Dispensing with Causality

sight. Planck retained his optimism and called upon the united forces of science and ethics.

I hope that even those of you who have little connection with physics have gained the impression that even a single science, if performed thoroughly and conscientiously, is able to unearth valuable treasures of aesthetical and ethical nature, and moreover, that precisely the deep crises in spiritual culture which we have initially mentioned as our starting point, at the end of the day only serve to prepare the federation into an new and higher union. (Ibid., p. 218)

Also Johannes Stark, who together with Philipp Lenard would become a leader of the Deutsche Physik under the nationalsocialist regime, contributed a booklet on The Present Crisis in German Physics (1922). To him, the crisis consisted in the ‘dogmatic nature’ of all modern physics, be it relativity or quantum theory. Stark found a resolute response in Die Naturwissenschaften penned by von Laue (1923). After rejecting Stark’s dislike of the bulk of modern physics and insisting that there exists no difference in value between pure and applied physics, von Laue concluded.

There is not doubt about the existence of a crisis in physics, and there is no doubt as well that it must be above all accredited to quantum theory. But the crisis is not limited to German science. It manifests itself in the same way in all countries participating in physical research, and it can be overcome only once science succeeds in solving the quantum riddle [Quantenrätsel]. There exists no other remedy. … All in all we wished that the book had remained unwritten, namely, in the interest of science in general, German science in particular, and not the least in the interest of the author. (Laue, 1923, p.

29f.)

As in Planck’s writings, Laue clearly posed the task to end the crisis by finding an appropriate quantum theory. It was Stark who felt an all-embracing sense of crisis, not the quantum physicists. For the quantum generation, Stark and Lenard were hardly a part of the milieu to seek rhetorical and ideological company with.

[Exner] therefore does the best to convince his (lay) readers of the implausibility of the existence of such a causal substratum, switching back and forth between, and largely confounding, the question of the validity of the laws of classical mechanics in the atomic domain and the validity of the principle of causality in the same domain. (Ibid., p. 75)

Yet, on pain of tautology the empiricist has to be specific about the character of causal laws, and entirely unknown causal micro-laws would be just empty gibberish.

Historically, Exner’s Lectures emerged to a substantial part from the courses he had taught at the Philosophical Faculty.²² Forman’s (lay) man on the street, at bottom, seems to be a rigid determinist who rejects to fill out questionnaires from the bureau of statistics.

Influential as Exner’s lectures indeed were, they have in many respects an archaic air. Exner is a curious mixture of the philosophical currents of the two preceding generations, a self-confessed mechanist-materialist yet clearly also a positivist in his view of scientific constructs. (Ibid., p. 75)

I wonder where Exner ever confessed to materialism. To be sure, he was a physicalist of Boltzmann’s breed. Forman’s talk of an ‘archaic air’ might be justified with respect to the poor coverage of Bohr’s theory of the atom despite a lot of spectroscopy; but on the other hand, the Lectures contain an ample discussion of nuclear physics, and in the second edition (1922) there are several pages about relativity theory, a topic not quite popular among German experimentalists.

The quasi-religious conversions to acausality … became a common phenomenon in the German physical community during the summer and fall of 1921. As it swept up in a great awakening, one physicist after the other strode before a general academic audience to renounce the satanic doctrine of causality. (Forman, 1971, p. 80)

There fell Richard von Mises (1922a and 1922b), “the loyal scion of Austrian positivism” (Forman, 1971, p. 80), Walter Schottky (1921), and Walter Nernst (1922).

Forman laments about von Mises’ “‘me too’ tone” (Forman, 1971, p. 82) and Nernst’s

“resolve to sink the law of causality by hook or by crook.” (Ibid., p. 84) I shall analyze the relevant evidence in Section 5.5. Suffice it to note that, strangely enough, Forman hears in all of them the “common theme of ignorabimus.” (Ibid., p. 86) But all three alleged renunciations of causality were motivated by the desire to get around DuBois-Reymond’s ignorabimus and to reject the idea of principal borders of human knowledge which had been a consequence of an untenable mechanist notion of causality, be in a Kantian or in a materialist setting.

