It is argued that the conclusions obtained by Renninger (Zeitschrift fur Physik 136, 251 (1953)), by means of an interferometer thought experiment, have important implications for a number of still ongoing discussions about quantum mechanics (QM). To these belong the ontology underlying QM, Bohr's complementarity principle, the significance of QM's wave function, the "elements of reality" introduced by Einstein, Podolsky and Rosen (EPR), and Bohm's version of QM (BQM). A slightly extended setup is used to make a physical prediction at variance with the mathematical prediction of QM. An english translation of Renninger's paper, which was originally published in german language, follows the present paper. This should facilitate access to that remarkable, apparently overlooked and forgotten, paper.
It is argued that the conclusions obtained by Renninger (Zeitschrift fur Physik 136, 251 (1953)), by means of an interferometer thought experiment, have important implications for a number of still ongoing discussions about quantum mechanics (QM). To these belong the ontology underlying QM, Bohr's complementarity principle, the significance of QM's wave function, the "elements of reality" introduced by Einstein, Podolsky and Rosen (EPR), and Bohm's version of QM (BQM). A slightly extended setup is used to make a physical prediction at variance with the mathematical prediction of QM. An english translation of Renninger's paper, which was originally published in german language, follows the present paper. This should facilitate access to that remarkable, apparently overlooked and forgotten, paper.
The article considers the implications of the experiment of A. Einstein, B. Podolsky, and N. Rosen (EPR), and of the exchange (concerning this experiment) between EPR and Bohr concerning the incompleteness, or else nonlocality, of quantum mechanics for our understanding of quantum phenomena and quantum probability. The article specifically argues that in the case of quantum phenomena, including those involved in the experiments of the EPR type, the probabilistic considerations are important even when the predictions concerned can be made with certainty, due to the impossibility, in general, to repeat any given quantum experiment with the same outcome. The article argue that this fact, not properly considered or taken into account by EPR, makes it difficult and ultimately impossible to sustain their argument, which it is consistent with Bohr’s counterargument to EPR and with his view of quantum phenomena and quantum mechanics.
The article offers a rereading of Bohr’s reply to the argument of A. Einstein, B. Podolsky, and N. Rosen (EPR) concerning the incompleteness, or else nonlocality, of quantum mechanics. Bohr shows EPR’s argument to be deficient on their own terms, by virtue of an ambiguity found in their application of the criterion of reality they propose to the phenomena in question. He also offers an alternative interpretation of the EPR experiment itself, the thought experiment proposed by EPR, as part of his overall interpretation of quantum mechanics as complementarity.
The Einstein-Podolsky-Rosen argument for the incompleteness of quantum mechanics involves two assumptions: one about locality and the other about when it is legitimate to infer the existence of an element-of-reality. Using one simple thought experiment, we argue that quantum predictions and the relativity of simultaneity require that both these assumptions fail, whether or not quantum mechanics is complete. EPR’s (1935) argument for the incompleteness of QM turns upon the following sufficient condition: “If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity.“ EPR apply this condition in a case where knowledge gained from measuring one quantum system allows one to predict with certainty a value for a physical quantity on another.
This article explores Bohr's argument, advanced under the heading of 'complementarity,' concerning quantum phenomena and quantum mechanics, and its physical and philosophical implications.In Bohr, the term complementarity designates both a particular concept and an overall interpretation of quantum phenomena and quantum mechanics, in part grounded in this concept.While the argument of this article is primarily philosophical, it will also address, historically, the development and transformations of Bohr's thinking, under the impact of the development of quantum theory and Bohr's confrontation with Einstein, especially their exchange concerning the EPR experiment, proposed by Einstein, Podolsky and Rosen in 1935.Bohr's interpretation was progressively characterized by a more radical epistemology, in its ultimate form, which was developed in the 1930s and with which I shall be especially concerned here, defined by his new concepts of phenomenon and atomicity.According to this epistemology, quantum objects are seen as indescribable and possibly even as inconceivable, and as manifesting their existence only in the effects of their interactions with measuring instruments upon those instruments, effects that define phenomena in Bohr's sense.The absence of causality is an automatic consequence of this epistemology.I shall also consider how probability and statistics work under these epistemological conditions.
This article reconsiders the double-slit experiment by establishing a new type of relationship between it and the concept of entanglement. While the role of entanglement in the double-slit experiment has been considered, this particular relationship appears to have been missed in preceding discussions of the experiment, even by Bohr, who extensively used it to support his argument concerning quantum physics. The main reason for this relationship is the different roles of the diaphragm with slits in two setups, S1 and S2, defining the double-slit experiment as a quantum experiment. In S1, in each individual run of the experiment one can in principle (even if not actually) know throughout which slit the quantum object considered has passed; in S2 this knowledge is in principle impossible, which impossibility is coextensive with the appearance of the interference pattern, once a sufficient number of individual runs of the experiment have taken place. The article offers the following argument based on two new concepts, an "experimentally quantum object" and an "ontologically quantum object." In S1 the diaphragm can be treated as part of an observational arrangement and thus considered as a classical object, while the object passing through one or the other slit is considered as an "ontologically quantum object," defined as an object necessary to establish a quantum phenomenon. By contrast, in S2, the diaphragm can, via the concept of Heisenberg-von-Neumann cut, be treated as an "experimentally quantum object," defined as an object treatable by quantum theory, even while possibly being an ontologically classical object. This interaction is not an observation but a quantum entanglement between these two quantum objects, one ontologically and one experimentally quantum. This argument is grounded in a particular interpretation of quantum phenomena and quantum theory, which belongs to the class of interpretations designated here as "reality without realism" (RWR) interpretations. The article also argues that wave-particle complementarity, with which the concept of complementarity is often associated, plays little, if any, role in quantum physics, or in Bohr's thinking, and may be misleading in considering the double-slit experiment, often explained by using this complementarity.
Quantum mechanics is one of our most successful physical theories; its predictions agree with experimental observations to an extremely high accuracy. However, the bare formalism of quantum theory does not provide straightforward answers to seemingly simple questions: for example, how should one model systems that include agents who are themselves using quantum theory? These foundational questions may be investigated with a theorist's tool – the thought experiment. Its purpose is to turn debates about the interpretation of quantum theory into actual physics questions. In this article, we give a state-of-the-art overview on quantum thought experiments involving observers, from the basic Wigner's friend to the recent Frauchiger-Renner setup, clarifying their interpretational significance and responding to objections and criticism on the way.