**Max Planck was notoriously unphotogenic. I mean just look at him, as a viewer said he looks like he had his humor surgically removed. But according to the people that knew him, Planck was actually a lovely person full of humor, energy, and kindness. As Albert Einstein put it in 1918, “living next to Planck is a joy.[1]” So who was Max Planck, and why and how did he revolutionize science? Ready? Let’s go!… **

Max Planck was born Karl Ernst Ludwig Marx Planck (whew) in Gottingen, Germany in 1858. Planck came from a family of scholars and theologians and always had an interest in science and music. He fondly recalled how he was inspired by his high school teacher who graphically described the first law of thermodynamics (that energy is conserved) by having his students imagine the energy in a heavy stone lifted onto a roof that one day might be, “loosened and drop on the head of some passerby.[2]” Planck felt that it was a “revolution” to find laws that “possess absolute, universal validity, independently from all human agency.[3]” He decided that “the quest for laws…. appeared to me as the most sublime scientific pursuit in life,[4]” and went to study Physics at the University of Munich. At Munich, he was told by a professor to drop Physics because “all the fundamental laws had already been discovered” but Planck ignored him and after three years at Munich, he moved to the University of Berlin, where some of the top scientists in the world were located, specifically Hermann von Helmholtz and Gustav Kirchhoff. Planck liked Helmholtz as a person but found his lectures to be miserably bad, so much that his classes dwindled to just three students, including Planck, who seemed to stay out of politeness. Kirchhoff was not much better and Planck realized that “my only way to quench my thirst for advanced scientific knowledge was to do my own reading on subjects which interested me; of course, these were the subjects relating to the energy principle.[5]” That is how he read the works of another German scientist named Rudolf Clausius. Clausius had created the idea of entropy in 1854, and the name entropy in 1865, and had defined the second law of thermodynamics to be: “the entropy of the universe tends to a maximum”. Planck was as excited about the second law as he was about the first and even wrote his Ph.D. on entropy in 1879 (when he was just 21 years old) although Helmholtz and Kirchhoff were unimpressed: “Helmholtz probably did not even read my paper… Kirchhoff expressly disapproved of its contents…[and] Clausius did not answer my letters. [6]” Undeterred, Planck dedicated his research for the rest of his life to the study of energy and entropy.

While in Munich, Max Planck fell in love with Marie Merck, the younger sister of one of his friends. He waited until he got a professorship before proposing. And waited. And waited. In his autobiography, Planck recalled, “I waited for years in vain for an appointment to a professorship… [and nursed] the desire to win, somehow, a reputation in the field of science.[7]” Six years later he finally lucked into a position at the University of Kiel (probably with the assistance of his father’s friend)[8]. In 1887, he finally married Marie Merck whereupon she had four children in rapid succession, (Karl in 1888, twins Emma and Grete in 1889, and Erwin in 1893). Planck was a devoted husband and father and preferred above all spending time with his family playing music or going on marathon hikes. He wrote to a friend, “How wonderful it is to set everything else aside and live entirely within the family.[9]” Planck’s former student Lise Meitner recalled, “Planck loved merry, relaxed company and his home was the center of such conviviality.[10]” She added that, even years later, Planck would play tag with his kids and students in the yard, “Planck participated with downright childish glee and great adeptness. It was almost impossible not to be tagged by him. And how visibly pleased he was when he had caught someone![11]”