The next prey of the milieu was Schrödinger whose 1922 Zurich inaugural address explicitly rehearsed the Exnerian viewpoint, but “by the fall of 1925 Schrödinger had converted back to causality for what were most probably personal-political reasons. He now conceived and developed the wave mechanics as a causal space-time description of atomic processes in opposition to the Copenhagen-Göttingen matrix mechanics.” (Forman, 1971, p. 104) But at least in 1929, Schrödinger, in an exchange with Planck on the occasion of his election as a member of the Prussian Academy of Sciences (Schrödinger, 1929), entertained the same views as in the Zurich speech, which he now sent to Die Naturwissenschaften without modifications (1922a).

Reichenbach fell only in 1925. Once again, the empiricist criterion of meaning allegedly leads straight to the Decline of the West. While in a earlier paper Reichenbach had left the decision about causality to physics, now he held that “even without the hypothesis of rigorous causality it is possible to give a quantitative description of the course of nature which does everything that physics can possibly do.” (Reichenbach, 1925, quoted according to Forman, 1971, p. 89) Thus probability became the most fundamental concept because in this way we need the minimal number of presuppositions. “Which is to say that existentialist philosophy, disguised as logical empiricism, has preempted the decision.” (Ibid., p. 90) Reichenbach was quite active in the Jugendbewegung;²³ maybe he even wore a beard to disguise his existentialism? But suddenly in 1925 he decided to have it shaved off in Occam’s barber shop for a better adaptation to the milieu. More interesting is Forman’s conclusion of all such “quasi-religious experience” because it already opens up the possibility of alternative histories discussed in Section 2.1.

When our converts attempted to demonstrate the necessity for this renunciation of causality, their arguments, as often as not, ought logically to have led to the opposite conclusion. From this I think one must infer that they fully anticipated that any argument advanced by a physicist as a demonstration of the failure of causality would be received by their audience with uncritical approval.

(Forman, 1971, p. 90f.)

While Forman does not offer a glimpse of how these logical conclusions run, he narrative now calls upon the “Unrenegates against the Tide” who stood up to protect the banner of causality. “Among the first … was Mach’s old bulldog, Joseph Petzoldt.” (Ibid., p. 91) Closer scrutiny (Section 5.5.3.) will teach that Petzoldt defended precisely the Machian concept of causality which he, rightly, considered to be reconcilable with quantum mechanics.

Planck himself repeatedly raised his voice in favor of the transcendental character of the law of causality (Planck, 1923, 1929 & 1930) and hoped that quantum mechanics would ultimately return to a causal description. While Forman’s description of Planck’s stand is fairly accurate, I have serious doubts whether “Planck and Einstein were in complete agreement.” (Forman, 1971, p. 94) Einstein’s position in causal matters was rather sophisticated. Independently of Smoluchowski, he had obtained a solution of the problem of Brownian motion, and even before, he openly availed himself of statistical methods for black-body radiation while Planck did do so only stepwise (See Kuhn, 1987, Ch. VII). Moreover, Einstein was seeking, without success though, a unified field theory rather than hoping for a simple return of causality.

Similar motivations might also have guided Weyl²⁴; after all, the issue whether the basic description of nature be continuous (as in field theory) or discrete (as in quantum mechanics) was not just an internal physical question but a philosophical classic of the same rank as causality.

Forman now turns his attention to the situation circa 1924. Without resorting to the milieu as the reason how such a heresy could have been possible, he states that now “the atomic physicists were becoming convinced of the fundamental inadequacy of the extant theory of the atom.” (Ibid., p. 97) It seems that Forman now detects a second adaptation to the milieu, the failure of mechanics, such that “the

23 See the paper of Hans-Ulrich Wipf in (Danneberg/Kamlah/Schäfer, 1994).

24 In private communication, Erhard Scholz has expressed serious doubts about Forman’s account of Weyl.

antimechanical and anticausal movements coalesced, reinforcing one another.” (Ibid., p. 98) Whatever “mechanics” is to mean here – perhaps the strict validity of energy conservation –, the most important result in 1924 was the Bohr-Kramers-Slater (BKS) theory in which energy conservation held only statistically. Of course, it was once again the milieu which guaranteed the theory a “widespread assent” until it was experimentally refuted by the experiments of Geiger and Bothe who already in 1925 showed that energy was conserved in each individual process. And again, Schrödinger’s (1924a) “moral feelings” made him “clutching at it with both hands.”

