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Scientific contributions of Max Planck
by Jeremy Paschke
Amid the revelries of a new millennium, I invite students of science to commemorate a revolution in thought: an insight that changed the world. The year 2000 marks the 100 year anniversary of Max Planck’s creation of quantum physics. “The introduction of the quantum theory,” said Planck, “led not to the destruction of physics, but to a more profound reconstruction, in the course of which the whole science was rendered more universal.”
Quantum theory, the theory that nature apportions her energy in small, indivisible bundles, was a watershed. It tied physics to chemistry by their roots, and modern theorists say the quantum undergirds all aspects of nature. This story narrates when, where, and why quantum theory appeared and tells about the free thinker who gave this gift to humanity.
Those who knew Max Karl Ernst Ludwig Planck before 1900 would never have pinned him as a revolutionary. Self described as, “peaceful and disinclined to questionable adventures,” Planck carried out his studies humbly, excelling in all subjects but not showing any particular genius. He played the piano with professional grace, and he nearly chose the life of a concert pianist instead of a theoretical physicist. Planck’s advisors at gymnasium (high-school) counseled him against a career in physics, saying there was nothing left to discover, no further glory to win. Planck instead followed his passion for science, and by 1897, he became the recognized authority on his favorite topic: thermodynamics. As a leader of the Berlin Academy of Sciences, Planck’s name stood at the forefront of German science.
The looming physical puzzle at the end of the nineteenth century — the puzzle that eventually gave rise to the quantum revolution — concerned the radiation of hot glowing objects. Any object with a high temperature emits electromagnetic radiation. When the temperature becomes extremely high, that radiation enters the visible spectrum and glows red, orange, blue, or white depending upon the object’s temperature. Ceramic plate manufacturers witnessed this phenomenon as early as the 1700s. By noting the color of their oven, plate-makers could tell the oven’s temperature, and that determination held true regardless of the oven’s material composition. When Planck learned that intensity of radiation depended only on frequency (color), he had to find out why.
Planck viewed the axiomatic link between energy intensity and frequency as a cornerstone in comprehending nature. He said that, “in physics, we labor not for the day, not for the momentary success, but, as it were, for eternity.” Planck dove into the radiation problem, hoping to glimpse the eternal, to find a formula that would apply at all times to all cultures, even, “nonterrestrial and nonhuman ones.” Planck’s wish for pure knowledge then, formed the first reason to study radiation, but there was also a second, more pragmatic reason for Planck’s study of radiation.
In 1879, Thomas Edison evacuated the air around a wire filament and excited that filament with electricity until it glowed in the color spectrum, thus creating the first incandescent light. The world quickly caught on to Edison’s invention, and the German bureau of standards (Physikalische Technische Reichanstalt) wanted to know the optimal frequency to power their new system of electric lights. Possessing a mathematical formula that related frequency to energy intensity would inform technicians what frequencies wasted the most energy from a light bulb, and hence what frequencies they should avoid.
From 1894 until 1900, Planck struggled with the problem. He conjured up a formula that fit the data, but had no clue what the formula meant. “After some weeks of the most intense work of my life,” Planck recalled, “light began to appear to me and unexpected views revealed themselves in the distance.” In his mind’s eye, Planck envisioned the glowing object as a huge collection of atoms. As the electrons within the atom vibrated, the object radiated light as electromagnetic waves. By summing up the energy contribution from each vibrating atom, Planck scribed a formula for the total intensity of a glowing object based on the vibrational frequency alone.
On October 19, 1900, Planck addressed his colleagues at the Berlin Academy and drew their attention to a, “new formula that I [Planck] consider to be the simplest possible . . . from the point of view of the electromagnetic theory of radiation.” Contrary to traditional assumptions, Planck’s new formula only allowed atoms to vibrate at certain frequencies. Hence, atomic energies could not take on any value possible; energy was relegated to integer multiples of a singular quanta. In Planck’s quantum world, energy does not increase along a smooth, continuous line, like an elderly gentleman ambles down a sidewalk. In the quantum world, energy hops from one level to another, like a child playing hopscotch, and never lands in the unmarked regions. Why nature prefers to hop rather than amble is riddle that vexed Planck until the day he died.
