Werner Heisenberg and Albert Einstein

by Gerald Holton
Jefferson Physical Laboratory
Harvard University
Cambridge, MA 02138

Werner Heisenberg is suddenly in the news again, this time thanks to the controversial new play "Copenhagen" by Michael Frayn. [1] It centers on the ambiguous reasons for Heisenberg's visit to his early mentor, Niels Bohr, in 1941, in German-occupied Copenhagen. Among its high points are the speculations of what might have transpired at that time during their evening walk, which Bohr had ended abruptly, terrified by something Heisenberg had said to him.

Remaining content with half-knowledge

The play brings together those three quite different worlds of science, history and theater, and rightly has been highly recommended. However, there is of course the danger that the intermingling of playwright, actors, physics and history of science, might in some minds strengthen the all-too-common failing to confuse the play, a work of fiction, with a documentary. Just think how many people believe they know all about Galileo from watching Bertolt Brecht's tale. One must never forget, as Samuel Taylor Coleridge put it in his Biographia Literaria of 1817, the task of the poet and dramatist is to create the "willing suspension of disbelief" and "poetic faith." These notions are not far from what John Keats, about that time, famously referred to as the "Negative Capabilities" of great authors, namely their ability, in his words, of "being in uncertainties, mysteries, doubts, without any irritable reaching after fact and reason"--in short, of their "remaining content with half-knowledge." [2]

Scientists and historians also often have to be content with only half knowledge, at least for a time. Let me give you a poignant example. It concerns precisely that private conversation between Heisenberg and Bohr during their walk in 1941. Heisenberg's gave his most familiar version in his letter to the journalist Robert Jungk, who published it in 1958 in his book Brighter than a Thousand Suns: A Personal History of the Atomic Scientists. [3]

The main thrust of Heisenberg's letter was that the researchers in his "Uranverein" in 1941, in Heisenberg's words, "knew that one could produce atom bombs but overestimated the necessary technical expenditure at the time." Still, the physicists engaged in such work could have "decisive influence on further developments, since they could argue with the government that atom bombs would probably not be available during the course of the war." Hence it was thought by Heisenberg and colleagues that a talk with Niels Bohr "would be of value." During that evening walk, "this talk probably started with my question whether or not it was right for physicists to devote themselves in wartime to the uranium problem...." Heisenberg added that Bohr was shocked by this train of thought, assuming "that I had intended to convey to him that Germany had made great progress in the direction of manufacturing atomic weapons," and that Heisenberg was unable "to correct this false impression."
Even though Heisenberg himself had started that letter with the remark, "I may be wrong after such a long time," and even though Jungk later called the notion of passive resistance owing to moral compunction by German scientists during their war work on the exploitation of nuclear energy "a myth," many have taken the letter and similar statements to be the definitive description of that meeting, and of the role of Heisenberg and his fellow researchers in general.

However, in 1985, when I was in Copenhagen to speak at a meeting in honor of Niels Bohr's memory, I was approached by Erik Bohr. He showed me a letter written by his father, Niels, saying it had been found after Bohr's death, folded in his copy of the book by Jungk. That letter was addressed to Heisenberg. It took serious issue with Heisenberg's published version of the meeting, in quite firm language--so much so that Niels Bohr apparently decided not to mail it after all.

I was asked what should be done with the letter. I advised, as I do usually, to save it and to put it in the archives. That's where it is now, part of the "Bohr Political Correspondence" file which the family has decided not to release to the public until fifty years after Niels Bohr's death, that is, in 2012. Therefore it would be inappropriate for me to say more about it now. Thus, even on the question of what happened during that walk, those who rely mainly on Heisenberg's version and that of his followers, as the "Copenhagen" play does in the end, will remain with half-knowledge for perhaps another dozen years. In the meantime, Jeremy Bernstein's book, Hitler's Uranium Club [4] is an excellent source for understanding the ambitions of Heisenberg's "Uranverein," and reasons for their ultimate failures.

