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.
Beginnings
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 enrolled at the Maximilians Gymnasium – a school for academically able children. One of his teachers, the mathematician Hermann Müller, noticed 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 age 21
In 1874, age 17, and now a freshman at the University of Munich, Planck spoke to Professor Philipp von Jolly about the merits physics. Jolly famously replied:
“In this field [physics] almost everything is already discovered, and all that remains is to fill a few insignificant gaps.”
Undeterred, Planck chose to study physics. One day he was destined to find evidence to prove the absurdity of his professor’s beliefs. In fairness to Philipp von Jolly – and although it’s hard to believe today given the rapid march of science and technology – many physicists of that era shared Jolly’s view: they believed they had already discovered and understood most of what there was in the universe to be discovered and understood!
At university Planck discovered he did not enjoy experimental work. His mathematical talent found its natural home in the world of theoretical physics.
He continued to enjoy music. He sang in the university choir and composed 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 university friends including the mathematician Carl Runge. While walking, the students discussed science, mathematics, and their views of the world.
Lake Como in northern Italy, one of the places Max Planck and his friends walked. Hiking amid spectacular scenery became one of Planck’s lifelong pleasures.
Runge raised a question about whether Christianity and religion did more harm than good – 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 and rejected atheism, but became very tolerant of alternative philosophies and religions.
Berlin and Thermodynamics
In the winter semester of 1877, age 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 – age 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 understood 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 age 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 more deeply into thermodynamics. He continued making progress in this difficult field, but made no major breakthroughs.
At age 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 – 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 Disagrees 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.
Hot metal glows, emitting electromagnetic radiation.
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 noticed the colors of light 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
Compare the curve expected from classical thermodynamic theory at a temperature of 5000 K (black line) versus that observed in experiments (blue line). They are very different! Also shown in green and red are curves at somewhat lower temperatures.
Quantum Theory
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 the energy carried by electromagnetic radiation must be divisible by a number now 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:
“…it seemed so incompatible with the traditional view of the universe provided by Physics that it eventually destroyed the framework of this older view. For a time it seemed that a complete collapse of classical Physics was not beyond the bounds of possibility; gradually, however, it appeared, as had been confidently expected by all who believed in the steady advance of science, that the introduction of Quantum Theory led not to the destruction of Physics, but to a somewhat profound reconstruction…”
The Planck Scale
The Planck Scale was born in 1899. It replaced the Earth-centered measurement system of:
- a kilogram – the mass of a liter of water
- a meter – one ten-millionth of the distance from the North Pole to the Equator
- a second – 1⁄86400 of an Earth day
with new, universal units based on:
- the speed of light
- the Planck constant
- the gravitational constant
In the following century Stephen Hawking discovered that the Planck scale really does measure something fundamental about nature, showing that the smallest possible black hole has a mass of 1 Planck mass unit, a Schwarzschild radius of 1 Planck length unit, and a half-life of 1 Planck time unit.
Moreover, Jacob Bekenstein found that when any black hole takes in a single elementary particle containing 1 bit of information the area of the event horizon increases 1 square Planck length, revealing a remarkable link between the Planck scale and information.
Some Personal Details and the End
In March 1887, age 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 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 was 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.
“In 1905, when Einstein, than an unknown employee of the Swiss patent office in Bern, sent five revolutionary papers to the physics journal that Planck edited in Berlin, Planck immediately recognized them as works of genius and published them quickly without sending them to referees.”
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 was flattened in an allied air raid. All 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, age 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 held its presidency twice and gave 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.
“The laws of Physics have no consideration for the human senses; they depend on the facts, and not upon the obviousness of the facts.”
“[Planck’s] discovery became the basis of all twentieth-century research in physics and has almost entirely conditioned its development ever since. Without this discovery it would not have been possible to establish a workable theory of molecules and atoms and the energy processes that govern their transformations. Moreover, it has shattered the whole framework of classical mechanics and electrodynamics and set science a fresh task: that of finding a new conceptual basis for all of physics.”
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Further Reading
Max Planck
The universe in the light of modern physics
W.W. Norton & Company, Inc., New York, 1931
R. E. Oesper
James R. Partington (1886 – )
J. Chemical Education, Vol 15, 11, Nov. 1938 p 501
Max Planck
A survey of physical theory, translated by R. Jones and D. H. Williams
New York: Dover, 1960
Max Planck
Scientific Autobiography and Other Papers
Philosophical Library, 1968