How Max Planck Gave Birth to the Concept of Quanta


By Steven Gimbel, Ph.D., Gettysburg College

An anomaly in classical physics known as the blackbody radiation had befuddled scientists for a number of years in the late 19th century and early 20th century. The existing knowledge about the nature of light suggested a particular outcome, but in practice this wasn’t realized. This anomaly gave birth to what was known as the ultraviolet catastrophe.

Scattered ultraviolet light
Ultraviolet and infrared light scattering from a point source. (Image: Wacoma/Shutterstock)

What Is Ultraviolet Catastrophe?

The hypothesis was that if a hollow black metal sphere is heated, the metal will eventually start to glow as it gets hot and emits light in the form of electromagnetic radiation. The sphere being hollow, a part of the light will be emitted inside the sphere as well. Since the inner walls of the sphere are black as well, the light will be completely absorbed by the walls.

As the inner walls absorb more energy it will begin emitting light as well. This light, in turn, will be absorbed by the same walls, resulting in more energy being absorbed by them. A closed-loop will be formed and this process will continue. The classical physics theory predicted that the amount of energy inside the sphere will tend to infinity.

Thus in theory, if a hole were to be poked in such a sphere the blast of energy released should destroy any and everything in its path. Obviously, that’s not what happens. This experiment could be easily carried out at the time and the observation was that at any given temperature the energy would reach a peak and then drop off completely to zero. This happened when the ultraviolet range of the electromagnetic radiation spectrum was reached. This is the ultraviolet catastrophe.

Who Was Max Planck?

Nobel prize-winning German theoretical physicist Max Planck played a key role in the origin of quantum theory. Before going any further we need to learn a little more about Planck to understand why his approach to finding a solution for the ultraviolet catastrophe is considered radical and how that had a major impact on future theoretical physics.

He was what historian and philosopher of science, Thomas Kuhn described as a ‘normal scientist,’ who works within a paradigm and is reticent to change it. Planck didn’t aim to start a revolution or disturb the scientific principles and beliefs of that period.

Portrait of Max Planck. by an unknown photographer from 1933.
Portrait of Max Planck in 1933. He was 75 years old at the time. (Image: Unknown/Public domain)

This is starkly evident when the scientific approaches of Planck and his dear friend Albert Einstein are compared. They had polar opposite personalities. Einstein was a staunch anti-authoritarian and strongly against the emphasis on memorization within the German educational system. On the other hand, Planck thrived in this very environment. Hence, it’s ironic that it was the stature and influence of Planck that provided Einstein with legitimacy in the scientific community of that period.

Planck was meticulous in every aspect of his life. He planned every day to the last minute, which included practicing the piano. He did all his scientific work while standing at a tall accountant’s desk. Planck was soft-spoken, but he chose his words with the utmost care. The goal was to not waste any time or sentiment. His approach towards clothing, political beliefs, or ideology, as well as theoretical physics, was distinctly conservative. This conservative streak was put to pause when he decided to tackle the ultraviolet catastrophe with a radical method.

Butter Versus Eggs

Typically, scientists develop a theory and come up with certain predictions based on that theory. Then they match the predictions against the actual observations of the experiment to either dismiss the theory or further refine it. When faced with the discrepancy of the blackbody radiation and the resultant ultraviolet catastrophe, Planck decided to reverse engineer a solution.

He began work on determining the mathematical equation that would satisfy the empirically generated curve. What he discovered was that in order to solve this problem light emitted and absorbed by the hollow black sphere had to be considered as coming in packets instead of waves. Planck named these packets quanta.

In simple words, quantifying something means attributing it with a numerical representation. If, however, something is quantized then it’s given a numerical representation that can only be a whole number.

This is a transcript from the video series Redefining Reality: The Intellectual Implications of Modern Science. Watch it now, on Wondrium.

The easiest way to understand this is to think in terms of butter and eggs. The physical properties of butter allow you to purchase any amount of it. That amount is given a numerical value, but it doesn’t have to be a whole number. On the other, eggs can only be bought in whole numbers. You can’t buy half an egg or quarter of an egg. Hence, eggs are quantized. You can buy as many eggs as you please, but it will have to be a whole number.

Learn more about whether light is a wave or particle.

So, the reverse engineered mathematical equation Planck came up with suggested that emitted and absorbed light was, in fact, similar to eggs and not butter. However, this did not fit into Planck’s own conviction that all energy and matter was continuous and smooth.

Einstein Refines Planck’s Theory

The year that Einstein developed his theory of relativity, he was also working on a solution for the photoelectric effect. When ultraviolet light is directed towards metal, the electrons in it are expelled. It was already known that electrons in metal have a relatively weak bond to their respective atoms. Plus, the established view of the time considered light to be smooth waves. Hence, the light waves directed towards the metal would cause its surface to vibrate.

Image of Albert Einstein giving a lecture in Vienna.
Albert Einstein giving a lecture in Vienna, Austria in 1921. (Image: F. Schumtzer/Public Domain)

Learn more about how Einstein solved the general theory of relativity.

In elementary school, you must have watched your teacher demonstrate the experiment with two tuning forks or even done it yourself. When one tuning fork is struck and brought near a second tuning fork, the second starts vibrating as well. So if light, like sound, is a wave then it would have a similar effect on the metal. And the vibrations could release some of the loosely bound electrons.

The problem arose when brighter light was directed towards the metal. If the light is a wave, then brighter light would mean the wave is bigger. And the effect on the vibrating metal would be the release of electrons with greater velocity and greater energy. However, that’s not what happened. A greater number of electrons were emitted, but the velocity or energy didn’t increase.

Einstein applied Planck’s concept of light emitted and absorbed as quanta or particles instead of waves to the photoelectric effect. Now if the light is brighter, it would mean that more quanta of light is being directed at the metal. Note that Einstein is working on the assumption that light is moving at a constant speed. So, more quanta of light would release more electrons from the metal. And because the light is moving at a constant speed the electrons should be released at a constant velocity or energy as well.

By applying Planck’s theory of light being quantized to the photoelectric effect, Einstein successfully demonstrated that it was in fact reality. Light is like eggs and not butter, but only in some cases. Light behaves like a particle only when it’s emitted or absorbed. While it exhibits perfect wave behavior when it travels. So, the question remained – is light a particle or a wave? These are essentially two completely separate states of being. Does light exist in both states?

Common Questions About Max Planck and Quanta

Q: How Long Is Planck Time?

The Planck time, which is also known as the ‘quantum of time’, is the time taken by a photon traveling at the speed of light to traverse a distance equal to the Planck length. This is the shortest measurement of time with any meaning. Time cannot be divided any further. The value of Planck time is 10-43 seconds.

Q: How Many Max Planck Institutes Are There?

The Max Planck Society for the Advancement of Science (MPG) was originally known as the Kaiser Wilhelm Society. It was founded in 1911 and is a non-governmental research institute. Max Planck served as the president of MPG between 1930 and 1937. After his death in 1947, the institute was renamed the Max Planck Society. Currently, there are 83 institutions under MPG.

Q: When Did Max Planck Win the Nobel Prize?

Max Planck received the Nobel Prize in Physics in 1918 for his discovery of energy quanta. This discovery is one of the cornerstones of modern quantum mechanics.

Q: What Is the Best Definition of Quanta?

The simplest definition of quanta is it’s the smallest amount of physical entity that exists independently and can only be denoted in terms of discrete whole numbers.

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