Being captured by Einstein

The larger theme of the long-term relationship with Bohr, starting with their first meeting in 1922, has been well covered in the biographies by David Cassidy and Abraham Pais.[5] But to understand better Heisenberg's enormous talent and his responses to the challenges of history, it will be useful to take, as it were, a complementary point of view--to examine another deeply significant relationship with a major scientist, even if in the space available one can only sketch high points of the full trajectory.

At the center of this case are Heisenberg and Einstein. My interest in their interaction was aroused during my first, accidental encounter with Heisenberg himself. It happened in December 1965, when UNESCO was holding a grand conference in Paris devoted to Einstein's work. I was invited to lecture on his epistemology, chiefly on Einstein's pilgrimage, starting from his early positivism, strongly influenced by the philosophy of Ernst Mach, and ending in Einstein's rational realism, close to that of Max Planck. I had followed that change in Einstein's thoughts through reading his correspondence in the files, then located at the Institute in Princeton, where I was guiding Helen Dukas in the preparation of a scholarly accessible archive of his correspondence and manuscripts.

On finishing my UNESCO lecture, I left the podium, the next speaker came forward, and we met midway. It was Heisenberg. He seemed pleased, and in passing whispered to me, "We must talk afterwards." I shall return to this encounter later.

Among the main sources for what follows are Heisenberg's eloquent books and autobiographical articles, the unpublished transcripts of the twelve interviews he gave to the History of Quantum Mechanics Project, his unpublished letters to Einstein, and some thoroughly researched biographies. From these it emerges first of all that in the history of modern physics, no one but young Werner was so destined by the fates to be captured by Einstein's relativity theory. In his Gymnasium days, he read and loved Einstein's popular book on special and general relativity, which had just been published. He was not yet eighteen when he must have heard of the sensational November 1919 eclipse expedition results. Then, at the University of Munich, he studied under the guidance of Arnold Sommerfeld, and attended his lectures on relativity. Heisenberg was also captivated by Hermann Weyl's book, Raum-Zeit-Materie. And to top it off, one of his closest friends there was Wolfgang Pauli, who, while still a fellow student, was writing his Handbuch monograph on relativity theory. When Heisenberg moved to the University at Göttingen, he got more relativity theory from Max Born. In short, it came to him from all sides. Although Pauli wisely warned him to devote his future research to quantum physics instead of relativity, Heisenberg had no way to escape being fascinated by Einstein's work.

Early in his years at Munich University, as Heisenberg reported later, he and some friends went on a bicycle tour for several days around Lake Walchensee. At one point, while resting, the talk turned to Sommerfeld's relativity course. Heisenberg was especially struck by a remark made by his friend Otto Laporte, recalling it later as follows: "We ought only to use such words and concepts as can be directly related to sense perception....Such concepts can be understood without extensive explanation. It is precisely this return to what is observable that is Einstein's great merit. In his relativity theory, he quite rightly started with the commonplace statement that time is what you read on a clock. If you would keep to such commonplace meaning of words, you will have no difficulties with relativity theory. As soon as a theory allows us to predict correctly the result of observations, it gives us all the understanding we need." [6] This "instrumentalist" or "operational" view of Einstein's method was then quite common, and remained so for decades. We shall see that Laporte's long-remembered praise of it prepared for Heisenberg, unbeknownst to him, the ground for a key insight many years later, which changed physics forever.

In the summer of 1922, Sommerfeld arranged for Heisenberg to go to Leipzig, where Einstein was to give a lecture. It was to be their first meeting; but it turned into a surrealistic glimpse of things to come. On entering the crowded lecture hall, a handbill was forced on Heisenberg, signed by the physicist-Nobelist, Philipp Lenard, and eighteen other German scientists. It was a vicious attack on Einstein, whose theory, as Heisenberg recalled, "was said to be nothing but wild speculations, alien to the German spirit, and blown up by the Jewish press." [7]

Heisenberg has told about being shaken by this political attack on scientific truth--so much so that in his agitation he didn't even notice that the speaker on the distant platform was not Einstein but rather his courageous friend and colleague, Max von Laue, the one who also never felt he had to bend with the wind after the Nazis took power in 1933. Einstein had decided to cancel that lecture, knowing he was in mortal danger from Nazi rowdies; they had just recently assassinated his close friend, the Foreign Minister Walther Rathenau, and had published a list of future Jewish victims, on which Einstein's name appeared. It was a major reason for Einstein's leaving Germany for his around-the-world trip in 1922-23.

