This is the first in a series of articles exploring the birth of quantum physics.
We now live in the digital age. The landscape of technological wonders that surrounds us is something we owe to a hundred physicists who, at the dawn of the 20e century, were trying to figure out how atoms worked. They did not know what would become of their courageous and creative thinking a few decades later.
The quantum revolution was a very difficult process of letting go of old ways of thinking, ways that had framed science since Galileo and Newton. These habits were firmly rooted in the notion of determinism – simply put, scientists held that physical causes have predictable effects or that nature follows a simple order. The ideal behind this worldview was that nature had meaning, that it obeyed rational rules, like clocks do. Abandoning this way of thinking took enormous intellectual courage and imagination. It’s a story that needs to be told many times over.
The quantum era is the result of a series of laboratory discoveries during the second half of the 19e century that refused to be explained by the dominant classical worldview, a view based on Newtonian mechanics, electromagnetism and thermodynamics (the physics of heat). The first problem seems simple enough: heated objects emit some type of radiation. 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 is 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 studied not hot objects in general, but what happens to a cavity when it is heated. And this is where things got weird.
The problem they described became known as black body radiation, electromagnetic radiation trapped inside an enclosed cavity. Blackbody here simply means an object that produces radiation on its own, without anything entering. By studying the properties of this radiation by drilling a hole in the cavity and studying the radiation that escaped from it, it became clear that the shape and material of the cavity does not matter. Only the temperature inside the cavity counts. As 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 mostly filled with high-energy or high-frequency radiation. But that’s not what the experiments have revealed. Instead, they showed that there is a distribution of electromagnetic waves inside the cavity with different frequencies. Some waves dominate the spectrum, but not those with the highest or lowest frequencies. How is it possible ?
A quantum pint
The problem inspired German physicist Max Planck, who wrote in his Scientific autobiography that, “This [experimental result] represents something absolute, and since I had always regarded the search for the absolute as the highest goal of all scientific activity, I set to work with alacrity.
Planck struggled. On October 19, 1900, he announced to the Berlin Physical Society that he had obtained a formula which corresponded well to the results of the experiments. But finding the fit was not enough. As he would later write, “the very day I formulated this law, I began to devote myself to the task of giving it real physical meaning.” Why this cup and not another?
In working to explain the physics behind his formula, Planck was led to the radical assumption that atoms do not give off radiation continuously, but in discrete multiples of a fundamental quantity. Atoms deal with energy like we deal with money, always in multiples of a smaller amount. One dollar equals 100 cents and ten dollars equals 1,000 cents. All financial transactions in the United States are in multiples of one cent. For blackbody radiation with its many waves of different frequencies, each frequency emitted corresponds to a minimum proportional “hundred” of energy. The higher the frequency of the radiation, the higher its “hundred”. The mathematical formula for this “minimum penny” of energy reads as follows: E = hf, where E is the 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 blackbody curve. Radiation of a particular frequency can only appear as multiples of its fundamental “hundred”, which he later called quantum, a word which in late Latin meant a portion of something. As the great Russian-American physicist George Gamow once remarked, Planck’s quantum hypothesis created a world in which you could either drink a pint of beer or no beer at all, 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, forcefully driven by a deep sense of scientific honesty to come up with an idea he was not comfortable with. As he writes in his autobiography:
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“My vain attempts to integrate the… quantum… somehow into the classical theory continued for a number of years, and they cost me a great deal of effort. Many of my colleagues saw it as something bordering on tragedy. But I feel differently about it…I now knew that…quantum…played a much bigger role in physics than I had originally been inclined to suspect, and this recognition made me see clearly the need for introduce completely new methods of analysis. and reasoning in dealing with atomic problems.
Planck was right. The quantum theory he helped propose evolved into an even deeper departure from ancient physics than Einstein’s theory of relativity. Classical physics is based on continuous processes, such as planets orbiting the Sun or waves propagating on water. Our whole perception of the world is based on phenomena which evolve continuously in space and time.
The world of toddlers works in a completely different way. It is a world of discontinuous processes, a world where rules foreign to our daily experience dictate bizarre behavior. We are effectively blind to the radical nature of the quantum world. The energies we currently deal with contain such enormous numbers of energy quanta that their “granularity” obscures our ability to see them. It’s as if we live in a world of billionaires, where a penny is a perfectly 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 could not have foreseen this. Neither did Einstein, Bohr, Schrödinger, Heisenberg and the other quantum pioneers. They knew they had found something different. But no one could have foreseen how quantum would change the world.
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