Max Planck

Max Karl Ernst Ludwig Planck (German: [maks ˈplaŋk] ⓘ ; 23 April 1858 – 4 October 1947) was a German theoretical physicist whose groundbreaking discovery of energy quanta earned him the coveted Nobel Prize in Physics in 1918. [4] This revelation didn’t just win him an award; it irrevocably reshaped the very foundations of how humanity understood the universe.

Planck’s contributions to theoretical physics were numerous and profound, yet his enduring legacy, his indelible mark on the annals of science, rests primarily on his pivotal role as the architect of quantum theory and, by extension, one of the principal architects of modern physics . [5] [6] This new paradigm utterly revolutionized the prevailing understanding of both atomic and subatomic processes, forcing a re-evaluation of classical concepts that had held sway for centuries. He is perhaps most famously associated with the Planck constant , a fundamental physical constant of paramount importance for the entirety of quantum physics. From this constant, he elegantly derived a set of units , now universally recognized as Planck units , which are expressed solely in terms of other fundamental physical constants , providing a natural system of units for the universe itself. [7]

Beyond his revolutionary scientific endeavors, Planck also held significant administrative sway, serving twice as President of the esteemed Kaiser Wilhelm Society . In a fitting tribute to his colossal impact, this institution was posthumously renamed the Max Planck Society in 1948. Today, this society stands as a formidable scientific powerhouse, encompassing 83 institutions dedicated to an expansive spectrum of scientific disciplines, a testament to the enduring influence of its namesake.

Early life and education

Max Karl Ernst Ludwig Planck made his entrance into a world already teeming with intellectual ferment on 23 April 1858, in Kiel , Holstein. He was the son of Johann Julius Wilhelm Planck, a distinguished professor of law, and his second wife, Emma Patzig. At birth, he was given the rather formidable name of Karl Ernst Ludwig Marx Planck, with Marx designated as his “appellation name.” [8] However, by the tender age of ten, perhaps sensing the future weight of a singular identity, he opted for the more concise “Max,” a name he would carry with him for the remainder of his long and impactful life. [9]

Planck’s lineage was one deeply steeped in tradition and intellectual rigor. His paternal great-grandfather and grandfather both held esteemed positions as theology professors at the venerable University of Göttingen . His father, as noted, carved out a distinguished career as a law professor, first at the University of Kiel and later at the University of Munich . Even an uncle found his calling within the judicial system, serving as a judge. [10] This heritage provided a clear, if somewhat predetermined, path towards academic and intellectual pursuits.

He was the sixth child in a bustling household, with two older siblings stemming from his father’s initial marriage. Planck’s early years were punctuated by the unsettling drumbeat of war; among his most vivid and formative memories was the sight of Prussian and Austrian troops marching through Kiel during the brutal Second Schleswig War in 1864. [10] A childhood shadowed by conflict, perhaps, instilled a certain stoicism that would serve him later in life.

In 1867, the Planck family relocated to the vibrant city of Munich , where Max enrolled at the prestigious Maximiliansgymnasium. It was within these hallowed halls that his remarkable mathematical prowess first began to manifest, emerging with a clarity that belied his youth. [11] [12] He soon found a mentor in Hermann Müller, a mathematician who recognized the burgeoning genius within the young student. Müller took a particular interest in Planck, guiding him through the intricate landscapes of astronomy and mechanics , in addition to advanced mathematics . It was from Müller that Planck was first introduced to the profound and universal principle of conservation of energy , a concept that would resonate throughout his future work. He proved to be an exceptionally bright student, graduating early at the impressive age of 17. [13] This early exposure, facilitated by Müller’s mentorship, marked Planck’s initial and decisive foray into the demanding yet captivating realm of physics .

Despite his burgeoning scientific inclinations, Planck was also prodigiously gifted in music. He cultivated his talents with singing lessons, demonstrating proficiency on the piano, organ, and cello, and even venturing into the composition of songs and operas. However, ultimately, the siren call of physics proved irresistible, leading him to choose the rigorous study of the natural world over the harmonious complexities of musical composition. One can almost hear Emma Monday sighing at the pragmatic choice, “Another soul lost to the cold, hard facts of the universe, when there was potential for something… less predictable.”

In 1874, Planck matriculated at the University of Munich . Under the guidance of Professor Philipp von Jolly , Planck undertook what would prove to be the sole experimental investigations of his entire scientific career: a study into the diffusion of hydrogen through heated platinum . Yet, the pull of theoretical abstraction was too strong, and he soon transitioned to the realm of theoretical physics . Jolly, a well-meaning but ultimately myopic figure, famously cautioned Planck against this path. Planck himself would later recall, in 1878, Jolly’s assertion that physics was nearing its completion, describing it as a “highly developed, nearly fully matured science, that through the crowning achievement of the discovery of the principle of conservation of energy will arguably soon take its final stable form.” [14] The cosmic irony of this statement, delivered just two decades before Planck himself would detonate the very foundations of classical physics, is not lost on history, nor on those who appreciate the exquisite folly of human certainty. It’s a stark reminder that even the most brilliant minds can be utterly wrong about the future, particularly when it involves dismantling their own carefully constructed worlds.

Undeterred by such pronouncements of physics’ imminent demise, Planck journeyed to the University of Berlin in 1877 for a year of intensive study. There, he immersed himself in the teachings of eminent physicists Hermann von Helmholtz and Gustav Kirchhoff , as well as the formidable mathematician Karl Weierstrass . His observations of these luminaries were candid: Helmholtz, he noted, was often ill-prepared, spoke slowly, made incessant miscalculations, and had a penchant for boring his auditors. Kirchhoff, by contrast, delivered meticulously prepared lectures that, while precise, were dry and monotonous. Despite these initial impressions, Planck soon forged a close friendship with Helmholtz. During this pivotal period, he embarked on a largely self-directed study of the seminal writings of Rudolf Clausius , a profound engagement that ultimately steered him towards specializing in the burgeoning field of thermodynamics .