About the same time as the twins were born, Gustav Kirchhoff died and Planck was offered his position at the University of Berlin, where Planck stayed for the rest of his career. In Berlin, Planck became interested in using entropy to explain the experimental results of an experiment created thirty years earlier by the same Kirchhoff called blackbody radiation[12]. Planck recalled, “It was an odd jest of fate that… the lack of interest of my colleagues in [entropy] now turned out to be an outright boon. While a host of outstanding physicists worked on the problem… nobody paid any attention to the method adopted by me.[13]” It took Planck 6 years, but in 1900 he triumphantly published his results. But then a friend of Planck’s found that at relatively low energies, Planck’s equation didn’t work and Planck quickly made up a new equation, “which, as far as I can see at the moment, fits the observational data.[14]” Planck was happy his new equation worked but was distressed that it had no theoretical justification. So, in “an act of desperation” Planck used an idea he hated, Ludwig Boltzmann’s statistical method of looking at molecules and at entropy. In 1877, Boltzmann created an idea of “probability”, W, that depended on the “state distributions” of the energies of the molecules and stated it was related to the entropy. Previously, Planck didn’t like treating the second law, “merely as a law of probabilities.[15]” However, Planck was out of options, and used the following logic to relate entropy S with Boltzmann’s idea of probability, W: “Since the entropy S is an additive magnitude but the probability W is a multiplicative one, I simply postulated that S = k log W, where k is a universal constant.[16]” But then Planck had a problem. If the energy is, “considered to be continuously divisible quantity, this distribution is possible in infinitely many ways,[17]” which would make the entropy infinite! For that reason, Planck constrained the energy to be created in little energy packets with energy equal to a constant, h, times the frequency. Then he used the results from the blackbody experiments and got values for the constants k and h that are within 2% of the currently accepted values! Planck was very happy about his results, especially with his definition of the constant k (currently called Boltzmann’s constant), because with an accurate measure of k, Planck could derive nearly exact values for other constants, like the mass of a hydrogen ion and the charge on an electron. However, he didn’t think much of his equation making the energy into little packets where the “energy packets” or photons as we now call them, have quantized energy. Quantizing energy was, according to Planck, “a purely formal assumption, and I did not give it much thought.[18]” Planck did, however, try his best to “weld the elementary quantum of action h somehow into the framework of the classical theory. But in the face of all such attempts, this constant showed itself to be obdurate.[19]”

The rest of the scientific world politely ignored Planck’s startling claim and most believed in a competing theory that used statistical mechanics but didn’t require energy to be in little packages but predicted that as you went to the ultraviolet range, the radiation went up exponentially, a situation poetically called the ultraviolet catastrophe! Then, in March of 1905, Planck was the editor of Germany’s major scientific publication, Annalen der Physik, and received a paper by an unknown scientist named Albert Einstein that took Planck’s “formal assumption” of light being composed of energy packets as being a profound statement about the nature of light and used it to explain a variety of phenomenon, including the photoelectric effect. Surprisingly, Planck was completely unimpressed with Einstein’s development of his idea although he let the paper be published. A few months later, Planck received another paper from Einstein, this one on special relativity, and Planck was entranced.[20] In fact, Planck became the first person to give a public lecture about Relativity (crediting Einstein of course) and dedicated his research for the next several years to relativity theory, including proof that relativity could be derived from principles of least action. Years later Einstein recalled, “It is largely due to the determined and cordial manner in which [Planck] supported this theory that it attracted notice so quickly among my colleagues in the field.[21]” Planck and Einstein started writing letters to each other and Planck encouraged Einstein to focus on relativity and basically drop all the quantum stuff, a request that Einstein ignored at the time, working on both relativity and quantum ideas as well as working 6 days a week at the patent office. For example, in 1907, Einstein wrote 8 papers including one that used Planck’s idea of energy elements to understand how solids emit and absorb heat, *and* his first paper on general relativity!

By August 1909, Planck invited Einstein to come to Berlin and give his first public talk. Planck was probably hoping that Einstein would talk about relativity but, instead, he spoke about Quantum Mechanics once again pushing “a theory of light that can be understood as a kind of fusion of the wave and particle theories.” Years later, a member of the audience recalled that this talk went nowhere as “the chairman of the meeting was Planck, and he immediately said that it was very interesting but he did not quite agree with it.[22]”

A few months after Einstein’s first talk, tragedy hit the Planck household, Planck’s wife of 23 years, Marie Planck, died from tuberculosis. Max Planck buried his wife and told his cousin that her grave is where “my lost happiness is sleeping[23]”. Planck was 51 years old and his children were 21, the twins were 20 and the youngest was just 16 years old. Planck’s close relationship with his children became even closer, especially with his youngest, Erwin. Within a year and a half, however, Max Planck’s children were shocked when he married their cousin (his former wife’s 28-year-old niece) Marga von Hosslin. Marga continued to occasionally call her husband “uncle Max” even after the marriage (blech) but the marriage seemed to have been more than platonic as Marga Planck gave birth to Max Planck’s fifth child, Hermann, in December of 1914.

