Max Planck changed physics and our understanding of the world forever when he discovered that hot objects do not radiate a smooth, continuous range of energies as had been assumed in classical physics. Instead, he found that the energies radiated by hot objects have distinct values, with all other values forbidden. This discovery was the beginning of quantum theory – an entirely new type of physics – which replaced classical physics for atomic scale events.
Quantum theory revolutionized our understanding of atomic and subatomic processes, just as Albert Einstein’s theories of relativity revolutionized our understanding of gravity, space and time. Together these theories constitute the most spectacular breakthroughs of twentieth-century physics.
Of high intelligence, showing brilliance in mathematics, science and music, Planck was a deeply thoughtful, ethical man. He experienced a long life, living almost 90 years. In his later years he lived in Germany through the great depression and both world wars, suffering a succession of personal tragedies.
Max Karl Ernst Ludwig Planck was born in Kiel, on the north coast of Germany, on April 23, 1858. He had five older siblings.
His father, Johann Planck, was a law professor who came from an academic family. Max’s mother’s name was Emma Patzig. Her father was an accountant. Emma was lively and well-liked in the academic circles Max’s family moved in.
Max attended elementary school in Kiel. In 1867, when he was 9 years old, his family relocated over 500 miles to Munich in southern Germany, where his father had been offered a tempting professorship.
Max was enrolled at the Maximilians Gymnasium – a school for academically able children. As the years passed one of his teachers, the mathematician Hermann Müller, noticed that Max was rather gifted mathematically, so he offered him extra lessons in astronomy and mechanics. Max accepted the offer enthusiastically, and Müller taught his receptive young student how to visualize the laws of physics in his mind – a vital weapon in the armory of the great physicists.
Frequently it happens that students who are talented mathematically are also talented musically, and this was the case with Max Planck, who composed classical music, had perfect pitch, and played the cello and piano expertly. As if that were not enough, he also had a beautiful singing voice.
Before he left high school, Planck decided he would pursue science as a vocation while music would remain an enjoyable hobby. He would later recall why he chose to become a man of science:
“as a direct result of the discovery that pure reasoning can enable man to gain an insight into the mechanism of the world about us.”
University and a Ph.D. at the age of 21
In 1874, aged 17, a freshman at the University of Munich, Planck spoke to professor Philipp von Jolly at about the merits of studying physics. Jolly famously replied:
Undeterred, Planck chose to study physics. One day he would personally find startling evidence to prove the absurdity of his professor’s beliefs about the future of physics. In fairness to Philipp von Jolly – and although it’s hard to believe given the rapid march of science and technology today – many of the physicists of that era shared Jolly’s view: they believed they had already discovered and understood most of what was there was in the universe to be discovered and understood!
At university Planck discovered that he did not enjoy doing experiments much. His mathematical talent found its natural home in the world of theoretical physics.
He continued enjoying music, singing in the university choir and composing a mini-opera.
An Important Vacation
During the spring vacation of 1877, close to his twentieth birthday, Planck embarked on a hiking tour in northern Italy with friends from his university, including the mathematician Carl Runge. While walking, the students discussed science, mathematics and their views of the world.
Runge raised a question about whether Christianity and religion did more harm than good in the world – a question that shocked Planck, who had received a traditional Lutheran upbringing. Planck began to question his personal view of the world. He remained a Lutheran throughout his life. He rejected atheism, but became very tolerant of alternative philosophies and religions.
Berlin and Thermodynamics
In the winter semester of 1877, aged 20, Planck transferred for a year to Berlin’s Friedrich Wilhelms University where he was taught by two of the giants of physics – Hermann von Helmholtz and Gustav Kirchhoff.
In Planck’s opinion, each of these renowned men of science delivered lectures distinguished only by their dreariness.
Nevertheless, he and Helmholtz became great friends. Planck admired – indeed almost worshiped – Helmoltz for his scientific integrity, honesty, kindness, modesty and tolerance.
One of Helmholtz’s passions in physics was thermodynamics – the study of the relationships between temperature, heat, energy and work. Planck grew increasingly fascinated by thermodynamic theory.
He began his own program of work in the field, spending endless hours poring over papers written by Rudolf Clausius, one of thermodynamics’ founders.
Unlike the lectures he attended, he found Clausius’s work to be interesting, well-delivered and clear.
The Highest Honors and a First Job
After his year in Berlin, Planck returned to Munich in late 1878 where he passed his state exam allowing him to teach physics in high schools.
A few months later, in February 1879, he submitted a doctoral thesis concerning the second law of thermodynamics. Three months later he defended his thesis in an oral examination and – aged 21 – was awarded a Ph.D. in physics with the highest honors – summa cum laude.
Funnily enough, from the questions he was asked during his thesis defense, Planck drew the conclusion that none of the professors who interrogated him had any worthwhile understanding of his thesis!
A year later Planck successfully submitted a further thermodynamics thesis for his habilitation – a much more demanding qualification than the Ph.D., which allowed its holder to become a professor if such a job became available.
At the age of 22, Planck became a physics lecturer (unpaid) at the University of Munich. Without any salary, he continued living with his parents. His research focused on entropy – a quantity sometimes defined in a loose sense as a measure of the amount of disorder at the atomic level.
A Return to his Birthplace, then back to Berlin
Finally, almost on his 27th birthday, Planck became an associate professor of theoretical physics at the University of Kiel, where he probed ever deeper into the complex world of thermodynamics. He continued making progress in this difficult field, but made no major breakthroughs.
At the age of 31, in April 1889, Planck returned to Berlin to take over the lecturing duties of Gustav Kirchhoff, who had died in the fall of 1887.