In October 1878, Planck successfully navigated his qualifying examinations, and in February 1879, he staunchly defended his doctoral thesis, provocatively titled Über den zweiten Hauptsatz der mechanischen Wärmetheorie (On the second law of mechanical heat theory). Following this academic triumph, he briefly returned to his former school in Munich, where he imparted his knowledge of mathematics and physics.

By 1880, Planck had amassed the two highest academic distinctions available in Europe. His doctorate, awarded after the completion of his detailed paper on his research and theoretical insights into thermodynamics, was merely the first step. He then proceeded to present his venia legendi (habilitation) thesis, a work titled Gleichgewichtszustände isotroper Körper in verschiedenen Temperaturen (Equilibrium states of isotropic bodies at different temperatures), solidifying his credentials as a fully fledged academic.

Career and research

With his academic qualifications firmly in hand, Planck commenced his professional journey in 1880 as a Privatdozent (an unsalaried lecturer) at the University of Munich, patiently awaiting the elusive offer of a full academic position. Though his early work initially languished in relative obscurity within the academic community, he relentlessly pressed forward with his investigations into the complex realm of heat theory . In a striking instance of independent discovery, he meticulously developed the very same thermodynamical formalism as Josiah Willard Gibbs , entirely unaware of Gibbs’s prior, parallel achievements. The concepts articulated by Clausius , particularly those concerning entropy , formed the undeniable bedrock of Planck’s foundational work during this period.

April 1885 marked a significant turning point when Planck received an appointment as an Associate Professor of Theoretical Physics at the University of Kiel . Here, he continued to deepen his exploration of entropy and its intricate applications, particularly within the burgeoning field of physical chemistry . This rigorous work culminated in the publication of his seminal Treatise on Thermodynamics in 1897. [15] Demonstrating the breadth of his theoretical insights, he further proposed a robust thermodynamic underpinning for Svante Arrhenius ’s then-revolutionary theory of electrolytic dissociation , thereby lending crucial theoretical weight to experimental observations.

The year 1889 saw Planck ascend to a more prominent academic stature when he was named the successor to Kirchhoff’s esteemed position at the University of Berlin . [16] This appointment was, in all likelihood, facilitated by the astute intervention of his friend and former mentor, Helmholtz. By 1892, his dedication and intellectual acumen were recognized with a promotion to Full Professor. A testament to his growing reputation, in 1907, he was extended an offer to take over Ludwig Boltzmann ’s prestigious position in Vienna . However, Planck, demonstrating a clear preference for the intellectual environment he had cultivated, politely declined, choosing to remain in Berlin. His international standing continued to grow, and in 1909, while still a professor at the University of Berlin, he was honored with an invitation to serve as the Ernest Kempton Adams Lecturer in Theoretical Physics at Columbia University in New York City. The series of lectures he delivered there were subsequently translated and co-published by Columbia University professor A. P. Wills , further disseminating his ideas across the Atlantic. [17] His intellectual contributions were also recognized by his election to the prestigious American Academy of Arts and Sciences in 1914. [18] Planck’s illustrious career at Berlin concluded with his retirement on 10 January 1926, [19] a role into which he was succeeded by the equally brilliant Erwin Schrödinger , a symbolic passing of the torch in the quantum age. [20] Further recognition of his enduring scientific impact came with his election to the National Academy of Sciences in 1926 and to the venerable American Philosophical Society in 1933. [21] [22]

Professor at Berlin University

Upon assuming his professorship at the University of Berlin , Planck promptly joined the local Physical Society, a gathering of like-minded scientific minds. He would later reflect on this period, noting with a characteristic blend of dry observation and subtle complaint: “In those days I was essentially the only theoretical physicist there, whence things were not so easy for me, because I started mentioning entropy, but this was not quite fashionable, since it was regarded as a mathematical spook.” [23] The idea of entropy , a concept so fundamental to his work, was clearly met with a certain intellectual resistance, or perhaps simply a lack of comprehension, by a community still firmly rooted in more tangible, experimental physics. It seems even profound truths can be dismissed as spectral illusions if they don’t conform to the prevailing intellectual fashion.

Through his persistent initiative and organizational acumen, the various disparate local Physical Societies scattered across Germany coalesced in 1898 to form a unified entity: the German Physical Society (Deutsche Physikalische Gesellschaft , DPG). Planck’s leadership was further recognized when he served as its president from 1905 to 1909, guiding the nascent organization through its formative years.

A plaque at the Humboldt University of Berlin now commemorates his long tenure, stating: “Max Planck, discoverer of the elementary quantum of action h , taught in this building from 1889 to 1928.”

As a lecturer, Planck delivered a comprehensive, six-semester course on theoretical physics. Lise Meitner , a future luminary in her own right, famously described his lectures as “dry, somewhat impersonal,” yet the English participant James R. Partington offered a contrasting, almost reverent, assessment: “using no notes, never making mistakes, never faltering; the best lecturer I ever heard.” Partington’s account further paints a vivid, if slightly bizarre, picture of the packed lecture hall: “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.” Such dedication, or perhaps sheer scientific enthrallment, is a rare commodity. Despite his compelling lectures, Planck did not actively cultivate a large “school” of disciples in the traditional sense; the number of his direct graduate students remained relatively modest, totaling only about 20. However, the intellectual caliber of these few was undeniably high, including future giants such as:

• 1897 Max Abraham (1875–1922)
• 1903 Max von Laue (1879–1960)
• 1904 Moritz Schlick (1882–1936)
• 1906 Walther Meissner (1882–1974)
• 1907 Fritz Reiche (1883–1960)
• 1912 Walter Schottky (1886–1976)
• 1914 Walther Bothe (1891–1957) [24]

Entropy

At the close of the 19th century, thermodynamics , then often referred to as the “mechanical theory of heat,” had evolved from its humble beginnings early in the century. Its initial impetus was a practical one: to comprehend the operational principles of steam engines and, more importantly, to enhance their efficiency. The 1840s proved to be a fertile decade for scientific insight, as multiple researchers, working independently, converged upon and formally articulated the fundamental law of conservation of energy , now widely known as the first law of thermodynamics . This established energy as an uncreatable, indestructible entity, merely transforming between forms.

