Werner Heisenberg: Philosopher of the Quantum Revolution

This essay explores Werner Heisenberg’s philosophy and religion, examining how his uncertainty principle reshaped modern science and influenced debates about faith, ethics, and the nature of reality

“Science deals with the objective, material world. It invites us to make accurate statements about objective reality and to grasp its interconnections. Religion, on the other hand, deals with the world of values. It considers what ought to be or what we ought to do, not what is. In science we are concerned to discover what is true or false; in religion with what is good or evil, noble or base.”1

The career of the German physicist Werner Heisenberg is one of the enduring enigmas of the 20th century. Poised at the intellectual and geographic nexus of quantum theory’s mercurial origin, Heisenberg made essential contributions to the emerging doctrine, developing the groundbreaking matrix formulation of quantum mechanics as well as the foundational Uncertainty Principle. For this work he was awarded the Nobel Prize in Physics for 1932. Like many of his cohort in the development of modern physics, Heisenberg was deeply concerned with the philosophical implications of the revolutionary quantum and relativity theories. Unlike most, however, his views were informed by both a solid grounding in philosophy and a deeply ingrained Christian world view.

The overarching mystery of Heisenberg’s life was his decision to stay in his native country as the storm clouds of National Socialism and the Second World War gathered. His international reputation as a first-rate physicist made Heisenberg briefly, to the select few who were aware of the possibility of an atomic bomb, one of the most feared men in history: the potential architect of a Nazi nuclear weapon. The reason this nightmarish vision did not come to pass – did Heisenberg sabotage the effort, or was he stymied by technical matters? - remains controversial to this day.

Formative Years 

Werner Karl Heisenberg was born on December 5, 1901, in the Bavarian town of Würzburg. He was the family’s second son, born a year after his brother Erwin.  The boys’ father, Dr. Kaspar Ernst August Heisenberg, was a teacher of classical languages in one of the local secondary schools. Although somewhat shy and sensitive, young Werner displayed a formidable work ethic from an early age. Even in childhood he found solace and expression in music, growing into an excellent classical pianist. As scholastic horizons broadened for Werner, and his enormous intellectual talents became manifest, music continued to hold a vital place in his heart, as it would throughout his life.

The Heisenbergs were a Lutheran family (Werner’s mother, Annie Wecklein, converted from Catholicism before marrying August in 1899). Lutheranism emerged in Germany in the early 16th century through Martin Luther’s Reformation, a revolt against what were seen as excesses and theological errors in the Catholic church. The Lutherans believed in salvation by faith alone and scripture as the sole authority; unlike the protestant Calvinists, however, they eschewed predestination. The bedrock of the denomination’s world view was a profound sense of duty, defined in the words of Luther himself: “A Christian is a perfectly free lord of all, subject to none. A Christian is a perfectly dutiful servant of all, subject to all… For though I am free from all men, yet have I made myself servant unto all, that I might gain the more.”2 By the time of Heisenberg’s birth, Lutheranism was the dominant religion in northern and central Germany, though Bavaria remained predominantly Catholic. As a state-supported church in many German regions, Lutheranism shaped cultural and ethical norms, influencing education and community life.

The Heisenbergs attended services regularly. With his appreciation for music, Werner was particularly enamored of the prayerful hymns, which he memorized. Even as a young boy, the ethical teachings of the church also left a strong impression.

In 1910, when Werner was eight years old, his father was appointed professor of medieval and modern Greek studies at the University of Munich. The family moved to that city, settling in the culturally vibrant Schwabing district. Werner entered the Maximilians-Gymnasium in 1911. Gymnasia were nine-year schools, focused on classical languages and literature, which were designed to prepare students for entering a university before going on to careers in the professions or academia. Werner excelled at the Gymnasium, obtaining high scores in nearly every subject. Although the focus of the school was on the classics, science and, especially, mathematics also fascinated him. Before long he had moved past the topics in the math curriculum. Werner’s instructor in the subject recognized his pupil’s brilliance. "He tried to interest me and give special problems to me,” Heisenberg later said. “He told me, 'Try to solve that and that.'"3 Werner taught himself differential and integral calculus, which were not subjects at the Gymnasium. During his final oral exams before graduation, Heisenberg astounded the examiner by solving the equations of projectile motion with air resistance taken into account. The examiner wrote, "He has already gone far beyond the goal of middle school mathematics."4

When World War One erupted in 1914 the lives of Heisenberg and his fellow students were profoundly affected. His father (who was a captain in the army reserves) and many of the all-male Gymnasium faculty joined the military and were sent to the front. Classes were severely curtailed, and the pupils became responsible, to an alarming degree, for their own education. The deleterious effects of the Allied naval blockade soon were seen in the streets and markets of Munich; food and fuel shortages became the norm. Later, Werner remembered finding solace during this trying period, what he described as “a sense of peace,” in the Lutheran hymns and prayers he had loved since early childhood.