Planck named the rift between energy levels — the distance between hopscotch squares — the, “elementary quantum of action.” The quantum of action ploughed its way through scientific fields, a, “mysterious messenger from the real world,“ said Planck, that, “insisted on turning up in every kind of measurement.” Thirty years later after creating his formula, an interviewer asked Planck how he guessed that nature quantized her energy. Planck admitted, “it was an act of desperation.” He had exhausted all the logical routes and could do nothing other than step into the unknown.
Five years after the first announcements of quantum theory, Albert Einstein used Planck’s elementary quanta to explain the photoelectric effect. Einstein was the first to introduce h, later called Planck’s Constant. New theories catch on slowly, and the development of twentieth-century physics owes much to the Planck-Einstein dialectic. Einstein applied energy quanta to light and increased the veracity of Planck’s original ideas. Planck reciprocated by supporting Einstein’s special relativity when he exposed it to a highly conservative and highly critical German audience. Planck’s friendship with Einstein extended beyond professional boundaries as they occasionally teamed up to make music together.
Planck worked on physics with colossal zeal, but he loved his family above all. “How wonderful it is,” said Planck in his youth, “to set everything else aside and live entirely within the family.” After twenty-three yeas of marriage, Planck’s first wife died in 1909. Nevertheless, Planck counted his blessings in his four healthy children.
Disasters began when Europe first slid into world war. In 1914 Planck’s eldest son, Karl, was killed on the German front. In the succeeding five years, two more of his children went to an early grave as his twin daughters both succumbed in childbirth.
Einstein described his friend as a man eaten by grief, yet one who still stood, “fully courageous and erect.” Rather than grow bitter and caustic towards life, Planck marshaled his spirits through work and the loving relationship with his only living son, Erwin.
Planck lived through two world wars, both times on the losing side, and while he often disagreed with German politics, Planck never relinquished his national pride. “It is a great feeling,” he said at the start of the Great War, “to be able to call one self a German.” In the optimist fervor of the first weeks of war, Planck signed his name to the Appeal to the Cultured Peoples of the World. The Appeal was a formal declaration that the leaders of German arts and sciences supported the acts of the German army. Planck regretted signing the Appeal. He called his signature a ”mental scruple” and complained it that weighed heavy on him for a year and a half.
Germany’s economy plummeted after the Great War, and the ensuing depression affected all citizens. As a leader of the physics community, Planck was prone to giving guest lectures and attending seminars at various universities in Germany. However, there were times when the inflation grew so bad that the cost of a hotel far exceeded Planck’s travel allowance. The 65 year old Planck would sit up all night in a train station just to keep his countrymen and himself up-to-date in science.
A ruined economy was one consequence of the Great War, but ruined mentalities made for further consequences. The excess of blood shed in trenches across Europe led to a morose view on life. After the war, science became the scapegoat for all the world’s pain; and the belief in causality — a belief that made science possible — was philosophically rejected. In place of traditional science and causality, the general populace substituted vogue topics such as magic and mysticism. Even working scientists joined in the flight from rationality. A majority of scientists conceded that scientific theories could tell the observer only about the outcomes of experiments. Theories, they said, tell us absolutely nothing about nature herself. Planck understood that science had given the world poison gas and the machine gun, but science also created telegraphs and vaccinations for polio. Not all scientific offspring were evil. Planck believed in causality, that a lawfulness directed everything that happened, and against a world of critics, he fought for his underdog; he fought for science.