Building upon this foundation, in 1850, Rudolf Clausius delivered his pivotal formulation of the second law of thermodynamics . This law, a cornerstone of physics, decreed that any spontaneous or “voluntary” transfer of energy is inherently unidirectional, flowing exclusively from a warmer body to a colder one, never the reverse. Across the Channel, in England, the eminent William Thomson, 1st Baron Kelvin arrived at an identical conclusion, underscoring the universal nature of this thermodynamic principle.

Clausius, driven by an insatiable intellectual curiosity, continued to refine and generalize his formulation, culminating in a profound re-articulation in 1865. To achieve this, he introduced a novel and abstract concept: entropy . He meticulously defined entropy as a quantitative measure of the reversible supply of heat relative to the absolute temperature. This definition provided a mathematical framework for understanding the directionality of natural processes.

The revised formulation of the second law, which remains inviolable in contemporary physics, was elegantly simple yet profoundly far-reaching: “Entropy can be created, but never destroyed.” This statement, a cosmic edict if ever there was one, delineates the fundamental irreversibility of the universe. Clausius, whose extensive body of work Planck devoured as a diligent young student during his time in Berlin, masterfully applied this newly discovered law of nature to a diverse array of phenomena, spanning mechanical, thermoelectric, and chemical processes, demonstrating its universal applicability.

In his 1879 doctoral thesis, Planck undertook the formidable task of synthesizing Clausius’s voluminous writings. With characteristic rigor, he meticulously highlighted inherent contradictions and subtle inaccuracies embedded within their original formulations, proceeding then to clarify and rectify them. More significantly, Planck boldly extended the validity of the second law to encompass all natural processes; Clausius, in his earlier work, had somewhat circumscribed its application to purely reversible and thermal processes. Furthermore, Planck delved deeply into the nascent concept of entropy, asserting that it was not merely a property inherent to a physical system, but simultaneously served as an unequivocal measure of the irreversibility of any given process. His conclusion was stark: if entropy is generated during a process, that process is inherently irreversible, precisely because, according to the second law, entropy cannot be annihilated. Conversely, in the rare ideal of reversible processes, entropy remains constant. He meticulously elaborated on this profound insight in 1887 through a series of treatises collectively titled “On the Principle of the Increase of Entropy.” [25]

Crucially, in his exhaustive study of entropy, Planck deliberately eschewed the then-prevalent molecular, probabilistic interpretation championed by figures like Boltzmann. His rationale was rooted in a fundamental skepticism: such interpretations, he contended, failed to provide an absolute proof of universality. Instead, he favored a more robust, phenomenological approach, grounded in observable phenomena rather than statistical inference. This methodological preference also fueled his initial skepticism towards atomism itself, a stance he would later, somewhat reluctantly, relinquish in the course of his monumental work on the law of radiation. Nevertheless, his early contributions powerfully demonstrated the immense explanatory and predictive capabilities of thermodynamics in resolving concrete physicochemical problems. [26] [27]

Planck’s sophisticated understanding of entropy included the profound realization that the maximum possible entropy within a system precisely corresponds to its equilibrium state. The logical corollary of this insight—that a comprehensive knowledge of the system’s entropy allows for the derivation of all laws governing thermodynamic equilibrium states—aligns perfectly with the contemporary understanding of such states. Consequently, Planck strategically focused his research on equilibrium processes. Building upon the groundwork laid in his habilitation thesis, he meticulously investigated phenomena such as the coexistence of different aggregate states and the equilibrium dynamics of gas reactions. This pioneering work, positioned at the cutting edge of chemical thermodynamics, garnered considerable attention, particularly given the explosive growth and practical importance of chemical research during that era.

Unbeknownst to Planck, Josiah Willard Gibbs had independently arrived at virtually all the same profound insights regarding the properties of physicochemical equilibria, publishing his findings from 1876 onwards. Planck remained unaware of these groundbreaking essays, which, adding to the intellectual isolation, were not translated into German until 1892. Despite this parallel discovery, the two scientists approached the subject from distinct philosophical and methodological standpoints. While Planck delved into the intricacies of irreversible processes, Gibbs’s focus lay primarily on the elegant description of equilibria. Though Gibbs’s approach ultimately gained wider acceptance due to its inherent simplicity and analytical power, Planck’s method is often lauded for its greater conceptual universality, providing a more expansive framework for understanding the relentless, one-way march of the universe. [28]

Black-body radiation

In 1894, Planck pivoted his formidable intellect towards one of the most perplexing and ultimately revolutionary problems confronting physics at the time: the enigma of black-body radiation . The problem had been elegantly articulated by Kirchhoff way back in 1859: “how does the intensity of the electromagnetic radiation emitted by a black body (a perfect absorber and emitter, often conceptualized as a cavity radiator) depend on the frequency of the radiation (its ‘color’) and the absolute temperature of the body?” This wasn’t merely a theoretical exercise; experimentalists had already extensively explored the phenomenon, meticulously mapping the observed spectral distribution. However, the theoretical explanations available at the time consistently failed to align with the empirical evidence. Wilhelm Wien ’s law, while offering a correct prediction for the behavior of radiation at high frequencies, conspicuously faltered when applied to lower frequencies. Conversely, the Rayleigh–Jeans law , another attempt to model the problem, managed to agree with experimental results at low frequencies, but catastrophically diverged at high frequencies, predicting an infinite output of energy – a phenomenon famously dubbed the “ultraviolet catastrophe .” This theoretical absurdity, a direct consequence of classical physics , suggested that a hot object should emit an infinite amount of energy in the ultraviolet range and beyond, which was clearly not observed. However, it is crucial to note, contrary to popular textbook narratives, that this particular “catastrophe” was not the primary impetus for Planck’s initial investigations. [29] His motivations were more deeply rooted in the quest for a fundamental, universal law.