(Ibid. p. 100) Two years later, he allegedly changed his mind when writing to Wien on 25 August, 1926: “Today I no longer like to assume with Born that an individual event of this kind is “absolutely random”, i.e., completely undetermined. I no longer believe today that there is much to be gained from this conception (which I championed so enthusiastically four years ago).”²⁵ Yet this passage needs more profound interpretation; it will be analyzed in Section 6.3.4.

“Causality’s last stand” was tenaciously defended by Wien who maintained

“that, even when the laws are statistical, causality must reign at the level of the elementary process” and that “physicists will not rest until they have subjected atomic processes to the law of causality.” (All ibid., p. 102f.) “The confidence and corresponding aggressiveness which Wien manifested on the issue of causality in the spring of 1926 derived chiefly from Erwin Schrödinger’s papers on wave mechanics which Wien was then publishing in his journal, the Annalen der Physik.” (Ibid., p 103f.) Unfortunately, the next issue of the Annalen would contain Schrödinger’s (1926b) proof that wave mechanics was equivalent to matrix mechanics. After Born’s statistical interpretation and with the equivalence of wave mechanics and matrix mechanics established, Heisenberg in the spring of 1927 could proclaim victory:

“quantum mechanics definitively establishes the fact that the law of causality is not valid.” (according to Forman, 1971, p. 105)

Here Forman’s story ends. Now there were rational reasons for the dismissal of causality on the basis “of a fundamentally acausal quantum mechanics” (Ibid., p. 110).

Many philosophers subsequently would jump on the band wagon of the new theory, however, they based their “nonsense announced with great fanfare … wholly and solely upon the manifestos against causality issued by physicists before that date.”

(Ibid., p. 111) But whatever stand scientist-philosophers had taken in the causality debate, all of them vigorously criticized these misinterpretations. (See Sections 8.5. &

8.7.).

In the end, Forman devotes himself to the really big picture.

[P]aralleling Ringer’s observation that early in the Weimar period the ‘modernist academics’ tended to be ‘methodologically adventurous’, one finds that, by and large, those physicists who were readiest to repudiate causality had either distinctly ‘progressive’ views by the standards of their social class and the German academic world, and/or had an unusually close interest in, or contact with, modern literature. [The second condition is probably to save the case von Mises.]… On the other hand, with the notable exception of Einstein, those who defended causality tended to be highly principle political conservatives and/or interested in classical literature. … And finally to the causalist camp one may add the outright reactionaries: Ernst Gehrcke, Erwin Lohr, Philipp Lenard, and Johannes Stark. (Ibid., p.

113)

The book’s rather feeble ending, which I have already cited at the beginning of this section, might have been conditioned by the insight that the last series of names poses a substantial danger for the stability of Forman’s whole edifice because they evidence a continuity with the struggles about relativity theory which Forman has to avoid at best to convincingly make his case. Gehrcke, Lenard, and Stark were the main opponents of Einstein’s relativity theory, and of modern ‘abstract’ physics altogether.

Although the clashes around Einstein had already begun before the war (e.g., Gehrcke vs. Born, 1913), they reached the peak right in the years 1920-1922, and were thus in perfect synchrony with the alleged adaptations to acausality. If it were true that the German physicists were more prone to sacrifice the concept of causality familiar from classical physics after they had realized that the familiar concepts of Euclidean space and absolute time were untenable, the adaptation diagnosed by Forman was by far less scandalous. This intimate connection between relativity and atomic theory becomes even closer if one takes into account Forman’s later extension of the adaptation thesis to the notion of Anschaulichkeit because the later stood in the center of the battles between Einstein and Lenard.

Before turning to Anschaulichkeit in Section 1.3 I shall provide some highlights of the debates ensuing from Forman’s original paper. My reading will be somewhat selective in order not to lose sight of this book’s main thread.