From his youth onward Heisenberg spent most of his free time with a circle of like-minded friends, hiking, skiing, and mountain climbing in Germany and the surrounding countries, often engaged in philosophical conversation while fighting hay fever. In later life he frequently reminisced about reading Plato’s Timaeus (in Greek) while hiking in the mountains, absorbing its profound considerations of cosmology and the nature of thought and reality.  At one organized youth event held at Prunn Castle in the Altmuhl Valley Werner had an epiphany. Hearing a solitary violinist playing Bach’s D minor Chaconne, he felt “all at once, and with utter certainty,” a link with what he felt to be “the central order” of all things. It was a moment he never forgot; an insight that provided spiritual foundation for the scientific work that lay ahead.6

After the war finally came to an end, Germany entered the unstable period that would eventually give rise to its most calamitous era. Werner, however, was focused on gaining an education in mathematics and the physical sciences. In preparation for matriculation at his father’s institution, the University of Munich, he worked on local farms in the Bavarian Highlands to earn tuition money. He later considered this an experience of great value to him and others who did similar work, “the raw wind was no longer alien to us; and we were not afraid to form our own opinions on the most difficult problems.”7

Heisenberg entered the University of Munich in the fall of 1920, planning to study pure mathematics. The chair of the department, however, blocked this when he found out the young man had been studying relativity. Luckily the professor in charge of physics, Arnold Sommerfeld, who was well known for developing (along with the eminent Dane, Niels Bohr) an early quantum model of the atom, immediately recognized Werner’s capabilities, and admitted him to the advanced seminar.

Heisenberg’s interests in physics revolved exclusively around the esoteric ideas that had emerged in the twentieth century – relativity, quantum theory, and the study of atomic phenomena. The bedrock of the discipline, the contributions of Newton, Maxwell, and the rest that would soon be called “classical physics,” only intrigued him to the extent that they provided the setting in which the novel concepts shone by contrast. Under Sommerfeld’s tutelage he would be given the latitude to indulge this focus and, although he would encounter resistance from other sources in academia for this, Heisenberg took advantage of the opportunities to demonstrate his exceptional talents. During the early days at the University of Munich he encountered another brilliant young physicist named Wolfgang Pauli who, despite having a nearly diametrically opposite personality, became a lifelong friend. Pauli would prove also to be a major force in the development of quantum theory.

In June 1922 Heisenberg travelled to the university town of Göttingen to hear Niels Bohr lecture at a colloquium the young man whimsically called the “Bohr Festival.” After one talk Heisenberg, intrigued but unawed by the great man, asked an insightful question. The impressed Bohr invited Werner to join him on a mountain hike. In the hills above the town the two men – master and possible apprentice – took in the views and immersed themselves in the first of what would be many conversations about physics and philosophy. Bohr explained to his young charge that the atomic model for which he was known worldwide had arisen from his wondrous observation that, despite any number of physical and chemical assaults that could affect it, matter never changed: “The same substances always have the same properties,” said Bohr. “The same crystals recur, the same chemical compounds, etc. In other words, even after a host of changes due to external influences, an iron atom remains an iron atom, with exactly the same properties as before. This cannot be explained by the principles of classical mechanics.”8

Werner recognized Plato’s eternal Forms in Bohr’s description of the atom and would nurture the analogy throughout his life.

 In the 1922-23 academic year Sommerfeld went to America as a visiting professor. During this time, he arranged for Heisenberg to study and work for a term with Professor Max Born in Göttingen, who was known as a master mathematical physicist. When this session was completed, Heisenberg returned to Munich, where he submitted a thesis and received his doctorate degree, facing some opposition due to his disinterest in the standard curriculum and proven clumsiness in the experimental lab. Heisenberg then returned to Göttingen in July 1924, this time as privatdozent and Born’s assistant.

The confusion that already enveloped the physics community regarding the atom deepened in these days with a report by the American experimentalist Arthur Holly Compton that x-rays scattered by electrons changed their wavelength in the interaction. This finding was both unexpected and, according to the current thinking, inexplicable. Because of these and other experimental results there was now a growing sense among the leaders in the discipline that, as Bohr had said to Heisenberg in the mountains above Göttingen, classical mechanics – the hitherto-unassailable science of moving bodies that dated back to the discoveries of Isaac Newton - was going to prove inadequate to describe events at the atomic level.

It was arranged that Heisenberg would spend the winter of 1924-25 with Bohr in Copenhagen. Here, Werner concentrated on a theory to explain Compton’s findings, working alongside the Dutch physicist Hans Kramer. Their approach was unorthodox: “Our efforts were devoted not so much to deriving the correct mathematical relationships as to guessing them from similarities with the formulae of classical theory.”9 The investigative endeavors were broken up by intermittent mountain hikes with Bohr. The philosophical and scientific topics that dominated these picturesque walks were punctuated now by Bohr’s expressed concerns over the rise of antisemitism in Germany, which a hopeful but naïve Heisenberg tried to assuage. In a pensive moment when political discussion had played out, Werner told Bohr about his emotional and cathartic glimpse into the central order of the universe years before, when the notes of Bach’s chaconne drifted over the Altmühl Valley. Bohr, who was deeply philosophical but intensely pragmatic, could only marvel at an experience so far outside his own.

In April 1925 Heisenberg returned to Göttingen. At this point, the conversations with Bohr and the unconventional angle of attack he had developed working with Kramer still fresh in his mind, he made the first of his monumental contributions to physics.

