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 (Jammer, p. 209).
Von Weizsäcker has described Grete Hermann’s contributions as including clarifying that
the impossibility of making certain predictions is not based on the fact that a causal chain
investigated turns out to be interrupted somewhere, but rather on the fact that the
different causal chains cannot be organized to form a unified picture embracing all
aspects of the process, so that it remains to the choice of the observer which of the
different causal chains is realized (Mehra, Vol. 6, part 2, p. 713).
Here is Grete Hermann’s phrasing of it: “The difficulties in which the partisans of
causality are placed by the discoveries of quantum mechanics, seem...not to arise from
the causality principle itself. They rather emerge from the tacit assumption connected
with it that the physical cognition grasps natural phenomena adequately and
independently of the observational connection. This assumption is expressed in the
prerequisite that every causal connection between processes yields a calculable action
due to the cause, even more, that the causal connection is identical with the possibility of
such a calculation.” She added: “Quantum mechanics forces us to dissolve this mixing of
different principles of natural philosophy, to drop the assumption of the absolute
character of the cognition of nature, and to use the causal principle independently of the
11
latter. By no means has it disproved causal law, but it has clarified its status and freed it
from other principles which must not be combined with it necessarily.” (quoted in Mehra,
Vol. 6, part 2, p. 713