Only the theory decides what one can observe

The first real meeting between our two protagonists occurred in 1924, when Einstein--at age forty-five about twice as old as Heisenberg--came briefly to Göttingen. The recent work of Bohr, Kramers and Slater, the BKS theory, was hot news. But because it relaxed the requirements in physics of strict causality and of energy and momentum conservation, Einstein wrote to Max Born that if this kind of science would persevere, "I would rather be a shoemaker or employee in a gambling casino than a physicist."

Against that background, Einstein and Heisenberg had a private talk in 1924, during a walk through the neighborhood. (By the way, what has happened to the life of scientists? Where have all those walks gone?) But, as Heisenberg wrote to his parents right away, "Einstein had a hundred objections" to the BKS theory. Coming from the scientist whose work

Heisenberg had been admiring since early youth, this rejection of the new way of doing physics at their first encounter must have shaken Heisenberg. But he would console himself, as he said in one of his later interviews, that his generation, having "grown up into a complete mess" in quantum physics, was in the happy position willingly to give up old schemes if necessary.

Next year, on September 25, 1925, Heisenberg's published his brilliant breakthrough to quantum mechanics, "On the Quantum Theoretical Reinterpretation of Kinematic and Mechanical Relations." He had arrived at it during two lonely weeks in the island of Helgoland, to which he had fled to recover from hay fever. At the very beginning, the abstract of the paper announced Heisenberg's fundamental guiding principle: to work with the observable properties of a spectrum, instead of with models built on unobservables such as the position and periods of electrons in the atom: "This work is an attempt to find foundations for a quantum-theoretical mechanics which is based exclusively on relations between quantities that are in principle measurable."

In one of his autobiographical essays, Heisenberg tells that his crucial insight to deal only with the observable frequencies and intensities of line spectra was an echo from the days when he had been struggling with relativity theory at the University in Munich. Before starting that paper in Helgoland, he remembered, "as our friend Otto during our bicycle tour presented the position of Einstein's viewpoint..., one must regard only the observable magnitudes as the indication of atomic phenomena." [8]

But if Heisenberg had any illusion that his article would be approved by Einstein, he would have been wrong. One of Heisenberg's five surviving letters in the Einstein archive, dated November 30, 1925, is evidently a reply to a note from Einstein (now lost) that had contained many objections. In his response, Heisenberg tried to hold out the hope of an eventual peaceful bridging between what he called Einstein's light quantum theory and "our quantum mechanics." Heisenberg also drew prominent attention to his having used only "observable magnitudes" in his theory. All to no avail.

The following year, 1926, is one of high drama in this growing but troubled relationship. In April, Heisenberg gave a two-hour lecture on his matrix mechanics before von Laue's famous physics colloquium at the University of Berlin. In the audience, with a whole group of potentates, was Einstein. It was their second meeting. Einstein, interested and no doubt disturbed by the lecture, asked Heisenberg to walk home with him--there is that walk again--and thus ensued a remarkable discussion, which Heisenberg later reconstructed and reported in many places, from 1969 on.

At that encounter, Heisenberg once more tried to draw attention to having not dealt with unobservable electron orbits inside atoms, but with observable radiation. He reports having said to Einstein: "Since it is acceptable to allow into a theory only directly observable magnitudes, I thought it more natural to restrict myself to these, bringing them in, as it were, as representatives of electron orbits." To this Einstein is said to have responded, "But you don't seriously believe that only observable magnitudes must go into a physical theory?" Heisenberg goes on, "In astonishment, I said: I thought that it was exactly you who had made this thought the foundation of your relativity theory....Einstein replied: Perhaps I used this sort of philosophy; but it is nevertheless nonsense (Unsinn)." And then came Einstein's famous sentence: "Only the theory decides what one can observe." [9]

All this must have come to Heisenberg as a scathing attack on what he regarded as his fundamental orientation, derived from reading Einstein's early works, and being guided by them from the start, right through his most recent triumph. But now, in this meeting, Einstein, whose development away from positivistic instrumentalism to a rational realism had escaped Heisenberg's notice, went on to explain at length how complicated any observation is in general, how it involves assumptions about phenomena which in turn are based on theories. For example, one almost unconsciously uses Maxwell's theory when dealing with a light beam that conveys experimental readings.

