.
The problem they described became known as black-body radiation, electromagnetic radiation trapped inside a closed cavity. The black body here simply means an object that produces radiation on its own, with nothing entering it. By studying the properties of this radiation by drilling a hole in the cavity and studying the escaping radiation, it became clear that the shape and material of the cavity does not matter. All that matters is the temperature inside the cavity. Because the cavity is hot, the atoms in its walls will produce radiation that will fill the space.
Physics at the time predicted that the cavity would be filled mostly with highly energetic or high-frequency radiation. But that was not what the experiments revealed. Instead, they demonstrated that there is a distribution of electromagnetic waves within the cavity with different frequencies. Some waves dominate the spectrum, but not those with the highest or lowest frequencies. How can it be?
A quantum pint
The problem inspired the German physicist Max Planck, who wrote in his Scientific autobiography that this [experimental result] it represents something absolute, and since I have always considered the pursuit of the absolute to be the highest goal of all scientific activity, I set to work with enthusiasm.
Planck struggled. On October 1, he announced to the Berlin Physics Society on November 19, 1900, that he had obtained a formula which was well suited to the results of the experiments. But finding the fit wasn’t enough. As he later wrote, “The very day I formulated this law, I began to devote myself to the task of investing it with real physical meaning.” Why does this fit and not another?
Working to explain the physics behind his formula, Planck was led to the radical assumption that atoms do not emit radiation continuously, but in discrete multiples of a fundamental amount. Atoms treat energy as we treat money, always in multiples of a minimal amount. One dollar equals 100 cents and ten dollars equals 1,000 cents. All financial transactions in the United States are in multiples of a cent. For black-body radiation with its many waves of different frequencies, each frequency released refers to a minimum proportional “hundredth” of energy. The higher the frequency of the radiation, the higher its “hundredth”. The mathematical formula for this “hundredth” of energy is E = hf, where E is energy, f is the frequency of the radiation and h is Planck’s constant.
Planck found its value by fitting his formula to the experimental curve of the black body. Radiation of a particular frequency can only appear as multiples of his fundamental “hundredth” of him, which he later called quantum, a word that in late Latin meant a portion of something. As the great Russian-American physicist George Gamow once observed, Planck’s quantum hypothesis created a world where you could have a pint of beer or no beer, but nothing in between.
Planck was far from satisfied with the consequences of his quantum hypothesis. In fact, he spent years trying to explain the existence of a quantum of energy using classical physics. He was a reluctant revolutionary, driven forcefully by a deep sense of scientific honesty to propose an idea with which he was not comfortable. As he wrote in his autobiography about him:
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“My futile attempts to adapt the… how much… somehow to the classical theory have continued for a number of years, and have cost me a great deal of effort. Many of my colleagues saw something bordering on tragedy in this. But I think otherwise… I now knew that the… quantum… played a much more significant role in physics than I was initially inclined to suspect, and this recognition made me see clearly the need for the introduction of totally new analyzes and reasoning in the treatment of atomic problems.
Planck was right. The quantum theory that he helped propound evolved into an even more profound departure from older physics than Einstein’s theory of relativity. Classical physics relies on continuous processes, such as planets orbiting the sun or waves traveling through water. Our entire perception of the world is based on phenomena that continuously evolve in space and time.
The world of toddlers works completely differently. It is a world of discontinuous processes, a world in which rules foreign to our daily experience dictate bizarre behavior. We are effectively blind to the radical nature of the quantum world. The energies we commonly deal with contain such an enormous number of energy quanta, that its “graininess” obscures our ability to see them. It is as if we lived in a world of billionaires, where a penny is an absolutely negligible amount of money. But in the world of the very small, the cent, or the quantum, reigns.
Planck’s hypothesis changed physics and ultimately the world. He couldn’t have foreseen it. Not even Einstein, Bohr, Schrödinger, Heisenberg and the other quantum pioneers. They knew they had found something different. But no one could have predicted the extent to which quantum would change the world.