5.1 JUMPING OVER ONTOLOGY: THE COPENHAGEN SETTLEMENT
The founding fathers of quantum mechanics did not read Einstein’s paper like we did, that is as a summons to take a position concerning the manner of being of a constituent of matter. For Bohr, the paper was simply an attack upon the foundations of the still uncertain theory he was putting together, an attack which had to therefore be repulsed at all costs. As Leon Rosenfeld recalled:
The EPR paper came down on us as a bolt from the blue. Its effect upon Bohr was remarkable. As soon as he heard my report of Einstein’s argument, everything else was abandoned: we had to clear up such a misunderstanding at once.
Reported in Jeremy Bernstein Quantum Profiles
As described in Chapter 3, Einstein’s argument consisted of two parts: the first part asserted that the state of a given system should not be allowed to depend on observations performed upon another system which was spatially separated from it; the second part argued that the theory must be incomplete because it assigned different wave functions to a given system depending on which measurement one chose to undertake upon another system which had interacted with it in the past but which was by now beyond physical communication with it. To Bohr, the second part of the argument represented the most immediate threat because it suggested that the theoretical description of the state of a quantum system would be ambiguous. He therefore simply passed over the former part of the argument and focused his attention upon the latter. He argued that, while different measurements performed upon the second system do indeed assign different wave functions to the first, there is no ambiguity involved because they do not do so simultaneously; each measurement we choose to make on the second system fundamentally differs from all the others we may care to undertake by setting up “different conditions for defining the possible types of predictions which regard the subsequent behavior of the first system”. In other words the state of a system does not objectively reside in that system but is provided to it by the measurement process, even if that process involved another, spatially separated, system. However, by thus rejecting Einstein’s insistence that two interacting systems which have been separated from each other beyond the possibility of further physical interaction should not be allowed to influence each other, Bohr is in fact rejecting the ontological consideration which, as we have argued above, was hidden behind that insistence. By not asking about the manner of being of an object which, unlike ordinary objects of experience, is devoid of any physical states prior to observing them and only acquires such states through observation, Bohr effectively jumped over the ontological issue. For him there was no ontological question to be answered.
Following his lead, the founding fathers accepted neither the assertion that their theory was an incomplete description of an object of experience, nor the suggestion that their theory described an object whose ontology should be explicitly explored. Instead, they took that part of the ontological position which asserted that the theory was complete, and added to it that part of the epistemological position which asserted that the objects described by the theory were objects of experience, and combined them into one. Since, however, they also held that the physical states of a quantum object are not objectively possessed by that object but are rather created for it through measurement, the resulting Copenhagen settlement carried within it a fundamental ontological contradiction: constituents of matter were both objectively real and not objectively real, where real is here to be taken in the sense in which Einstein used it as “something that is considered to be independent of its being observed”. In other words, they could be subjected to ordinary measurements seeking to ascertain their pre-existing physical properties despite the fact that those properties did not pre-exist the act of measurement. This useful ambiguity was noticed by Einstein in his letter to Popper:
The question may be asked whether, from the point of view of today’s quantum theory, the statistical character of our experimental findings is merely the result of interfering with a system from without, which comprises measuring it, while the systems in themselves—described by a ψ-function—behave in a deterministic fashion. Heisenberg flirts with this interpretation without adopting it consistently.
That the founding fathers could credibly maintain such an ontologically precarious position, and thus drive Einstein into eventual silence, testifies to the existence of that “highest form of musicality in the sphere of thought” which Einstein had recognized in their work before. Specifically, they allowed for the possibility that the constituents of mater might perhaps be ontologically different from ordinary objects of experience, but pointed out that it was not possible to tell whether that was true or not because one could not approach a quantum object with any measuring apparatus available in order to ask about its pre-observational state without thus uncontrollably disturbing it. This position was implemented by posting a gatekeeper at the entrance to the theory. It consisted of two assertions: one forbade anything which could not be ascertained by direct ordinary measurement from entering the theory; the other imposed limits upon the accuracy with which direct ordinary measurement could determine the physical properties of a constituent of matter, a limit which was reflected in Heisenberg’s uncertainty relations. Put behind this gatekeeper construct, the ontology of a constituent of matter thus remained safely unexplored. As Feynman wrote in 1965:
The uncertainty principle ‘protects’ quantum mechanics. Heisenberg recognized that if it were possible to measure the momentum and the position simultaneously with a greater accuracy, the quantum mechanics would collapse. The kind of relation given by Heisenberg must be true in order to keep the theory from getting into trouble.
Richard Feynman, Lectures on Physics, Volume III
The scientific community therefore misunderstood the ontological question as a demand for replacing the classical theory, with which they had been unsuccessfully trying to explain the results of atomic experience, with a new theory. If the electrons produced by them as particles scattered off a nickel crystal as von Laue waves, the classical theory they had developed to explain mechanical experience would break down when applied to electrons and a new mechanical theory would then have to be created to include them as well. Similarly, if heat radiation produced by them as electromagnetic waves displayed spectral properties characteristic of a collection of discrete energy quanta, the classical theory they had developed to explain optical experience would break down when applied to radiant heat and a new optical theory would then have to be created.
Under the circumstances, the theoretical difficulties the community encountered at the beginning of the 1900s appeared to them to be a crisis of the scientific project itself, a crisis that could only be overcome by appropriate theoretical modification. Failure to realize that they were in fact dealing primarily with an ontological turn, not a theoretical crisis, has thus driven physicists to connecting the theories associated with the scientific project before and after the turn to each other as if the later, quantum theory, was in fact a correction of the former, classical one, a correction designed to fix the former’s failure to explain when applied to a predicated measurement but which would coincide with the former when applied to an ordinary measurement. In fact, quantum theory was a program designed to jump over the ontological difference.