Perhaps this discussion helped Heisenberg eventually to embark on his own epistemological pilgrimage, ending later with a kind of neo-Platonism in the description of nature through the contemplation of symmetries. But in 1927, just before starting on his next breakthrough, later called the uncertainty principle paper, Heisenberg suddenly remembered Einstein's terrifying sentence, "Only the theory decides what one can observe." It was a key to Heisenberg's advance. As he put it in one of his interviews, "I just tried to turn around the question according to the example of Einstein."

But exactly at this point I should pause to return briefly to the unfinished story of my own encounter with Heisenberg in 1965 in Paris. For after giving his lecture, Heisenberg came over to tell me in detail about that meeting with Einstein in 1926, and what it had meant for him--all this long before he published anything about it. Indeed, as if to make sure I had it straight, Heisenberg followed up by sending me a letter in January 1966, in which he repeated the story, and added a rather striking conclusion: While the theory determines what can be observed, the uncertainty principle showed him that a theory also determines what cannot be observed. Thus, ironically, Einstein, through his 1926 conversation, had provided Heisenberg with some genetic material in the creation of the uncertainty principle article of 1927.

Descending along two tracks

From here on, we could follow the effect of Einstein on Heisenberg along two diverging tracks. Both start at a high level, but descend eventually into terrifying terrain below. One track is the scientific one. Despite all his misgivings, Einstein realized of course the brilliance of Heisenberg's work. For example, he nominated Heisenberg for a Nobel Prize for three years before Heisenberg was so recognized, even though Einstein to the end believed that Heisenberg's way of doing physics would ultimately turn out not to be true to the thoughts of the "Old One," not the "true Jacob."

The third meeting of the two men took place in October 1927, at the six-day-long Solvay Congress of physicists in Brussels. It was the scene of famous debates, mainly between Einstein and Schrödinger on the one hand, and Bohr, Heisenberg and their colleagues on the other. It soon became clear that the Copenhagen spirit had triumphed; each of Einstein's ingenious arguments were answered by Bohr before nightfall. This would go on day after day, until Paul Ehrenfest finally said, as Heisenberg recalled, "Einstein, I am ashamed for you."

During a later interview, Heisenberg added a shrewd point: "I would say that a change had taken place, which I can only express in terms of lawsuits. That is, the burden of proof was reversed....That made a complete change of view among the younger generation...." Ironically, the same kind of reversal of fortunes had happened long before, in the triumph of Einstein's relativity over his opponents. But Heisenberg's last surviving letter to Einstein, written a few months before the Brussels meeting, already showed the cocky self-confidence of the victors in that new struggle. Heisenberg writes there that while in the new quantum mechanics Einstein's beloved causality principle is baseless, "We can console ourselves that the dear Lord God would know the position of the particles, and thus He could let the causality principle continue to have validity."

Strangely enough, in 1954, in Princeton, a year before Einstein died, a final meeting with Heisenberg took place. Heisenberg found that Einstein's view had not changed since the Physics Congress of 1927. Despite all Heisenberg's persuasive skills, Einstein just said, "No, that's nothing. That's not the thing I am after. I don't like your kind of physics. I think you are all right with the experiments...but I don't like it."

The second track I mentioned concerning the later relation between the two men was of course the result of the emergence, in full force in 1933, of what had been foaming in Germany since the early 1920s from the mouth of the Beast. For a time, Heisenberg continued to mention Einstein in his lectures and publications. But the scene was now dominated by demons, for example by the raving articles published by Johannes Stark, calling Heisenberg in 1935 the "spirit of Einstein's spirit." The attacks on Heisenberg and on theoretical physics as such increased in the press, culminating on July 15, 1937 in the official journal of the SS, Das Schwarze Korps. That article, endorsed by Stark, called Heisenberg a "white Jew," while dismissing relativity and quantum theory as non-German, Jewish thinking.