Matrix Mechanics

As noted above, the prevailing theory of the atom at this time was the one initially proposed by Bohr, with amendments by Sommerfeld, during the previous decade. In this theory, atoms consisted of a tiny, positively charged nucleus surrounded by electrons. These negatively charged electrons orbited the nucleus like planets around the sun, held in their orbits not by gravity but by electrical attraction. But, unlike in our solar system, the electrons could only occupy certain orbits corresponding to the energy they contained. Not long before, Max Planck had shown that energy exists in discrete packets called “quanta.” Bohr applied the same concept to the atom and, because the electrons could only have certain “quanta” of energy, they were required to inhabit particular orbits.

Although it was conceptually simple, an asset to any theory, the model also harbored seemingly insurmountable difficulties. For example, according to classical electromagnetism theory an orbiting electron should gradually radiate away its energy as it moved around the nucleus, which was not a possibility in the model. So, while the Bohr-Sommerfeld concept was useful, it clearly was not the ultimate answer to the question of atomic structure. Long before obtaining his doctorate Heisenberg had worried and wondered about this. In extended conversations with Wolfgang Pauli, his friend mocked the idea of the miniature solar system: “Do you honestly believe that such things as electron orbits really exist inside the atom?" he asked with affectionate derision.10

In June of 1925 Heisenberg, deeply immersed in this conundrum, decided to escape the hay fever that was torturing him at Göttingen and vacation for ten days on the tiny island of Heligoland in the North Sea. It was here amidst the clear, bracing air that he experienced the towering insight that would solve the problem of the atom and introduce the unavoidable, and inconceivable, implications of quantum mechanics to the world. Heisenberg’s great leap was to recognize that what Bohr and all the others had been focusing on – himself included – was, in fact, the unknowable (Pauli had recognized this intuitively). Like classical physicists, they had worried about the position and momentum of the electron and the other components of the atom as if they were observable, like watching a thrown baseball on its path toward a batter. But this mental image of an electron was a phantom. No experimental apparatus could actually see an electron in its path around an atomic nucleus. Unlike the case of visible objects like baseballs, those aspects of these subatomic particles were only inferred from experimental phenomena that could be observed. Instead of trying to sculpt a physical model utilizing elements that could not be seen, Heisenberg determined to generate a concept using only the data that could be measured, in particular, the atomic spectra.*

*Elements were known to produce unique frequencies of electromagnetic radiation (their “spectra”) in certain circumstances. The Bohr-Sommerfeld atom accurately predicted the spectrum of hydrogen, although it failed when applied to other elements.

In his North Sea inner sanctum far from meddling mentors, Heisenberg employed the method he had used in Copenhagen: guessing what the natural law should look like based on classical mechanical analogies, using only the available data. In so doing, he came up with a bizarre array of frequencies and amplitudes. These arrays did not even exhibit the fundamental arithmetic property of commutation – the outcome of a product depended on the order in which the elements were multiplied - but they produced results that matched experimental spectral data. He left Heligoland elated but perplexed. He had found something - some deep truth - of that, he was sure. But what was it?

Back in Göttingen, Born, who was steeped in the mathematical methods of physics, recognized that Heisenberg’s scribbled arrays were what mathematicians call matrices, of which non-commutation is a well-recognized aspect. He also realized that his young protégé – Heisenberg was only 23 – had uncovered a fundamental aspect of nature: the non-intuitive way objects at the subatomic level behave. They called the theory “quantum mechanics.” Heisenberg’s first paper on his discovery, "On the Quantum-Theoretical Reinterpretation of Kinematic and Mechanical Relations," was submitted just 60 days after he returned from Heligoland and published in September 1925.11 In the months that followed, another protégé of Born’s, Pascual Jordan, and the Englishman Paul Dirac made important contributions to the new theory.

Heisenberg returned to Copenhagen in 1926 to cultivate his ideas in the stimulating presence of Bohr and his team (by now Heisenberg was fluent in Danish as well as English – a major advantage). Bohr’s response to the fundamental mystery of quantum physics – how energy and other properties of physical objects exhibited smooth continuity at the macroscopic scale but were discontinuous in the atomic setting – had begun to crystalize, even before the discovery of quantum mechanics, in his concept of complementarity. It was becoming clear to Bohr that these different manifestations of matter and energy – sometimes behaving like a wave, sometimes like a discrete particle – were not mere accidents of the observation process or reflections on the limitations of experimental technology, but a genuine facet of nature. Although an electron, for example, may behave as a particle in one experimental setting and a wave in another, to Bohr these manifestations, although mutually exclusive, were not contradictory but complementary. Only when these contradictory aspects were taken together was the true nature of the examined object revealed. As Bohr remarked, and Heisenberg frequently reiterated over the years, the real difficulty with quantum physics lay not in the concept, but in the weakness of human language to express it – or human minds to comprehend it.

Uncertainty 

In 1926 Heisenberg met Einstein in Berlin while giving a lecture on the new quantum mechanics. As Bohr had done in Göttingen four years before, Einstein took the opportunity to seek out the young man after the presentation and discuss his ideas in a one-on-one setting. He found it difficult to grasp the rationale of Heisenberg’s “observables-only” approach to the atomic structure problem. Did Werner and his colleagues think that the motion of the electron in an atom was inherently unknowable? He pointed out that electron paths were visible in cloud chambers.* Did these young Turks think that confinement of the electron to an atom somehow eliminated its path? Heisenberg had no answer.