Planck’s initial foray into solving this intractable problem, proposed in 1899, stemmed from what he termed the “principle of elementary disorder.” This conceptual framework allowed him to derive Wien’s law by making a series of specific assumptions about the entropy of an ideal oscillator. The resulting formulation was known as the Wien–Planck law. Yet, the brutal arbiter of scientific truth – experimental evidence – soon delivered a disheartening verdict: the new law failed to accurately describe the observed phenomena, much to Planck’s palpable frustration.

Undeterred, Planck meticulously revised his approach. This time, he arrived at the first iteration of what would become the universally celebrated Planck black-body radiation law , a formula that, with astonishing precision, perfectly described the experimentally observed black-body spectrum across all frequencies. He first presented this revolutionary equation at a meeting of the DPG on 19 October 1900, with its formal publication following in 1901. It’s worth noting that this initial derivation did not yet incorporate the concept of energy quantization , nor did it rely on statistical mechanics , a field towards which Planck harbored a distinct theoretical aversion.

However, in November 1900, Planck found himself compelled to revisit and refine this initial version. Now, he reluctantly embraced Boltzmann ’s statistical interpretation of the second law of thermodynamics , viewing it as a necessary, albeit philosophically uncomfortable, means to achieve a more profound, fundamental understanding of the principles underpinning his radiation law. Planck’s inherent conservative nature made him deeply suspicious of the philosophical and physical implications inherent in Boltzmann’s statistical approach. His recourse to these methods was, as he later candidly admitted, “an act of despair… I was ready to sacrifice any of my previous convictions about physics.” The universe, it seemed, was demanding a price for its secrets, and that price was Planck’s cherished classical worldview.

The absolutely central, and utterly revolutionary, assumption underpinning his revised derivation, unveiled to the DPG on the now-historic date of 14 December 1900, was the extraordinary proposition, now known as the Planck postulate , that electromagnetic energy is not continuous but could only be emitted or absorbed in discrete, indivisible packets—or quantized form. In simpler terms, energy could only exist as integer multiples of a fundamental, elementary unit. This unit is elegantly expressed by the equation:

$$E = h\nu$$

Here, E represents the energy of a single quantum, h is the now-ubiquitous Planck constant —also known as Planck’s action quantum (a quantity he had, in fact, implicitly introduced in 1899)—and ν (nu) denotes the frequency of the radiation. It is crucial to grasp that the elementary units of energy under discussion are represented by the product hν, not simply by ν. Modern physicists, building on Planck’s foundation, now refer to these discrete packets as photons , and each photon of a given frequency ν possesses its own unique and specific energy. The total energy at that particular frequency is, therefore, the product of hν and the number of photons present at that frequency.

Initially, Planck himself considered this notion of quantization to be merely “a purely formal assumption… actually I did not think much about it…” Such is the understated beginning of a revolution. Yet, this very assumption, so utterly incompatible with the elegant continuity of classical physics , is now universally regarded as the birth of quantum physics itself, and undoubtedly stands as the single greatest intellectual accomplishment of Planck’s illustrious career. (It is worth noting, for historical context, that Ludwig Boltzmann had, in a theoretical paper published in 1877, briefly entertained the abstract possibility that the energy states of a physical system might, in some contexts, be discrete rather than continuous.) The discovery of the Planck constant, h, further empowered him to define a new, universal set of physical units —such as the Planck length and the Planck mass —all meticulously derived from fundamental physical constants. These Planck units now form the very bedrock upon which much of contemporary quantum theory is constructed. In a rare moment of personal reflection, Planck, in a conversation with his son in December 1918, described his discovery with a profound sense of its significance, calling it “a discovery of the first rank, comparable perhaps only to the discoveries of Newton.” [30] In fitting recognition of this monumental contribution to a wholly new branch of physics, he was awarded the Nobel Prize in Physics for 1918, though he physically received the award the following year, in 1919. [31] [32]

Following this initial, reluctant leap, Planck wrestled for years with the profound implications of energy quanta, striving, often in vain, to reconcile them with the classical framework he so deeply respected. “My unavailing attempts to somehow reintegrate the action quantum into classical theory extended over several years and caused me much trouble,” he lamented. Even many years later, other esteemed physicists, including John William Strutt, 3rd Baron Rayleigh , James Jeans , and Hendrik Lorentz , clung to classical notions, attempting to nullify the quantum by setting the Planck constant to zero in their equations. But Planck, with the stubborn certainty of one who had stared into the abyss of a new reality, knew with absolute conviction that this constant possessed a precise, non-zero value. His exasperation was palpable: “I am unable to understand Jeans’ stubbornness – he is an example of a theoretician as should never be existing, the same as Hegel was for philosophy. So much the worse for the facts if they don’t fit.” [33] It’s a classic human failing, to reject the truth simply because it’s inconvenient or defies one’s preconceived notions.

Max Born , a quantum pioneer himself, offered a poignant and insightful summation of Planck’s intellectual journey: “He was, by nature, a conservative mind; he had nothing of the revolutionary and was thoroughly skeptical about speculations. Yet his belief in the compelling force of logical reasoning from facts was so strong that he did not flinch from announcing the most revolutionary idea which ever has shaken physics.” [1] Planck, it seems, was dragged kicking and screaming into revolution, but his commitment to empirical truth was stronger than his personal comfort.