There followed a one-year-long attempt by Heisenberg to obtain exoneration from a family acquaintance, Heinrich Himmler himself. That finally succeeded, although with the stipulation that in the future Heisenberg would "clearly separate for your audiences, in the acknowledgment of scientific research results, the personal and political characteristic of the researcher." Privately, Himmler had his eye on Heisenberg as a possible researcher on Himmler's own crazy "World Ice Theory," of which I will spare you the details. But any future playwright dealing with a version of the Heisenberg-Einstein relation will not be able to avoid including the cries, off stage and ever more distant, of the unmentioned millions who had also loved their homeland but had no way to make a deal with Himmler, or to bribe an SS man bent on murder.

At any rate, the use of the equation E=mc2 continued to be quite allowable to German scientists. Indeed, it remained their Holy Grail, from the very beginning of the Uranium Club. The several crucial mistakes ruled out producing a bomb, but allowed them to hope, under Heisenberg's leadership, to exploit nuclear energy for powering the war machine if they could build a reactor. They had been called into action by the German government well before the Allies got going in an organized way on their research.

Recasting the portrait of Einstein

At the end of that second track, we are at last in peacetime. Heisenberg is now securely installed as the leader of a new generation of German physicists, as he had hoped all along he would be. But just then there appeared, in two of Heisenberg's lectures, passages that signaled the depth to which his relationship with Einstein had fallen--as, on a parallel path, it had earlier with Niels Bohr.

Shortly after Einstein had died in 1955, Heisenberg published an article at a popular level, entitled, "The Scientific Work of Einstein." [10] It started out generous in its assessment of Einstein's contributions, but found a serious fault with him. The flaw, in Heisenberg's view, was presented in these words: "that Einstein, to whom war was hateful, should have been moved by the infamous practices under Nazism to write a letter to President Roosevelt in 1939, urging that the United States vigorously set about the making of atomic bombs..." which eventually "killed many thousands of women and children...." [11]

That bitter statement was at the very least a major exaggeration. As often reprinted and analyzed, the famous letter of August 1939 which Einstein had signed had been written just as the German war machine was poised to start its Blitzkrieg--and, as we now know, four months after Paul Harteck and Wilhelm Groth had asked the German War Office to investigate nuclear explosives. Far from urging that the United States vigorously set about the making of atomic bombs, the letter Einstein signed was, in the words of the letter, "a call for watchfulness and, if necessary, quick action." It noted that two laboratories had found that splitting an uranium nucleus yielded approximately two neutrons. Therefore, the letter added, "it is conceivable" that nuclear chain reactions might be possible and, if so, might be used to produce "extremely powerful bombs." The watchfulness which the letter urged was the more important as "Germany has actually stopped the sale of uranium from the Czechoslovakian mines which she has taken over."

As to proposing actual steps, the letter asked only to establish contact between the Administration and the physicists, to appoint a person as liaison and help raise funds for experimental work in university laboratories, "if such funds be required," through contacts with private donors and industrial laboratories. In what followed, all of $6,000 was made available to Enrico Fermi at Columbia University. Einstein declined the invitation to be a member of a group to coordinate further research.

In March 1940, hearing from Peter Debye, who had direct knowledge that research on the use of uranium was indeed going on in Germany, Einstein signed a second letter to Roosevelt to convey that information. It, too, produced little action. In fact, the American Administration did not gear up seriously until it received in October 1941 the so-called Maud Committee report, the British account for a way to explore how a bomb could be made.

In early 1945, Leo Szilard got Einstein to write a third letter, simply a letter of introduction to Roosevelt, without Einstein's being told the need for it. Szilard hoped to use this letter to reach Roosevelt with a memorandum of his own, conveying Szilard's doubts of "the wisdom of testing and using bombs." But Roosevelt died before this plea reached him.