* Cloud chambers are experimental devices which were introduced in the early 1900s. They consist of closed environments filled with supersaturated water vapor. When ionized particles pass through the chamber they induce condensation of water droplets, revealing the path of the particles.

That same year the Austrian Erwin Schrödinger introduced a new theory of atomic behavior based on a wave function he had devised. Soon he proved that his own formulation and Heisenberg’s matrix version were mathematically equivalent. Schrödinger initially believed that his equation described actual waves of matter (and hoped that this would eliminate the quantum discontinuities he found intellectually repulsive) but Max Born soon showed that the formalism in fact reflected mathematical probabilities. That is, solving the Schrödinger equation did not, for example, provide the position of a particle, but instead gave the probability that the particle would be in a particular place at a given time. This discovery carried profound implications, soon to be revealed.

Meanwhile, the cloud chamber question posed by Einstein gnawed at Heisenberg. Einstein’s point was that there was no reason an electron’s path inside an atom should be inherently unknowable when it could be easily seen outside the atom, moving through one of these cloud chambers.

In February 1927 Bohr took a break from his work to go skiing. As Heisenberg had demonstrated in Heligoland, when he was left alone for a time, he often found inspiration. A second revelation came in Copenhagen when he realized that what seemed to be the path of the electron in the cloud chamber was actually not: the sequence of condensing water droplets appeared in response to the electron’s motion, but only approximated its path: “The right question should therefore be: can quantum mechanics represent the fact that an electron finds itself approximately in a given place and that it moves approximately with a given velocity, and can we make these approximations so close that they do not cause experimental difficulties?”12 Heisenberg expressed this formally in what became known as the Uncertainty Principle: the momentum (velocity multiplied by mass) and position of an object cannot both be known at the same time with arbitrary certainty. Born’s probability interpretation of Schrödinger’s wave equation now made sense.

When Bohr returned to Copenhagen, Heisenberg excitedly told him of his discovery. The great professor was bewildered at first but soon realized that the Uncertainty Principle fitted his own complementarity concept perfectly. Together, these ideas – the wave-particle duality of matter within a statistical formulation of behavior defined by the Uncertainty Principle - constituted what became known as the Copenhagen Interpretation of quantum mechanics.

The Implications of Quantum Mechanics 

Heisenberg had studied philosophy throughout his life, from the Ancient Greeks through to the twentieth century positivists. Accordingly, the implications of this new vision, beyond laboratory experimental outcomes, were not lost on him. Here was a revolutionary break from the most foundational, intuitional concepts of the past. He and the other theorists of this quantum generation had asserted that a completely deterministic view of the universe was not possible. It had previously been thought that, in principle at least - the practicalities would be impossible to overcome, of course - if one knew the position and velocity of all the particles in the universe in the present one could predict where they would be at a future time, based on the physics outlined by Newton, Maxwell, and the others. Therefore one could, in principle, know everything that would happen in the future. This was the old “clockwork” model that had seemed philosophically tidy over the centuries. The new physics swept this determinism away in a flash flood of mathematical matrices and waves. Now it had been shown that not only could one not know the exact future position and momentum of a single particle (let alone all of them) one could not know these things even in the present.

As unsettling as this was, the new uncertainty did open the door to a philosophical concept that had been difficult to reconcile with the deterministic universe: free will. Although the processes of human thought and consciousness were (and remain) fundamentally mysterious, most who considered the issue believed that they occurred due to physical and chemical phenomena not fundamentally different from those understood in other, less emotionally charged settings. Hence, in a deterministic universe, all thought and consciousness could, theoretically, be predicted from a comprehensive knowledge of the present (or the past). Free will was, therefore, an illusion: all had been ordained ages ago. The religious implications were enormous: for example, the predestination of the widely influential Protestant sect Calvinism, in which the actions and beliefs of individuals are of no consequence to their salvation, dovetails smoothly with a deterministic world. Heisenberg’s Uncertainty Principle, in disposing of rigid determinism, did not necessarily light the way to unrestricted free will, but it cleared some of the philosophical obstacles.

Predictably, and almost immediately, the Copenhagen Interpretation came under fire. In October 1927 the fifth Solvay Conference, a congress of eminent scientists underwritten by a Belgian industrialist, was held in Brussels. Here, the new ideas were discussed at length. By far the most notable and vocal critic of the Copenhagen Interpretation was Albert Einstein. In one of history’s great ironies the discoverer of relativity, which disrupted the underpinnings of the most basic intuitional assumptions regarding space and time, could not accept the wave-particle duality and inherent probabilistic nature of the universe that defined the views of Bohr, Heisenberg, Born, and the others. As Heisenberg later remarked:

Einstein was quite unwilling to accept the fundamentally statistical character of the new quantum theory. Needless to say, he had no objections against probability statements whenever a particular system was not known in every last detail – after all, the old statistical mechanics and thermodynamics had been based on just such statements.  However, Einstein would not admit that it was impossible, even in principle, to discover all the partial facts needed for the complete description of the physical process.  "God does not throw dice" was a phrase we often heard from his lips in these discussions.  And so, he refused point-blank to accept the uncertainty principle and tried to think of cases in which the principle would not hold.13

Einstein conceived of increasingly-sophisticated “thought experiments” designed to catch the Copenhagen Interpretation of quantum mechanics in a logical impossibility, but in Brussels - and for years to come - Bohr was always able to demonstrate either that Einstein’s reasoning was flawed or that the theory defied his machinations in some other way (Schrödinger, for his part, also attempted to disrupt the probabilistic viewpoint with his famous cat thought experiment). In the meantime, every laboratory experiment that was performed confirmed the predicted results of quantum mechanics.