Einstein and the theory of relativity

In 1905, a singular year in the history of science, three epoch-making papers penned by a then-unknown patent clerk, Albert Einstein , burst forth onto the pages of the esteemed journal Annalen der Physik . Among the select few who immediately grasped the monumental significance of the special theory of relativity , Planck stood out. It was largely thanks to his considerable influence and advocacy that this radical new theory rapidly gained widespread acceptance and recognition within Germany’s conservative scientific circles. Planck’s contributions were not limited to endorsement; he also played a significant role in extending the special theory of relativity, notably by recasting its principles in terms of classical action , thereby bridging the new physics with established frameworks. [34]

However, Planck’s progressive stance on relativity did not extend to all of Einstein’s revolutionary ideas. Einstein’s audacious hypothesis of light quanta, later dubbed photons —a concept born from Heinrich Hertz’s 1887 discovery (and further elucidated by Philipp Lenard ) of the perplexing photoelectric effect —was initially met with staunch rejection by Planck. He, the reluctant father of the quantum, found himself unwilling to completely abandon Maxwell ’s elegant and well-established theory of electrodynamics . His resistance was visceral, fearing a regression: “The theory of light would be thrown back not by decades, but by centuries, into the age when Christiaan Huygens dared to fight against the mighty emission theory of Isaac Newton …” [35] The irony, of course, is that this very “throwing back” was precisely what was needed to advance.

The intellectual stalemate persisted until 1910, when Einstein presented compelling new evidence, highlighting the anomalous behavior of specific heat at low temperatures as yet another phenomenon utterly resistant to explanation by classical physics. This mounting tide of contradictions prompted Planck and Walther Nernst to take decisive action: they organized the inaugural Solvay Conference in Brussels in 1911. It was at this pivotal gathering that Einstein, through the sheer force of his arguments and the weight of empirical evidence, finally succeeded in swaying Planck to accept the reality of light quanta.

Meanwhile, Planck had been appointed dean of the University of Berlin, a position of considerable power and influence. This afforded him the opportunity to extend a prestigious invitation to Einstein, bringing him to Berlin and establishing a new professorship specifically for him in 1914. This move not only enriched the university but also cemented a deep personal bond between the two scientific titans. They soon became close friends, often meeting to share their mutual love for music, a harmonious counterpoint to their revolutionary scientific debates.

First World War

With the sudden eruption of the First World War in 1914, Planck, like many of his contemporaries, initially found himself caught up in the prevailing wave of patriotic fervor and public excitement. He penned sentiments reflecting this initial enthusiasm, writing that, “Besides much that is horrible, there is also much that is unexpectedly great and beautiful: the smooth solution of the most difficult domestic political problems by the unification of all parties (and) … the extolling of everything good and noble.” [36] [37] This initial, almost naive, embrace of collective national purpose would later be viewed with a more critical lens.

In a move that would later draw considerable criticism, Planck also affixed his signature to the infamous “Manifesto of the 93 intellectuals .” This document was a thinly veiled piece of polemic war propaganda, a stark contrast to Einstein’s unwavering pacifistic stance, which nearly led to his imprisonment, spared only by the shield of his Swiss citizenship. The swift descent of intellectual elites into nationalistic fervor during times of crisis is, regrettably, a recurring theme in human history.

Despite his initial alignment with the prevailing nationalistic sentiment, Planck demonstrated a commitment to scientific integrity that transcended national boundaries. In 1915, while Italy maintained its neutrality in the brutal conflict, Planck successfully championed a scientific paper originating from Italy. His efforts ensured that this work received a well-deserved prize from the Prussian Academy of Sciences , an institution where Planck held one of the four permanent presidencies. This small act, amid the raging storm of war, underscored his belief in the universal and apolitical nature of scientific endeavor.

Post-war and the Weimar Republic

The tumultuous years following the First World War plunged Germany into an era of profound political and economic instability. In this chaotic landscape, Planck, now recognized as the preeminent authority in German physics, issued a pragmatic and almost stoic directive to his beleaguered colleagues: “persevere and continue working.” It was a call to maintain intellectual continuity amidst societal collapse, a grim recognition that the pursuit of knowledge must endure, even when everything else seems to crumble.

In October 1920, demonstrating his unwavering commitment to the future of German science, Planck, alongside the controversial but brilliant Fritz Haber , co-founded the Notgemeinschaft der Deutschen Wissenschaft (Emergency Organization of German Science). The primary objective of this vital organization was to provide crucial financial support for scientific research, which had been severely hampered by the war and its aftermath. A significant portion of the funds distributed by the Notgemeinschaft was painstakingly raised from international sources, a testament to the global scientific community’s recognition of Germany’s intellectual heritage and the urgent need for its revival.

During this period of intense rebuilding, Planck occupied numerous influential positions, reflecting his unparalleled standing within the German scientific establishment. He held leading roles at the University of Berlin, the prestigious Prussian Academy of Sciences, the German Physical Society, and the Kaiser Wilhelm Society . The latter, a testament to his enduring legacy, would famously be renamed the Max Planck Society in 1948. Despite his immense administrative responsibilities, the dire economic conditions plaguing Germany during this era meant that even a figure of Planck’s stature found it incredibly challenging to secure the resources necessary to conduct his own research. In 1926, his international recognition continued with his election as a foreign member of the Royal Netherlands Academy of Arts and Sciences . [38]

During the fraught interwar period, Planck also engaged in the political sphere, becoming a member of the Deutsche Volks-Partei (German People’s Party ). This party, associated with the pragmatic and internationally minded Nobel Peace Prize laureate Gustav Stresemann , generally advocated for liberal domestic policies while pursuing more revisionistic aims in international relations. However, Planck’s political views were not without their conservative, even elitist, leanings. He openly expressed his disagreement with the introduction of universal suffrage , believing that the masses were ill-equipped for such responsibility. Later, with the chilling clarity of hindsight, he articulated the view that the rise of the Nazi dictatorship was a direct consequence of “the ascent of the rule of the crowds.” [39] A tragic misreading of the specific nature of totalitarianism, perhaps, or a deeply cynical observation on the inherent instability of democratic movements.