Meanwhile, in 1906, a German scientist named Walther Nernst, came up with an astonishing idea while trying to create nitrogen-based fertilizer: Nernst decided that two Chemistry terms become identical asymptotically at absolute zero which seemed to imply that the entropy would also asymptotically go to zero at absolute zero temperature (although Nernst hated talking about entropy). Within months this “theorem” was being referred to as a “Third law of Thermodynamics[24]”. Planck became the first theoretician to support Nernst’s theories and by 1910, Max Planck proved that Nernst’s initial thought that the entropy would always go to zero at absolute zero temperature wasn’t true if an object was a mixture of more than one component, which rewrote the third law to be: “The entropy of a pure solid or pure liquid approaches zero at 0 Kelvin”. A few years later, Nernst created his final version of the third law of thermodynamics: “It is impossible for any procedure to lead to … t=0 in a finite number of steps. ^{[25]}” Scientists to this very day are still arguing about the proper form of the law.

Anyway, in the spring of 1910, Nernst read Einstein’s 1907 paper about using quantum mechanics to study solids and realized that it could be used as a theoretical validation of his “heat theorem”. By this time, Einstein had finally gotten a low-level job as a professor in Zurich, and in March of 1910 Nernst went to visit the young scientist and see for himself if Einstein was a genius or a crackpot. Nernst was impressed with Einstein and within weeks Nernst started talking with a wealthy soda magnet named Ernest Solvay about creating a conference on Quantum issues staring Einstein. Nernst then sent a letter to Max Planck detailing the whole plan but Planck was skeptical that the time was right, writing, “Such a conference will be more successful if you wait until more factual material is available. [26]” However, Planck added, that “whatever is done in this regard I shall take the greatest interest, and I promise my fullest collaboration in any project of this kind. Because I can say without exaggeration that for ten years nothing in physics, without a break, has absorbed, excited, and attracted me as these quanta of action.[27]” Nernst ignored the request to delay and personally set up the first international scientific meeting, called the Solvay Conference beginning in October of 1911. Neither Planck nor Nernst knew that Einstein was less pleased about the conference than Planck was. He had been trying to meld quantum ideas to Maxwell’s equations to no avail, and wrote a friend, “the more success the quantum theory has, the sillier it looks. How non-physicists would scoff if they were able to follow the odd course of developments. [28]” But Planck and Nernst were too famous to refuse so he put together what he told his friend was some “twaddle[29]” for the conference. Einstein thought the conference was a disaster but the rest of the attendees were fascinated. Both Planck and Nernst realized that Einstein was a science superstar and they quickly started a push to get him to Berlin. By July of 1913, Max Planck and Walther Nernst traveled personally to Zurich to offer Einstein a plum position at the University of Berlin, which he eagerly accepted so that he could skip teaching and he could be near his cousin with whom he was having an affair (the next year Einstein separated from his wife, Mileva, and in 1919 he officially divorced Mileva and married his cousin Elsa). When Einstein moved to Berlin, he and Planck became incredibly close friends and would spend many hours playing music together.

At the same time that Planck and Nernst were offering Einstein a job, a young Danish man named Niels Bohr heard about the Solvay Conference and it inspired him to create a new model of atoms using Planck’s constant, h. Bohr’s paper hit the scientific community like a thunderbolt. Scientists had known since the mid-1800s that hot gasses don’t make rainbows but instead make distinctive frequencies of light, in what is called spectral lines or spectroscopy. It seemed logical that if the light comes in little quantized energy packets, and the light from hot gasses comes in specific quantized colors that these two things should be related. In fact, way back in February of 1908, Planck wrote in his notebook that he was “fully convinced that the problem of spectral lines is intimately tied to the question of the nature of the quantum,[30]” but no one had gotten anywhere with it. What made Bohr’s model so different is that he stated that the energy of light produced told you nothing about the energy of the electron itself but only told you about the quantum change of energy as it jumped from one energy level to another. With Bohr’s new model, you could predict the color produced by glowing hydrogen and by glowing helium ions (helium with one electron). Even this was astonishing to scientists, 7 years later Planck gushed that Bohr discovered, “the long-sought key to the entrance gate into the wonderland of spectroscopy…. And now that way was opened, a sudden flood of new-won knowledge poured over the whole field including the neighboring fields in physics and chemistry.[31]” Quantum Mechanics were about to transform all of Physics, and Planck would be at the center of it, sometimes inspiring and even financing and sometimes restraining the pioneering work from 1914-to 1933 of Heisenberg, Schrödinger, Born, Laue, Meitner, Hann, and more. In the midst of that Planck also suffered (oh how he suffered) through a sea of personal tragedies, two world wars, and the depression between them. And that story is next time on the lightning tamers.