As early as the Solvay Conference the younger cohort of physicists had discussed what it all meant, stimulated by Einstein’s many references to God throwing dice. Some were disturbed that a modern scientist, not to mention one of the world’s most famous, would unabashedly use the word “God.” P.A.M. Dirac vocally derided the very concept of a deity as a man-made phantom, echoing the Marxist view of religion as an opiate for the masses. This caused Pauli to quip memorably that the Englishman’s view was “There is no God, and Dirac is his prophet.” Heisenberg, expressing what he portrayed as the views of the devout Christian Max Planck but summarizing his own perspective expressed elsewhere, remarked: “Science deals with the objective, material world. It invites us to make accurate statements about objective reality and to grasp its interconnections. Religion, on the other hand, deals with the world of values. It considers what ought to be or what we ought to do, not what is. In science we are concerned to discover what is true or false; in religion with what is good or evil, noble or base.”14 In short, religion and science formed their own complementarity pair – the wave and particle of all philosophy. Later, Pauli pushed Heisenberg on his belief in the “central order,” and what that expression really meant. Paraphrasing his friend’s inquiry, then adding the answer, Heisenberg replied: “’Can you, or anyone else, reach the central order of things or events, whose existence seems beyond doubt, as directly as you can reach the soul of another human being?’ I am using the term 'soul' quite deliberately so as not to be misunderstood. If you put your question like that, I would say yes. ... the word 'soul' refers to the central order, to the inner core of a being whose outer manifestations may be highly diverse and pass our understanding.”15

Heisenberg accepted a faculty position at the University of Leipzig in 1929, becoming the youngest professor in Germany. Soon afterwards he toured the United States as a visiting lecturer. He found the American scientists, many of whom became lasting friends, more receptive to the radical new ideas than their European counterparts. The enormous size of the country and its seemingly boundless energy for progress captivated him, as well. After returning to Leipzig, where - like Bohr - he began to gather his own students and colleagues, in 1932 Heisenberg was awarded the Nobel Prize in Physics for the discovery of quantum mechanics (he had been nominated by Einstein).

Bohr and Heisenberg continued their close relationship even as the younger man attained a comparable level of prestige. Their in-person interactions always tended to mix physics and philosophy; sometimes they strayed into other branches of science and the potential impact of their discoveries there. On one occasion the subject of Darwin’s theory of natural selection came up. Neither of the physicists could be sure how quantum physics could be applied to biology, but both felt certain that the strange new rules would remain applicable in living creatures. They wondered about the foundational elements of evolutionary theory, though, apart from any quantum mechanical considerations. Bohr pointed out that the theory was, in fact, twofold: natural selection favoring the fittest examples within a species, which seemed incontrovertible, and random genetic mutation leading to new forms of an organism with potentially enhanced fitness, which was far less self-evident. As Heisenberg remembered it, in Bohr’s view the thought that such a random process could lead to complex organisms, even within the “geological” times postulated, was “absurd.”

By this time, the political situation in Germany was deteriorating quickly. Just as in his days in Munich, Heisenberg spent most of his leisure hours enjoying the camaraderie of his youth group friends, hiking, skiing, and conversing. Now, to his horror, a parallel National Socialist organization, the Hitler Youth, had arisen, espousing the ideals of the German life and culture in a perverse and menacing way. They derived their name from the rising political figure whose name was on everyone’s lips. The Hitler Youth were symptomatic of the widespread and growing anti-Semitism and militant nationalism in the country.

Dozens of Heisenberg’s colleagues were forced to emigrate from Germany rather than face the degradations of the ascendant Nazi political movement. The threat reached even within the rarefied world of physics, where the strange new concepts of relativity and quantum mechanics were deemed “Jewish science,” to be rejected by the establishment. Jewish professors and students were ostracized and modern scientific concepts shunned, or their teaching banned outright. Heisenberg himself was called a “white Jew” (an ethnic German with Jewish behavior) in an SS newspaper for being a proponent of the new physics. He was prohibited from succeeding Sommerfeld to the Chair of Physics in Munich.

Most of the important German scientists, Jewish and otherwise, fled the Third Reich. Many of these, and others in England and America who had not been under threat, queried Heisenberg as to why he remained in Germany. His response combined a scientific patriotism that transcended politics with paternal responsibility to the fledgling physicists under his tutelage. It also reveals Heisenberg’s adherence to the ethical bond derived from his Lutheran background: the concept of faith in duty noted earlier:

I said to myself that a country’s reconstruction must start with its young people, and that it was my task to train these young people in the best possible way, even under difficult conditions. I felt that by staying I could perhaps create small islands of permanence in the midst of chaos, places where serious scientific work could still be done. To abandon my students and colleagues, to leave them to their fate, seemed to me a kind of betrayal. I believed that German science, which had given so much to the world, should not be allowed to perish completely, and that by remaining I could help to preserve something of value for the future.16

Heisenberg was aware then, as later, that there was a price to pay whatever he did. Stay and risk being swallowed up by a regime revealing itself with each passing day as the essence of evil (or, still worse, become subsumed within it) or abandon his pupils and beloved country when they needed him most. He had no way of knowing that a scientific wild card of the gravest import was on the horizon, one that would ratchet the significance of his decision to stratospheric heights.