Quantum mechanics

As the 1920s drew to a close, a new generation of brilliant physicists—Niels Bohr , Werner Heisenberg , and Wolfgang Pauli —had meticulously developed and articulated the Copenhagen interpretation of quantum mechanics. This interpretation, with its probabilistic nature and unsettling implications for determinism, represented a profound departure from classical intuition. Yet, to the surprise of some, it was met with a degree of resistance from Planck himself, who, alongside Erwin Schrödinger , Max von Laue , and even Albert Einstein , found himself unable to fully embrace its philosophical implications. Planck, the reluctant revolutionary, harbored a distinct hope that the more intuitively appealing wave mechanics would soon render quantum theory—his own intellectual offspring—unnecessary, a historical irony that is almost too perfect.

However, this was not to be the case. Subsequent experimental evidence and theoretical advancements only served to underscore the enduring, central importance of quantum theory, even in the face of the philosophical revulsions held by Planck and Einstein. In this intellectual struggle, Planck found himself experiencing the profound truth of an observation he himself had made years earlier, during his own arduous battle against entrenched classical views: “A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.” [40] A harsh, yet undeniably accurate, assessment of intellectual progress, and a testament to the stubborn inertia of the human mind.

Nazi dictatorship and the Second World War

When the chilling shadow of the Nazi regime descended upon Germany in 1933, Planck, then 74 years old, found himself a reluctant, horrified witness to the systematic dismantling of the scientific community he had helped to build. He watched as countless Jewish friends and esteemed colleagues were ruthlessly expelled from their positions, publicly humiliated, and forced into exile. Hundreds of brilliant scientists fled Nazi Germany , seeking refuge from an encroaching darkness. Planck, clinging to his earlier mantra, again urged his remaining colleagues to “persevere and continue working,” and even implored those contemplating emigration to remain in Germany, perhaps in a vain hope that the crisis would be transient. Nevertheless, he did quietly assist his nephew, the economist Hermann Kranold , in emigrating to London after his arrest, a small act of defiance against the growing tide of persecution. [41]

Otto Hahn , a prominent chemist, approached Planck, urging him to rally well-known German professors to issue a public condemnation of the barbaric treatment of Jewish academics. Planck’s response, delivered with a stark, almost cynical realism, revealed his deep understanding of human nature and ambition: “If you are able to gather today 30 such gentlemen, then tomorrow 150 others will come and speak against it, because they are eager to take over the positions of the others.” [42] A brutal truth, perhaps, but one that highlighted the paralyzing self-interest that often accompanies moments of moral crisis. Under Planck’s cautious leadership, the Kaiser Wilhelm Society (KWG) largely sought to avoid open confrontation with the Nazi regime, making an exception only in the case of the Jewish scientist Fritz Haber . In May 1933, Planck, demonstrating remarkable courage, requested and was granted a direct interview with the newly appointed Chancellor of Germany, Adolf Hitler . During this meeting, he attempted to reason with the dictator, arguing that the “forced emigration of Jews would kill German science and Jews could be good Germans.” Hitler’s chillingly dismissive reply—“but we don’t have anything against the Jews, only against communists”—effectively “took from him every basis for further negotiation,” [43] as, in Hitler’s warped worldview, “the Jews are all Communists, and these are my enemies.” [44] The futility of reason in the face of unreasoning hatred was laid bare. Haber, tragically, died in exile the following year, in 1934.

One year later, Planck, who had served as the KWG’s president since 1930, orchestrated a somewhat defiant official commemorative meeting for Haber. He also ingeniously succeeded in secretly enabling a number of Jewish scientists to continue their vital work within the institutes of the KWG for several years, a quiet act of sabotage against the regime’s oppressive policies. However, by 1936, his term as president of the KWG drew to a close, and the Nazi government, by now deeply suspicious of his independent spirit, exerted considerable pressure on him to refrain from seeking another term.

As the political climate in Germany grew progressively more hostile and irrational, Johannes Stark , a fervent proponent of the pseudo-scientific movement known as Deutsche Physik (“German Physics,” or more chillingly, “Aryan Physics”), launched vitriolic attacks against Planck, Arnold Sommerfeld , and Heisenberg. Their unforgivable crime, in Stark’s eyes, was their continued adherence to and teaching of Einstein’s “Jewish” theories, earning them the derogatory label of “white Jews.” The “Hauptamt Wissenschaft” (the Nazi government office for science) even initiated a ludicrous investigation into Planck’s ancestry, baselessly claiming he was “1/16 Jewish,” an accusation Planck vehemently denied. [45] The absurdity of racial purity tests applied to scientific thought was a new low.

In 1938, Planck marked his 80th birthday, a milestone celebrated by the DPG with a special ceremony. During this event, the prestigious Max-Planck medal—established as the DPG’s highest honor in 1928—was fittingly awarded to the French physicist Louis de Broglie . As the year drew to a close, the last vestiges of the Prussian Academy’s independence were brutally stripped away as it was absorbed by the Nazis, a grim part of their systematic process of Gleichschaltung , or forced coordination. Planck, in a final act of principled defiance against the regime’s intellectual stranglehold, resigned his presidency. Despite the escalating dangers and personal risks, he continued to travel frequently, delivering numerous public talks, including his insightful discourse on “Religion and Science.” Five years later, at the remarkable age of 85, he was still physically robust enough to climb 3,000-meter peaks in the majestic Alps , a testament to an indomitable spirit.

