Assuming – a big if! – that scientific works of lasting value survive from our era, future historians of science will be confronted with a quandary. Not just that telephone calls, e-mail, social media posts, zoom sessions and so forth represent a less permanent medium than the handwritten letter, people nowadays are not accustomed to pursuing a correspondence and committing one’s thoughts on intellectual matters to writing in this form, as one was formerly. Thus, the gradual unfolding of one’s train of ideas and the circumstances forming the context in which it takes place will be more poorly documented than would have been the case in the past.
Fortunately, we don’t have to worry about the issue in regard to physics during the first half of the twentieth century. The present volume reprints the letters that have survived of the long correspondence between Max Born and Albert Einstein from 1916 to the latter’s death in 1955, along with explanatory commentary by Born. Ever since they met in Berlin, when Einstein had just received a prestigious appointment there and Born was an up-and-coming assistant, the two remained close lifetime friends although they saw each other last in 1933. Their correspondence radiates with human warmth, contains communications between their wives and testifies to Einstein’s respectful friendship with Mrs. Hedi Born, with whom he exchanged letters and poetry. As one could expect, there is a lot on physics, less on politics, a little on philosophy of science and practically nothing on religion or theology.
The interested reader may follow the evolution of the correspondents’ respective views on current subjects such as the status of democracy under the Weimar republic, Communism during the Soviet era, anti-Semitism in academia, the rise of Nazism, Palestine and the Zionist question and, after the second world war, the question of collective responsibility for Nazism on the part of German citizens [see pp. 250, 254, 266-267]. Born, acquainted through his wife with many German Quakers who suffered more under Nazism than his own family had, was inclined to withhold judgment on his fellow Germans while Einstein was not. What looms largest in the first half, up to their emigration from Germany during the early 1930’s, is academic politics, esp. concerning appointments, though there is also some attention to the problem of securing funding and on how to manage under hyperinflation. In later years, the two would fondly recall, for instance, their adventure during the 1919 Berlin revolution, when they intervened with the provisional government in an attempt to protect the university against radical student measures that would have taken away academic freedom—without success, by the way. One notices right away how much less formal university administration was in those days; they were the old boys’ network until their expatriation by the Nazis. There are comments in passing on Wolgang Pauli, Werner Heisenberg and Hermann Weyl (whom Born interacted with as his assistants and later colleagues), among others. In what may come as a surprise, one hears a fair amount about the Hilbert-Brouwer affair [pp. 137-142]. The two pure mathematicians took their clash over philosophy of mathematics quite seriously. The elderly Hilbert was then at the height of his engagement with his formalist program, by which he hoped to provide mathematics with a secure and logically compelling foundation, and saw in L.E.J. Brouwer’s intuitionism a long-term threat to a vital intellectual culture since it would rule out many of the constructions by which he himself had made his name, three decades before. The practical consequence was that Hilbert fired Brouwer from his editorship of the Mathematische Annalen (there were, of course, unavoidable political overtones too, in that Brouwer, though Dutch, tended to support a National-Socialist line in Germany).
After the dismissal of Jewish academics from their posts in Germany from 1932/33 onwards, there is a good amount of back-and-forth on finding suitable positions in western countries for those who were dislocated, young physicists in particular. Einstein himself was secure: on a trip to America, he merely stayed on at the newly founded Institute for Advanced Study in Princeton and, fearing for his safety, never returned to Germany to recover his property before it was seized, while Born wandered about for a while until he was offered a permanent post in Edinburgh, Scotland. But the problem was severe and even with his name recognition Einstein was very limited in what he could accomplish.
One’s main impression upon perusing this epistolary exchange: how catholic their interests were; Born, known as an atomic physicist, discusses the general theory of relativity (on which he gave lectures and published an expository account), celestial mechanics, quantum electrodynamics, condensed matter and even pure mathematics (as an outsider). It is impressive besides that Born was still active in textbook writing during the 1950’s: he published highly regarded treatments of the theory of crystal lattices (1954) and the principles of optics (1959). The physics itself was less technical in those days and one could, if one wished to, try one’s hand at interesting outstanding questions outside one’s specialty.