World War Two

 In January 1937, even as war clouds began to gather over Europe, Heisenberg met Elisabeth Schumacher, the daughter of a prominent economics professor, at a Leipzig society gathering where he played piano. A whirlwind courtship ensued, and the couple were engaged just a few weeks later. Thirteen years his junior, Elisabeth was vivacious, intelligent and loved music. Like Werner, she was Lutheran. The pair were married that April and had twins Maria and Wolfgang the following January. The Heisenbergs would have five more children over the next dozen eventful years, his family providing Heisenberg a strong grounding during the tumultuous events ahead.

Not long after the Heisenberg twins were born news of an exciting and ominous experimental result swept through the community of physicists worldwide. The German chemists Otto Hahn and Fritz Strassmann, aided by the Austrian physicists Lise Meitner and Otto Frisch, had discovered nuclear fission. In their experiments, these scientists had bombarded uranium with neutrons, inducing splitting of the uranium nucleus with the release of energy and production of the lighter element barium. The ancient dream of the alchemists – the transmutation of elements – had been achieved. Of greater significance, physicists everywhere immediately realized that this nuclear fission, as it was called, could release more neutrons, which could then split other nuclei and create a chain reaction. Harnessed correctly, such a reaction would release prodigious amounts of energy. If it were done slowly, as a “reactor,” a near-limitless power source was at hand, suitable for peaceful purposes. If done quickly, though, it would constitute a bomb of immense destructive force. Quick calculations proved that a single “atomic bomb” had the potential to destroy an entire city.

When the long-anticipated European war finally commenced with the Nazi invasion of Poland in September 1939 Heisenberg was called to Berlin, along with many of his colleagues. They were assigned roles in the scientific establishment to support Germany’s war effort. The group Heisenberg led, which included such luminaries as the talented physicist Carl von Weizsäcker, was tasked with exploring the potential uses of atomic energy in a military capacity. The team was called the Uranverein (“Uranium Club”).

Early in these considerations Heisenberg recognized that the technical obstacles to assembling an atomic bomb were enormous. His calculations, mirrored by others in faraway places like Los Alamos, New Mexico, revealed that uranium, the obvious choice for the bomb’s “fuel,” presented serious difficulties. The predominant natural form of the element, uranium 238, would not produce sufficient secondary neutrons to generate a chain reaction. Its rare isotope, uranium 235, was suitable, but very difficult to isolate. Considering these and other hurdles, Heisenberg concluded that Germany did not have the industrial or technological capability to acquire sufficient amounts of the material to make a bomb in a time frame that would have an impact on the course of the war (indeed, he believed that was the case for both sides until he found out otherwise in August 1945). The Uranverien group focused its work, instead, on attempting to construct a nuclear reactor as a possible power source.

The work encountered both theoretical and practical difficulties and progress was slow. As the war rolled on along distant fronts and his experiences as a youth played out again in dismal reprise, Heisenberg sent a letter to Elisabeth confiding the comfort his well-loved Lutheran hymns still provided: “In these dark times, I find strength in the hymns of my youth, which remind me that there is a plan beyond what we can measure.” My work on the atom feels like a search for that plan.”17

In September 1941 a scientific congress was arranged in Copenhagen by the German authorities that now occupied Denmark. Heisenberg asked Niels Bohr for what he knew would be a most uncomfortable meeting. With his former mentor’s overriding worries come to full fruition, the old days of leisurely mountain hikes and musings on esoterica were long gone. The topic of atomic fission was, moreover, unavoidable. As Heisenberg remembered it, he informed Bohr that the Germans were, in fact, working on atomic energy. He emphasized, though, that technical difficulties of constructing a bomb on the fission principle were too great to overcome in a finite timeline: the Uranverein was limiting its efforts to building nuclear reactors. Even then, work was slow and laborious. Bohr remembered the conversation differently, recalling only the essential information that the Germans were working on atomic energy. In later years Heisenberg believed that his old mentor was so horrified to learn that his protégé was working in the field that he failed to grasp that an atomic bomb was not the goal. While that may be the case (and each man adhered to his version for the remainder of his life) Bohr could hardly be blamed for missing the point - if he did - surrounded as he was by the Wehrmacht.

Perhaps of greater significance, Bohr escaped from Denmark in September 1943 and eventually made it to Los Alamos, New Mexico, where the American effort to construct a nuclear weapon was well underway. There he would deliver direct information that the Germans, under the mighty Heisenberg, were themselves actively working on atomic fission. Little could have motivated the leaders of the Manhattan Project to exceed the effort they were already making, but this news certainly would not have eased their resolve. How Oppenheimer and his subordinates would have responded to word that the Germans did not believe a bomb was possible before war’s end can only be speculated.*

*An American spy, the former major league baseball player Moe Berg, was dispatched to Europe in December 1944 to observe Heisenberg and, if he were thought to be close to succeeding in the presumed German effort at an atomic bomb, assassinate him (this appears not to have been related to any reports of Bohr’s). Berg heard Heisenberg at a lecture in Zurich and was convinced that the Germans were not close to success.