One philosophical issue at stake was the tension between causality and free will. If the world is causal, argued skeptics of science, then humans have no freedom of action — our hands are tied by the deterministic laws of physics. Planck countered these arguments by claiming that self-reflection distinguished itself from scientific analysis. By contemplating ourselves contemplating an act, humans disrupt the causal chain and assert themselves as free actors in life’s drama. “Each one of us is an integral part of the world in which we live,” assured Planck.
Although he rhapsodized on the merits of science, Planck never dared suggest that scientific knowledge was absolute knowledge. The “apprehension of true reality,” he said, is a, “goal which is theoretically unobtainable.” True reality always lies ahead of us, always beyond our cognitive grasp. Planck compared science to art and said that humanity could advance only because of, “the creative force of the imaginative intellect.”
Planck’s noble dedication to teaching and learning pure science propelled him to the vanguard of German academia. He was the secretary of the Berlin Academy and the president of the Kaiser-Wilhelm-Gesellschaft, two influential positions in Germany. When Hitler became Reich Chancellor on January 30, 1933, Planck held far too much responsibility to flee the country. He could not admonish Germany from abroad like his friend Einstein did, calling the fatherland a place where “civil liberty and tolerance” no longer existed. As hordes of German intellectuals emigrated to make a new start, Planck stayed behind to salvage what he could in the name of science. Although he was not a Jew himself, Planck sympathized with the Jewish plight. His “warm and peaceful attentiveness” supported less fortunate physicists such as Paul Ehrenfest. As the Nazis accumulated greater power, Planck often wished he could retreat from official matters, but he was stuck; everyone counted on his help.
In May 1933, Planck met for a short interview with the Fuhrer himself. He tried to convince Hitler that forced emigration of the Jews was bad for German science and that Jews could be good Germans, but Hitler would hear none of it, and the dictator curtly showed Planck to the door. War raged again on German fronts, yet Planck continued to lecture in his quiet, humble, wise manner, “calling forth the divineness of life and its government by law.”
In mid-winter, 1944, an Allied bombing raid soared over Berlin and obliterated Planck’s home. Libraries, correspondence, and diaries, were all decimated. Later that same year, the Nazis captured Erwin Planck and judged him guilty of conspiring to assassinate Hitler. Max Planck did all he could to free his son; but on February 18, 1945, the inevitable happened, and the Nazis executed Erwin Planck. The elder Planck never recovered from this terrible blow, and he lived for three more years before dying of a stroke on October 4, 1947, just days before his ninetieth birthday.
To preface a lecture back in 1932, Einstein told his audience that if an angel of the Lord should descend to the temple of science and sweep out everyone there for impure reasons, Max Planck would be among the few remaining. Planck did not practice science to invent labor saving devices or improve the instruments of war, nor did he practice theoretical physics to vainly have his surname attached to some arbitrary constant. All the prestige and power of his offices never inflated Planck’s ego. He studied physics with unsullied motives, and was ever humble before the mysteries of nature. Planck wanted to learn about the world, purely and simply, and that is the reason why Einstein admired him.
Planck’s life gives us more lessons than scientific ones, however. He suffered Job-like catastrophes, yet maintained a positive countenance and a love for life. Strong in body, Planck continued to climb mountains well into his eighties. Strong in mind, Planck’s creativity granted him a glimpse beyond the dominant scientific paradigm and into the new world of the quantum. Strong in spirit, he fought ignorance and dogmatism on all fronts. Planck was a complete type of man, and that is the reason why, 100 years after his greatest triumph, we can all admire him.
The historical information and the quotations used in this article come from the following sources:
Hans Kangro, Planckís Original Papers in Quantum Physics, translated by D. ter Haar and Stephen G. Brush, (London, Taylor and Francis, 1972).
John Heilbron, The Dilemmas of an Upright Man, (Berkeley, University of California Press, 1986).
Emilio Segre, From X-rays to Quarks, (Berkeley, University of California Press, 1976).
Max Planck, The New Science, Preface by Albert Einstein, (Greenwich, Meridian Books Inc., 1959).
