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Jarvis323
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As a quarantine hobby I've been learning about Grete Hermann and the early history of QM. I find her early philisophical contribution to be interesting, but I lack the background to put it into a modern context.
A brief description can be found in the arxiv paper: Grete Hermann: An early contributor to quantum theory C. L. Herzenberg.
Here are some relevant parts with the main interest points in bold. What do you think?
A brief description can be found in the arxiv paper: Grete Hermann: An early contributor to quantum theory C. L. Herzenberg.
Here are some relevant parts with the main interest points in bold. What do you think?
The term relational quantum mechanics refers to an interpretation of quantum theory which eliminates the concepts of the absolute state of a system, the absolute value of its physical quantities, or an absolute event. Relational theory describes only the way that systems affect each other in the course of physical interactions, and state and physical quantities always refer to the interaction, or the relation, between systems. However, the theory is regarded as being complete. In this approach, the physical content of quantum theory is be understood as an expression of the entire set of relations connecting all different physical system (SEP).
As noted earlier, Hermann’s point of departure in examining the philosophical foundations of quantum mechanics was from the empirical fact of the unpredictability of precise results in measurements on microphysical objects (Jammer, p. 208). The most apparent way out of such a situation would be by searching for a refinement of the quantum state description in terms of additional parameters (“hidden variables”). But that approach had appeared to be denied by quantum theory, at least according to John von Neumann’s (erroneous) analysis of quantum theory (Jammer, p. 208).
However, as discussed below, Hermann established the presence of a serious error in von Neumann’s supposed proof of the impossibility of hidden variables in quantum mechanics. Since Hermann rejected von Neumann’s supposed proof, she raised the question of what could justify the denial of additional parameters to the theory. Just because an adequate set of such parameters had not yet been developed did not appear to be adequate justification, as it might be said to violate the ‘principle of the incompleteness of experience’ (Jammer, p. 208).
Hermann stated that the only sufficient reason for renouncing as futile any search for the causes of an observed result would be that the causes are already known (Jammer, p. 208). As Max Jammer has pointed out, the dilemma faced in quantum mechanics then was this: Either the theory provides the causes which determine uniquely the outcome of a measurement (in which case, why should we not be able to predict the outcome?) or the theory does not provide such causes (but then how can the possibility of discovering them in the future be categorically denied?). Hermann saw the solution of this dilemma in the relational (or as she called it, the ‘relative’) character of the quantum mechanical description (Jammer, p. 208).
In her analysis, Hermann renounced the classical principle of objectivity. She replaced the idea of objectivity with the concept of dependency on instrumental measurement together with the idea that the physical process leading to a result can be causally reconstructed from the factual result of a measurement of a physical process. By this approach, Hermann explained why the theory prevents predictability without excluding a subsequent or after-the-fact identification of the causes of the particular outcome. Hermann presented a detailed explanation of how this could be achieved by examining in considerable detail a particular case, that of the von Weizsacker-Heisenberg thought experiment, which was an earlier precursor of the better known Einstein-Podolsky-Rosen thought experiment (Jammer, p. 208).
Hermann excluded the possibility of additional parameters on the grounds that quantum mechanics, though predictively indeterministic, could be considered retrodictively a causal theory (Jammer, p. 209). This situation can be rephrased to say that any additional causes or parameters would overdetermine a process and thus lead to a contradiction, since, with the final result of a measurement in sight, the causal sequence that led to the observed result could be reconstructed (Jammer, p. 209). Hermann distinguished between causality and predictability and emphasized the fact that they are not identical; she stated that “The fact that quantum mechanics assumes and pursues a causal account also for unpredictable occurrences proves that an identification of these two concepts is based on a confusion” (Hermann, quoted in Jammer, p. 209; Lenzen). This allows for the possibility that physical processes may be strictly determined even though exact prediction is not possible (Lenzen).
Grete Hermann showed that causality was retained in the sense that after an interaction, causes could be assigned for a particular effect. Von Weizsäcker expressed Hermann’s conclusion by the statement that the persistence of classical laws can be applied to assign causes of past events but not to future events (Lenzen, p. 282).
Heisenberg seems to have approved of Grete Hermann’s resolution of this dilemma; she quoted him as saying to her “That’s it, what we were trying so long to clarify!” (Hermann letter quoted in Jammer, p. 208). However, Hermann’s claim of retrodictive causality has been criticized by several authors, including Jammer, Stauss, and Buchel (Jammer, p. 209). Jammer regards her analysis of the von Weiszäcker-Heisenberg thought experiment as allowing the observed result without requiring it (Jammer, p. 209).
Hermann’s views seem to emphasize the asymmetry between explanation and prediction in quantum mechanics as opposed to their symmetry in classical physics. This analysis was subsequently extended by others, including Norwood Russell Hanson, who appears to have emphasized that after a quantum event has occurred, a complete explanation of its occurrence can be given within the total quantum theory, but that it is in principle impossible to predict in advance those features of the event that can be explained after the fact...