[1] Albert Einstein to Hedwig Born (February 8, 1918) Einstein, A *The Correspondence of Albert Einstein: Volume 8* p. 467

[2] Planck, M *Scientific Autobiography and Other Papers* (1946) p. 12

[3] Planck, M *Scientific Autobiography and Other Papers* (1946) p. 12

[4] Planck, M *Scientific Autobiography and Other Papers* (1946) p.1

[5] Planck, M *Scientific Autobiography and Other Papers* (1946) p. 13

[6] p 19 “Scientific Autobiography and Other Papers” Max Planck

[7] p 3-4 “Scientific Autobiography and Other Papers” Max Planck

[8] As described on page 123 of “Planck: Driven by Vision, Broken by War” Brown

[9] Max Planck found in Heilbron, J *Dilemas of an Upright Man* (2000) p. 47

[10] Lise Meitner (1958) found on the Max Planck institute website www.max-planck.mpg.de

[11] Lise Meitner (1958) found on the Max Planck institute website www.max-planck.mpg.de

[12] described on pg 34 of “Scientific Autobiography and Other Papers” Max Planck

[13] Planck, M *Scientific Autobiography and Other Papers* (1946) p. 13

[14] Planck, M “On an Improvement of Wien’s Equation for the Spectrum” English translation from Haar, D. *The Old Quantum Theory* (1967) p. 81

[15] Planck, M *Scientific Autobiography, and other papers* (1947) p. 25

[16] Planck, M *Scientific Autobiography and Other Papers* (1947) p. 41

[17] Planck, M “On the Theory of the Energy Distribution Law of the Normal Spectrum” English translation by D. Haar *The Old Quantum Theory* (1967) p. 85

[18] Planck, M referenced in Kantorovich, A *Scientific Discovery: Logic and Tinkering* (1993) p. 164

[19] Planck, M *Scientific Autobiography and Other Papers* (1947) p. 32

[20] Planck, M *Scientific Autobiography and Other Papers* (1947) p. 33

[21] Albert Einstein (1915) found in Stone, A *Einstein and the Quantum *(2013) p. 88

[22] Paul Epstein quoted in Stone, A *Einstein and the Quantum* (2013) p. 140

[23] Max Planck (October 1909) found in Brown, B *Planck: Driven by Vision* (2015) p. 74

[24] *Yale Alumni Weekly* (Sept 1906), p. 184

[25] p 342 “A Survey Of Thermodynamics” Bailyn (1994) American Institute of Physics NY

[26] Max Planck to Walther Nernst (June 11, 1910) found in Barkan, D *Walther Nernst and the Transition to Modern Physical Science* (2011) p. 187

[27] Max Planck to Walther Nernst (June 11, 1910) found in Mehra, J *The Solvay Conferences on Physics* (2012) p. 5

[28] Albert Einstein to H Zannger (May 20, 1912) found in Pais, A *Subtle is the Lord: The Science and the Life of Albert Einstein* (1983) p. 399

[29] Albert Einstein to Michele Besso (September 11, 1911) translated in Einstein, A, Beck A, and Havas, P *The Collected Papers of Albert Einstein, Vol. 5* Princeton University Press p. 205

[30] Max Planck (February 1908) found in Kumar, M *Quantum: Einstein, Bohr and the Great Debate *(2008) p. 15

[31] Max Planck, “Nobel Prize Lecture 1920”

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