Throughout, Einstein himself was carefully shielded from direct knowledge of the Allied project. It even resulted in a moment of comedy. In late December 1941, Vannevar Bush tried to get advice from Einstein on building diffusion plants. But having been given only vague details, Einstein's reply was useless. Bush was asked by the intermediary if one could give Einstein more information. No, he cried, don't tell him one more thing, or he will guess the rest of the project, and might blab. The voluminous files the FBI kept on Einstein show that J. Edgar Hoover was deeply suspicious about Einstein. While on one side of the Atlantic, Heisenberg had once been called a "white Jew," Einstein on the other side was considered a red one.

At any rate, Heisenberg's remarks made in 1955 about Einstein were not to be an isolated exaggeration in that dark story, in which Heisenberg himself had been far more deeply involved. Heisenberg's second, more detailed attack on Einstein came in June 1974, when Heisenberg spoke, of all places, in the so-called Einstein house in Ulm, Germany. He began once more with a generous survey of Einstein's work on relativity, and repeated some of the points made in earlier publications, including an account of Einstein's rejections.

But then Heisenberg said he would have to add something, "in order not to leave the portrait of Einstein all too incomplete." He went on as follows: Einstein "wrote three letters to President Roosevelt, and thereby contributed decisively to setting in motion the atom bomb project in the United States. And he also collaborated actively, on occasion, in the work on this project." [12]

If there is to be some day a play based on the relation between these two men, the playwright will perhaps note that these astonishing exaggerations, uttered in Einstein's birth town, were part of a Heisenberg lecture with the title "Encounters and Conversations with Albert Einstein." Indeed, in that last talk, Heisenberg, two years before his death, had his final encounter with the person whom he had once called his Vorbild, his model; the person who for good and ill had unknowingly been the cause both of deep insights and of fierce insults throughout Heisenberg's scientific and personal life; whose acceptance Heisenberg had sought again and again, and always in vain. Those rejections by Einstein, at each encounter, had left on Heisenberg a bitter scar, which bled once again at that Einstein-House lecture. Niels Bohr, to his death in 1962, was also deeply saddened by Einstein's constant refusal. And as to Einstein himself, he often cursed the quantum he himself had set loose, only to have it haunt him in the form of a physics initiated largely by Bohr and Heisenberg.

It all had started so well. But in that future play, as the curtain falls on these three extraordinary men, even the evil spirit which has been watching them from the wings of the stage, and which had haunted that whole terrible century, will, after all, shed a tear for humanity.

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© Copyright Gerald Holton, 2000

This paper was presented at the Symposium, "Creating Copenhagen," held on 27 March 2000 at the Graduate Center of the City University of New York.

[1]The script of the play has been published: Michael Frayn, Copenhagen (London: Methuen, 1998). A new printing, with a few changes and a much expanded Postscript, is being released in 2000.

[2]Lionel Trilling, The Selected Letters of John Keats (New York: Farrar, Straus and Young, Inc., [date], p.28).

[3]New York: Harcourt, Brace & Co., pp. 102-04.

[4]Published by American Institute of Physics, 1996, and distributed by Springer Verlag (175 Fifth Avenue, New York, NY 10010). Also useful is the documentation in Part II of Paul L. Rose, Heisenberg and the Nazi Bomb Project (Berkeley: University of California Press, 1998).

[5]David C. Cassidy, Uncertainty: The Life and Science of Werner Heisenberg (New York: W. H. Freeman and Co., 1992); Abraham Pais, "Subtle is the Lord...": The Science and Life of Albert Einstein (New York: Oxford University Press, 1982).

[6]Heisenberg, Der Teil und das Ganze (Munich: R. Piper & Co., 1969), p. 49. My translation.

[7]Ibid, p. 67.

[8]Ibid, p. 88.

[9]Ibid., pp. 91-92.

[10] In Universitas, vol X, 1955, no. 9, pp. 897-902; reprinted in his book Across the Frontiers (New York: Harper & Row, 1974), first published as Schritte über Grenzen (Münich: R. Piper & Co., 1971).

[11]Across the Frontiers, p. 6.

[12]W. Heisenberg, Encounters with Einstein (Princeton: Princeton University Press, 1989, p. 120).