Over the course of the war the Uranverein made little significant progress toward producing a functioning reactor, much less a nuclear bomb. Surprisingly, Heisenberg managed to accomplish some pure theoretical research and published work on elementary particles. After Germany’s surrender, Heisenberg and some of his colleagues (including Carl von Weizsäcker and Otto Hahn) were captured by the British and Americans and brought to Farm Hall in England, where they were held and surreptitiously surveilled. Allied intelligence was still hoping to determine – even after the European war was over – how close the Germans had come to creating a fission weapon. It was here, on August 6, 1945, that the German scientists first heard, to their great shock, of the successful detonation of the atomic bomb at Hiroshima by the American military (Hahn, in particular, as the discoverer of nuclear fission, was devastated at the news). After the surprise had worn off, Heisenberg counted himself and his colleagues in the Uranverein fortunate in their lack of success in constructing an atomic bomb. As he wrote Elisabeth: “I know many of the British and American colleagues who worked on it, some of them are my pupils, and they have my sympathy, because their name is now tied to this atrocity.”18 Many years later he said, “Looking back, I see that our work on uranium was limited by both technical and ethical considerations. I felt a duty to my country, but not to its war aims in that direction. We proceeded cautiously, perhaps too cautiously, but that caution saved us from a greater burden.”19

Heisenberg was returned to Germany in January 1946 and resumed his work at the Kaiser Wilhelm Institute for Physics in Göttingen, which was renamed the Max Planck Institute for Physics soon after. He began the long process of resurrecting German science from the ashes of both the prewar talent exodus and the literal destruction wrought by the Allied conquest. In 1952 Heisenberg and Bohr had a bittersweet reunion, each recalling their portentous 1941 meeting in his own way before burying the painful past, if only superficially, to continue their personal and professional relationship. The close camaraderie, though, was lost forever.

Post War Life

 In the postwar years many questioned Heisenberg’s motivations for remaining in Germany during the Nazi regime. Some never forgave him for it. He remained steadfast in his justification, however, refusing to apologize for actions he professed to be driven by noble intent. As to his work in the Uranverein, he was quoted as saying, “I told Bohr in 1941 that we were working on atomic energy, but I emphasized the immense difficulties of a bomb. I did not want to contribute to such a weapon under the Nazis, and our work was directed toward peaceful applications. The failure was partly due to these obstacles, which I did not try to overcome with all my might.”20

In 1958, the Max Planck Institute relocated to Heisenberg’s hometown of Munich, and he continued as its director, also becoming – twenty years after the Nazis prevented it – his mentor Sommerfeld’s successor as Professor of Physics at the University of Munich. The progress of physics continued, and Heisenberg focused his talents on work in the fields of subatomic particles, turbulence, and superconductivity. He also joined his colleagues in the quest for the Unified Field Theory, a Holy Grail that would synthesize all of physics in a single, grand concept (this pursuit continues today). Though a reluctant public figure, he was vocal in the political arena about nuclear proliferation, arguing, among other things, against the arming of the Federal Republic of Germany with atomic weapons.

In the 1955-1956 academic year Heisenberg was asked to deliver the Gifford Lectures at the University of St. Andrews in Scotland. These were later published as the book Physics and Philosophy: The Revolution in Modern Science. True to its title, this work related both the foundation of quantum physics, including the groundbreaking re-interpretation of reality within the Copenhagen Interpretation, and its relation to philosophical thought from the ancients onward. In this book Heisenberg wrote that “What we observe is not nature itself, but nature exposed to our method of questioning.”21 He drew a comparison with the concepts of the great German nineteenth century philosopher Immanuel Kant, who emphasized that we do not experience the world itself, but how our senses perceive it. Recalling his many profound discussions with Niels Bohr, and the inadequacies of human language – and even the human mind - to grasp and convey the ideas revealed by quantum physics and relativity, Heisenberg also noted “The existing scientific concepts cover always only a very limited part of reality, and the other part that has not yet been understood is infinite. Whenever we proceed from the known into the unknown we may hope to understand, but we may have to learn at the same time a new meaning of the word 'understanding.'”22

In 1971 Heisenberg wrote a semi-autobiographical book called (in English) Physics and Beyond, which expanded on some of the ideas of Physics and Philosophy, this time in the classical format of the philosophical dialogue. Here, amongst the reminiscences, he reflected on the juxtaposition of science and religion, as he had while a young man at the Solvay Congress more than four decades before: “The problem of values is nothing but the problem of our acts, goals and morals. It concerns the compass by which we must steer our ship if we are to set a true course through life. The compass itself has been given different names by various religions and philosophies: happiness, the will of God, the meaning of life—to mention just a few.” He also remembered “Pauli once asked me if I believed in a personal God. I replied that if one understands 'God' as the central order of things, then yes, I believe in it. But this order is not something we can grasp fully with our intellect; it is something we can only approach through faith and intuition.”23