The Second World War brought with it an escalating barrage of Allied bombing missions against Berlin, forcing Planck and his wife to seek temporary refuge in the comparative safety of the countryside. In 1942, he penned a poignant reflection: “In me an ardent desire has grown to persevere this crisis and live long enough to be able to witness the turning point, the beginning of a new rise.” However, the war’s devastation would soon strike closer to home. In February 1944, his cherished home in Berlin was utterly obliterated by an air raid, a catastrophic loss that annihilated all his scientific records, correspondence, and the tangible history of a lifetime’s work. His rural retreat, too, soon found itself imperiled by the relentless advance of Allied armies from both the East and West.

The ultimate tragedy, however, was yet to unfold. In 1944, Planck’s second son, Erwin , to whom he was particularly close, was arrested by the ruthless Gestapo for his involvement in the attempted assassination of Hitler, the infamous 20 July plot . He was subsequently subjected to a sham trial and condemned to death by the notorious People’s Court in October 1944. Erwin Planck was hanged at Berlin’s Plötzensee Prison in January 1945. This devastating loss, the brutal execution of his son, inflicted an irreparable wound upon Planck, effectively destroying much of his remaining will to live. [46] The universe, having granted him such profound insights, had also demanded an unbearable personal toll.

Personal life and death

In March 1887, Max Planck entered into matrimony with Marie Merck (1861–1909), the sister of one of his former schoolmates. The couple then established their first home together in a sublet apartment in Kiel. Their union blessed them with four children: Karl (born 1888), the twins Emma (born 1889) and Grete (born 1889), and finally Erwin (born 1893).

Following their time in a Berlin apartment, the Planck family eventually settled into a spacious villa nestled in the affluent Berlin-Grunewald district, at Wangenheimstrasse 21. This neighborhood was a hub for intellectual luminaries, with several other distinguished professors from the University of Berlin residing nearby. Among these esteemed neighbors was the theologian Adolf von Harnack , who would become one of Planck’s closest and most cherished friends. The Planck home swiftly evolved into a vibrant social and cultural salon, a magnet for intellectual exchange and convivial gatherings. Numerous renowned scientists, including Albert Einstein , Otto Hahn , and Lise Meitner , were frequent and welcome visitors, drawn by the stimulating conversation and warm hospitality. The tradition of jointly performing music, a cherished pastime, had already been firmly established in the home of Planck’s former mentor, Hermann von Helmholtz , and continued as a beloved ritual in the Planck household.

After many years of shared happiness, Marie Planck tragically passed away in July 1909, possibly succumbing to tuberculosis . The loss was profound, leaving a void in Planck’s life.

In March 1911, Planck embarked on a second marriage, this time to Marga von Hoesslin (1882–1948). By December of that same year, his family expanded with the birth of his fifth child, Hermann.

The advent of the First World War brought with it an onslaught of personal tragedies for Planck. His second son, Erwin, was taken prisoner by the French forces in 1914, while his eldest son, Karl, was tragically killed in action during the brutal Battle of Verdun . The sorrow continued to mount: Grete, one of his twin daughters, died in 1917 while giving birth to her first child. Heartbreakingly, her twin sister, Emma, met the same fate two years later, after having married Grete’s widower. Both granddaughters, born into such sorrow, survived and were named after their mothers. Planck, a man of immense inner strength, endured these successive losses with a stoicism that belied the immense emotional toll.

The final, and perhaps most devastating, blow came in January 1945. His son Erwin Planck , to whom he had been particularly devoted, was sentenced to death by the notorious Volksgerichtshof (People’s Court) due to his participation in the failed attempt to assassinate Hitler in July 1944. Erwin was executed on 23 January 1945. [47] This unspeakable loss, the brutal state-sanctioned murder of his son, shattered much of Planck’s remaining will to live.

Following the devastating conclusion of World War II , Planck, his second wife, and their son Hermann were relocated to the home of a relative in Göttingen . It was in this city that Max Planck, the father of quantum theory, passed away on October 4, 1947, at the age of 89. He lies buried at the Stadtfriedhof (City Cemetery) in Göttingen, a final resting place for a life that redefined our understanding of existence. [48]

Religious views

Max Planck was a lifelong adherent of the Lutheran Church in Germany. [49] Despite his personal faith, he cultivated a remarkable intellectual openness and demonstrated considerable tolerance towards a wide spectrum of alternative viewpoints and religions . [50] In a thought-provoking lecture delivered in 1937, entitled “Religion und Naturwissenschaft” (“Religion and Natural Science”), Planck thoughtfully discussed the profound importance of religious symbols and rituals. He argued that these elements were directly intertwined with a believer’s capacity to worship God, but simultaneously cautioned that such symbols, by their very nature, could only ever provide an imperfect and incomplete illustration of the divine. He was equally critical of atheism, which he perceived as excessively focused on the derision of these symbols, while also issuing a clear warning against believers who might over-estimate the literal importance of such symbolic representations. [51] [50]

In a particularly profound and often-quoted statement from 1944, Planck articulated a perspective that bridged the scientific and the spiritual: “As a man who has devoted his whole life to the most clear headed science, to the study of matter, I can tell you as a result of my research about atoms this much: There is no matter as such. All matter originates and exists only by virtue of a force which brings the particle of an atom to vibration and holds this most minute solar system of the atom together. We must assume behind this force the existence of a conscious and intelligent spirit [orig. Geist]. This spirit is the matrix of all matter.” [52] This declaration, coming from a physicist who had delved into the fundamental nature of reality, speaks volumes about his conviction that a deeper, intelligent principle underpins the physical universe.

Planck further elucidated his conviction that the concept of God holds intrinsic importance for both religion and science, albeit in distinct capacities: “Both religion and science require a belief in God. For believers, God is in the beginning, and for physicists He is at the end of all considerations… To the former He is the foundation, to the latter, the crown of the edifice of every generalized world view.” [53] This perspective elegantly positions God as both the origin point for faith and the ultimate, unifying principle sought by scientific inquiry.