In 1892 Planck became a full professor of theoretical physics. By all accounts his students found his lectures much more interesting than Planck had found his predecessor’s. One of his students, the British chemist James Partington, described Planck’s lectures:
“using no notes, never making mistakes, never faltering; the best lecturer I ever heard. There were always many standing around the room. As the lecture-room was well heated and rather close, some of the listeners would from time to time drop to the floor, but this did not disturb the lecture”.
Two of Planck’s Ph.D. students would later win Nobel Prizes in physics: Max von Laue and Walther Bothe.
The scene was now set for Planck’s momentous discovery – the discovery that would totally transform the way we think about our world – quantum theory.
Max Planck’s Contributions to Science
Most theoretical physicists make their mark when they are young. Max Planck was 42 when he finally left an indelible mark on the world.
The problem he solved in 1900 was prompted by puzzlement over the electromagnetic spectrum emitted by hot objects.
Classical Physics Disagreed with Reality
When things get hot they radiate energy. For example, if you were to observe a blacksmith heating a horseshoe, you’d notice that when the shoe gets hot it glows a red color, and when it gets even hotter it glows white.
Physicists considered the case of a black body – a body which absorbs all electromagnetic radiation that falls on it. When it is heated, a black body radiates energy in the form of electromagnetic waves. These waves have a broad range of wavelengths such as visible, ultraviolet and infrared light.
BUT, in the 1800s people had noticed that the colors of light actually radiated in experiments did not agree with those predicted by theory. In scientific language, there was a mismatch between the wavelengths radiated by hot objects and the wavelengths predicted by classical theories of thermodynamics.
The graph below shows the problem. The black curve shows the predicted behavior of a black body at a temperature of 5000 K. The blue line shows the actual behavior.
Black-body Radiation Intensity vs Wavelength
In order to match theory with observations Planck made a revolutionary proposal. If you’re not already familiar with quantum theory, to understand what he proposed, it might help to think about a times table – for example the three times table – 3, 6, 9, 12, 15… in which only numbers divisible by 3 are allowed and all other numbers are forbidden.
Planck’s idea was that energy is emitted in a similar manner. He proposed that only certain amounts of energy could be emitted – i.e quanta. Classical physics held that all values of energy were possible.
This was the birth of quantum theory. Planck found that his new theory, based on quanta of energy, accurately predicted the wavelengths of light radiated by a black body.
Planck found that the energy carried by electromagnetic radiation must be divisible by a number called Planck’s constant, represented by the letter h. Energy could then be calculated from the equation:
where E is energy, h is Planck’s constant, and ν is the frequency of the electromagnetic radiation. Planck’s constant is a very, very small quantity indeed. Its small size explains why the experimentalists of the time had not realized that electromagnetic energy is quantized. To four significant figures, Planck’s constant is 6.626 x 10-34 J s.
Planck had not intended to overthrow classical physics. His intention was to find a theory that matched experimental observations. Nevertheless, the implications of his discovery were momentous. Quantum theory – the realization that nature has ‘allowed’ and ‘forbidden’ states – had been born and the way we interpret nature would never be the same again.
Planck was awarded the 1918 Nobel Prize in Physics for:
“the services he rendered to the advancement of Physics by his discovery of energy quanta.”
Planck himself would later write:
Some Personal Details and the End
In March 1887, aged 28, Planck married Marie Merck. The couple had four children: Karl, Grete, Emma and Erwin.
Tragically, Planck would live to see the death of his wife and all of their children. His wife, Marie, died in 1909 from tuberculosis. Karl was killed in battle in 1916 during World War 1. Grete died in childbirth in 1917, then Emma died also in childbirth in 1919. (Their babies survived.) Erwin was executed by the Nazis in 1945 for his suspected part in a plot to kill Adolf Hitler.
Two years after the death of his first wife, Planck married Marga von Hösslin. They had one son, Hermann. Both Marga and Hermann outlived Planck.
Like the famous mathematician David Hilbert, Planck was rather old (74 years of age) when the Nazis came to power in 1933, and he continued living in Germany under the Nazis. Hilbert and Planck deplored the Nazi’s behavior and their policies.
Planck had been one of the first scientists to recognize the brilliance of Albert Einstein’s work. He cleared the way for Einstein to move to Berlin to become a professor there in 1914. Later the two would meet up and enjoy themselves tremendously playing music together.
When the Nazis took control of Germany, Planck was distressed by the need for Einstein and increasing numbers of Jewish scientists to flee from Germany. In 1938 the Nazis took over the Prussian Academy. Planck resigned as the Academy’s president.
At all times, the elderly Planck remained patriotic to Germany, walking a moral tightrope, hoping that the Nazis would come to their senses and act in a way befitting a proper German government. His hopes were increasingly dashed, culminating with the execution of his son Erwin for ‘treason’ in January 1945.
Early in 1944, Planck’s home in Berlin had been flattened in an allied air raid. All of his personal papers and scientific records were destroyed.
When the war in Europe ended in May 1945, Planck, his wife, and his remaining son Hermann found refuge with a relative in the famous German university town of Göttingen. It was there that two years later Max Planck died, aged 89, on October 4, 1947. Today he lies buried in Göttingen’s old City Cemetery. Marga’s and Hermann’s graves lie beside his.
In 1948, Germany’s Kaiser Wilhelm Society was renamed, becoming The Max Planck Society as a tribute to the man who had held its presidency twice and who had given birth to quantum theory. Today The Max Planck Society is one of the most successful scientific organizations in the world, running over 80 scientific institutions. Since the 1950s research workers from the Max Planck Institutes have been awarded four Nobel Prizes in physics, eight in chemistry, and six in medicine.
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"Max Planck." Famous Scientists. famousscientists.org. 17 May. 2016. Web. <www.famousscientists.org/max-planck/>.
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