What the contemporary reader is apt to find of greatest interest in these letters is the decades-long debate between Einstein and Born on the interpretation of quantum mechanics in particular. Therefore, we shall reproduce some of the notable passages at length. Einstein volunteers his opinion to Born early on, in 1926:
Die Quantenmechanik ist sehr achtung-gebietend. Aber eine innere Stimmer sagt mir, daß das doch nicht der wahre Jakob ist. Die Theorie liefert viel, aber dem Geheimnis des Alten bringt sie uns kaum näher. Jedenfalls bin ich überzeugt, daß der nicht würfelt. [pp. 129-130]
In unserer wissenschaftlichen Erwartung haben wir uns zu Antipoden entwickelt. Du glaubst an den würfelnden Gott und ich an volle Gesetzlichkeit in einer Welt von etwas objectiv Seiendem, das ich auf wild spekulativen Wege zu erhaschen suche. Ich glaube fest, aber ich hoffe, daß einer einen mehr realistischen Weg, bezw eine mehr greifbare Unterlage finden wird, als es mir gegeben ist. Der große anfängliche Erfolg der Quantentheorie kann mich doch nicht zum Glauben an des fundamentale Würfelspiel bringen, wenn ich auch wohl weiß, daß die jüngeren Kollegen dies als Folge der Verkalkung auslegen. Einmal wirds sich ja herausstellen, welche instinkive Haltung die richtige gewesen ist. [p. 204]
Meine physikalische Haltung kann ich Dir nicht so begründen, daß Du sie irgendwie vernünftig finden würdest. Ich sehe natürlich ein, daß die prinzipiell statistische Behandlungsweise, deren Notwendigkeit im Rahmen des bestehenden Formalismus ja zuerst von Dir klar erkannt wurde, einen bedeutenden Wahrheitsgehalt hat. Ich kann aber deshalb nicht ernsthaft daran glauben, weil die Theorie mit dem Grundsatz unvereinbar ist, daß die Physik eine Wirklichkeit in Zeit und Raum darstellen soll, ohne Spukhafte Fernwirkungen. Allerdings bin ich nicht fest davon überzeugt, daß es wirklich mit der Theorie eines kontinuierlichen Feldes gemacht werden kann, obwohl ich hierfür eine bisher recht vernünftig erscheinende Möglichkeit gefunden habe. Die rechnerischen Schwierigkeiten sind jedoch so groß, daß ich ins Gras beißen werde, bevor ich selbst eine sichere Überzeugung hierüber erlangt habe. Aber davon bin ich fest überzeugt, daß man schließlich bei einer Theorie landen wird, deren gesetzmäßig verbundene Dinge nicht Wahrscheinlichkeiten sondern gedachte Tatbestände sind, wie man es bis vor kurzem als selbstverständlich betrachte hat. Zur Begründung dieser Überzeugung kann ich aber nicht logische Gründe, sondern nur meinen kleinen Finger als Zeugen beibringen, also keine Autorität, die außerhalb meiner Hand irgendwelchen Respekt einflößen kann. [p. 215]
Aside from general deterministic philosophicial grounds, the reason why Einstein insists on locality in quantum mechanics seems to be a residue of his work on the special theory of relativity from which he could not free himself:
Aber das, was wir als existierend (›wirklich‹) denken, soll irgenwie seit-räumlich loaklisiert sein. D.h. das Reale in einem Raumteil A soll (in der Theorie) irgendwie unabhängig ›existieren‹ von dem, was in einem anderen Raumteil B als real gedacht wird. Wenn ein physikalisches System sich über die Raumteile A und B erstreckt, so soll das in B Vorhandene eine irgendwie von dem in A vorhandenen unabhängige Existenz haben. Das wirklich in B Vorhandene soll also nicht davon abhängen, was für eine Messung im Raumteil A ausgeführt wird; es soll auch unabhängig davon sein, ob in A überhaupt eine Messung gemacht wird oder nicht. [p. 223]
To which Born aptly responds,
Der entscheidene Satz ist dann der, wo er sagt, »daß die Physik eine Wirklichkeit in Zeit und Raum darstellen soll, ohne Spukhafte Fernwirkungen«. Auch ich hatte dies für ein Postulat gehalten, das unbedingte Gültigkeit beanspruchen darf. Aber die Tatsachen der physikalischen Erfahrung haben mich belehrt, daß auch diese Forderung kein Prinzip a prior ist, sondern eine zeitgebundene Regel, die durch eine allgemeinere ersetzt worden muß und kann. [p. 217]
In a mini-essay by Einstein [pp. 229-234] he seems to forget Planck’s quantum of action and, in regard to localizability, to demand too much of physical theory: why couldn’t there be subtle correlations embedded at long distances since we presume the two now spatially separated bodies were once in causal interaction with each other? (As Born points out in his reply pp. 235-236.)
Why Born doesn’t expect quantum mechanics to be replaced the way Einstein does:
Was nun meine Erwartung eines »besseren« betrifft, so bin ich allerdings ganz andrer Meinung als Du. Der Fortschritt in der Physik ist doch immer eindeutig vom Anschaulichen zum Abstrakten gegangen. So wird es wohl auch bleiben. Die Quanten-Mechanik und Quanten-Feldtheorien versagen in entscheidenden Punkten. Aber alle Anzeichen scheinen mir darauf hinzudeuten, daß man sich auf Dinge gefaßt machen muß, die uns älteren Leuten nicht behagen. [p. 237]
Einstein condensed his incredulity towards the completeness of quantum mechanics into a thought-experiment [pp. 252-253], to which Born replies that Einstein’s views on quantum mechanics are inconsistent with what he did in the general theory of relativity [see esp. p. 300 for why]!
Born on Schrödinger, de Broglie, Bohm [pp. 270-271]. Einstein’s verdict on Bohmian quantum mechanics: ‘Der Weg scheint mir zu billig’ [p. 258], despite that, superficially, it would seem to adhere to Einstein’s deterministic spirit – the old master’s dislike reflects, no doubt, the fact that it is such an ad hoc kludge cobbled together out of disparate elements without being illuminated by any satisfying general principles. If it’s so great why haven’t the Bohmians been able to solve the measurement problem in 70 years?
Lastly on pp. 286-287 we get the clearest statement yet by Einstein of his point of view re. quantum mechanics. At this juncture, the exasperated Born decided to pull in Pauli for a judgment, who was then Einstein’s assistant in Princeton and would be able to discuss the matter with him in person. Pauli’s letter to Born, reproduced here, adjudicates the dispute with his trademark critical perspicacity [pp. 289-291 and 293-297], and largely sides with Born – who, in his commentary, reflects on why he and Einstein were talking past each other. In no way did their divergence of views on the scientific question impair their friendship, however.
One wavers between three and four stars. The scientific discussion itself ends up being less trenchant than one might have hoped, for the most part. But, on the other hand, the warmth of their friendship stands out on every page. Let them have the benefit of the doubt! Read this correspondence more to find out what a scientific career was like in those turbulent days than to acquire knowledge of physics or philosophy; for this turn instead to reviews by historians of science who take the trouble to distill stray remarks scattered across decades of letters into coherent positions – unless one be attracted by the prospect of working with the primary sources!