Furthermore, Planck, with the unwavering conviction of a man of science, wrote,

…“to believe” means “to recognize as a truth”, and the knowledge of nature, continually advancing on incontestably safe tracks, has made it utterly impossible for a person possessing some training in natural science to recognize as founded on truth the many reports of extraordinary occurrences contradicting the laws of nature, of miracles which are still commonly regarded as essential supports and confirmations of religious doctrines, and which formerly used to be accepted as facts pure and simple, without doubt or criticism. The belief in miracles must retreat step by step before relentlessly and reliably progressing science and we cannot doubt that sooner or later it must vanish completely. [54]

This statement firmly aligns Planck with a rational, scientific worldview that rejects supernatural intervention, asserting that scientific progress will inevitably erode the foundations of miraculous belief.

The noted historian of science John L. Heilbron characterized Planck’s views on God as fundamentally deistic . [55] Heilbron further recounts that when directly questioned about his religious affiliation, Planck clarified that while he had always been profoundly religious, he did not subscribe to a belief “in a personal God, let alone a Christian God.” [56] This nuanced position underscores his intellectual independence, embracing a spiritual dimension without conforming to conventional theological dogma.

Posthumous honors

The world, ever eager to commemorate genius once it has departed, has bestowed numerous honors upon Max Planck, ensuring his name echoes through time. For additional details, consult the List of things named after Max Planck .

• In 1953, the German Post Office Berlin issued a special 30-pfennig stamp, featuring Max Planck’s portrait, as part of their distinguished series “Men from Berlin’s History.”
• From 1957 to 1971, the 2-DM coins of the Federal Republic of Germany proudly bore Max Planck’s portrait, circulating his image throughout the nation.
• In 1958, a commemorative plaque was unveiled in the forecourt of the Humboldt University of Berlin , marking his significant contributions.
• Also in 1958, the Max Planck Society presented a bust of Planck, originally created in 1939, to the Physical Society of the GDR . This bust has been prominently displayed in the exhibition room of the Magnushaus since 1991.
• In 1970, celestial bodies were named in his honor: the lunar crater Planck and the adjacent valley Vallis Planck now bear his name.
• In 1983, the GDR issued a 5-mark commemorative coin to celebrate his 125th birthday. While not intended for general circulation, it was primarily marketed for foreign currency, a collector’s item.
• In 1989, a Berlin commemorative plaque was unveiled at Planck’s former residence in Berlin-Grunewald , marking a physical location tied to his life.
• In 2008, to mark his 150th birthday, a special postage stamp and a 10-Euro silver commemorative coin were issued.
• In 2013, The Max Planck Florida Institute For Neuroscience opened its doors in Jupiter, Florida, extending his legacy to new scientific frontiers.
• In 2014, Google celebrated Planck’s 156th birthday with a custom Google Doodle on April 23, bringing his image to millions.
• In 2022, his bust was ceremoniously placed in Walhalla , a hall of fame honoring distinguished Germans, cementing his place in the nation’s pantheon of greats. [57]

Publications

• Planck, M. (1900a). “Über eine Verbesserung der Wienschen Spektralgleichung”. Verhandlungen der Deutschen Physikalischen Gesellschaft . 2: 202–204. Translated in
  • ter Haar, D. (1967). “On an Improvement of Wien’s Equation for the Spectrum” (PDF). The Old Quantum Theory. Pergamon Press . pp. 79–81. LCCN 66029628.
• Planck, M. (1900b). “Zur Theorie des Gesetzes der Energieverteilung im Normalspectrum”. Verhandlungen der Deutschen Physikalischen Gesellschaft . 2: 237. Translated in
  • ter Haar, D. (1967). “On the Theory of the Energy Distribution Law of the Normal Spectrum” (PDF). The Old Quantum Theory. Pergamon Press . p. 82. LCCN 66029628. Archived from the original (PDF) on 20 September 2016. Retrieved 5 April 2014.
• Planck, M. (1900c). “Entropie und Temperatur strahlender Wärme” [Entropy and Temperature of Radiant Heat]. Annalen der Physik . 306 (4): 719–737. Bibcode :1900AnP…306..719P. doi :10.1002/andp.19003060410.
• Planck, M. (1900d). “Über irreversible Strahlungsvorgänge” [On Irreversible Radiation Processes]. Annalen der Physik . 306 (1): 69–122. Bibcode :1900AnP…306…69P. doi :10.1002/andp.19003060105.
• Planck, M. (1901). “Ueber das Gesetz der Energieverteilung im Normalspektrum”. Annalen der Physik . 309 (3): 553–563. Bibcode :1901AnP…309..553P. doi :10.1002/andp.19013090310. Translated in
  • Ando, K. “On the Law of Distribution of Energy in the Normal Spectrum” (PDF). Archived from the original (PDF) on 6 October 2011. Retrieved 13 October 2011.
• Planck, M. (1903). Treatise on Thermodynamics. Ogg, A. (transl.). London: Longmans, Green & Co. OL 7246691M.
• Planck, M. (1906). Vorlesungen über die Theorie der Wärmestrahlung. Leipzig: J.A. Barth. LCCN 07004527.
• Planck, M. (1914). The Theory of Heat Radiation. Masius, M. (transl.) (2nd ed.). P. Blakiston’s Son & Co. OL 7154661M.
• Planck, M. (1915). Eight Lectures on Theoretical Physics. Wills, A. P. (transl.). Dover Publications . ISBN 0-486-69730-4.
• Planck, M. (1908). Prinzip der Erhaltung der Energie. Leipzig: B.G.Teubner. ISBN 978-0-598-83768-4.
• Planck, M. (1943). “Zur Geschichte der Auffindung des physikalischen Wirkungsquantums”. Naturwissenschaften . 31 (14–15): 153–159. Bibcode :1943NW…..31..153P. doi :10.1007/BF01475738. S2CID 44899488.

See also

• Biographies portal
• Physics portal
• German inventors and discoverers
• Photon polarization
• Statue of Max Planck
• Zero-point energy

Notes