On March 24, 1973, Heisenberg gave a speech before the Catholic Academy of Bavaria. With the mantle of elder scientific statesman comfortably wrapped around him, the great scientist took the opportunity to reiterate his thoughts on the complementarity of science and religion:

In the history of science, ever since the famous trial of Galileo, it has repeatedly been claimed that scientific truth cannot be reconciled with the religious interpretation of the world. Although I am now convinced that scientific truth is unassailable in its own field, I have never found it possible to dismiss the content of religious thinking as simply part of an outmoded phase in the consciousness of mankind, a part we shall have to give up from now on. Thus, in the course of my life I have repeatedly been compelled to ponder on the relationship of these two regions of thought, for I have never been able to doubt the reality of that to which they point.24

In an interview with the German magazine Der Spiegel late in life, Heisenberg reflected “I have always felt that the more deeply one penetrates into the laws of nature, the more one is led to a sense of reverence for the Creator. This is not a scientific conclusion, but a personal conviction that has grown with my work.”*25

*The following quote has been attributed to Heisenberg: “The first gulp from the glass of natural sciences will turn you into an atheist, but at the bottom of the glass God is waiting for you.” A documentary source for this remark, which is reminiscent of Bacon’s quote "A little philosophy inclineth man’s mind to atheism; but depth in philosophy bringeth men’s minds about to religion," has proven elusive. In a 1948 lecture, Heisenberg’s friend and colleague Carl von Weizsäcker made a similar remark: “The first gulp from the cup of knowledge is separating us from God, but on the bottom of the cup God is waiting for those who search for him.” von Weizsäcker referred to this as “an old saying.” He also said “Heisenberg often spoke of a harmony in nature that he felt was rooted in his Christian upbringing. He saw the uncertainty principle not as a limit, but as a window to something greater, which he hinted might be God’s will.” 26

Heisenberg died of kidney cancer at his home in Munich on February 1, 1976.

The career of Werner Heisenberg is a complex saga full of dazzling achievements and baffling decisions. Had he emigrated from Germany in the 1930s like so many of his colleagues, Heisenberg might be celebrated worldwide by the public for his epoch-making scientific contributions in the same manner as Albert Einstein. Yet the fact remains that he did not emigrate, he did work on atomic energy in Germany during World War Two, and, despite moving and entirely consistent exculpatory statements by friends, family, and the man himself, questions of his motivations will inevitably persist. For our purposes, though, Heisenberg stands as the most devotedly philosophical great scientist of the twentieth century, his wisdom shaped by Plato and Kant as much as Newton and Maxwell; his morality sunk firmly in the bedrock of a deeply ethical German Lutheranism. Heisenberg’s life affirmed the complementarity he described - science probing reality, religion guiding values - a harmony at the heart of his contradictory legacy.

The Copenhagen Interpretation of quantum mechanics Heisenberg developed and championed with Niels Bohr remains the prevailing view on the subject. Every experimental result in the century since its conception has confirmed its predictions.

 End Notes

1. Attributed to Werner Heisenberg, Physics and Philosophy: The Revolution in Modern Science (New York: Harper & Row, 1958).

2. Martin Luther, “The Freedom of a Christian,” trans. W. A. Lambert, rev. Harold J. Grimm, in Luther’s Works, vol. 31, ed. Helmut T. Lehmann (Philadelphia: Fortress Press, 1957), 344.

3. Werner Heisenberg, Physics and Beyond: Encounters and Conversations, trans. Arnold J. Pomerans (New York: Harper & Row, 1971), 29.

4. Ibid.

5. David C. Cassidy, Uncertainty: The Life and Science of Werner Heisenberg (New York: W. H. Freeman, 1992), 45.

6. Heisenberg, Physics and Beyond, 41–43.

7. Ibid., Physics and Beyond, 49.

8. Ibid., Physics and Beyond, 56.

9. Ibid., Physics and Beyond, 65.

10. Cassidy, Uncertainty, 117.

11. Werner Heisenberg, “Über quantentheoretische Umdeutung kinematischer und mechanischer Beziehungen,” Zeitschrift für Physik 33 (1925): 879–893.

12. Heisenberg, Physics and Philosophy, 20–22.

13. Ibid., 34.

14. Ibid., Physics and Philosophy, 48.

15. Heisenberg, Physics and Beyond, 155.

16. Ibid., Physics and Beyond, 211.

17. Quoted in Cassidy, Uncertainty, 391.

18. Heisenberg, quoted in Cassidy, Uncertainty, 432.

19. Heisenberg, Across the Frontiers, trans. Peter Heath (New York: Harper & Row, 1974), 248.

20. Heisenberg, Physics and Beyond, 219.

21. Heisenberg, Physics and Philosophy, 58.

22. Ibid., 55.

23. Heisenberg, Physics and Beyond, 210.

24. Heisenberg, Across the Frontiers, 213.

25. Interview with Werner Heisenberg in Der Spiegel, 1974, as cited in Cassidy, Uncertainty, 443.

26. Carl Friedrich von Weizsäcker, quoted in Cassidy, Uncertainty, 421.

Copyright 2025 Craig A. Miller