The observer effect Massimiliano Sassoli de Bianchi Laboratorio di Autoricerca di Base, 6914 Carona, Switzerland * (Dated: June 19, 2012) Founding our analysis on the Geneva-Brussels approach to the foundations of physics, we provide a clarification and classification of the key concept of observation. An entity can be observed with or without a scope. In the second case, the observation is a purely non-invasive discovery process; in the first case, it is a purely invasive process, which can involve either creation or destruction as- pects. An entity can also be observed with or without a full control over the observational process. In the latter case, the observation can be described by a symmetry breaking mechanism, through which a specific deterministic observational process is selected among a number of potential ones, as explained in Aerts’ hidden measurement approach. This is what is called a product test, or product observation, whose consequences are that outcomes can only be predicted in probabilistic terms, as it is the case in typical quantum measurements. We also show that observations can be about in- trinsic (stable) properties of the observed entity, or about relational (ephemeral) properties between the observer and observed entities; also, they can be about intermediate properties, neither purely classical, nor purely quantum. Our analysis allows us to propose a general conceptual characteri- zation of quantum measurements, as observational processes involving three aspects: (1) product observations, (2) pure creation aspects and (3) ephemeral relational properties. We also discuss the important concept of non-spatiality and emphasize some of the differences and similarities between quantum and classical/relativistic observations. I. PREMISE The purpose of the present article is to provide a clarification and classification of the concept of obser- vation, as used in physical sciences. To do so, we shall make extensive use of the findings of the Geneva-Brussels school in the foundations of physics, which counts within its founders Josef Maria Jauch, Constantin Piron and Diederik Aerts (see for instance Refs. 1–10 and the refer- ences cited therein). Therefore, as a whole, the present essay can also be considered as a non-technical review of some of the results and intuitions developed in the last decades by this school, and more particularly by Diederik Aerts, in his creation-discovery view [6, 7] and hidden measurement approach [7, 11]. However, in the present article we shall make some di- dactical choices that slightly differ from those employed by Aerts and collaborators. The most important one is about the concept of observation, which is the cen- tral theme of the present work. In the ambit of the creation-discovery view, it is usually stated that quan- tum measurements are not just observations, as they can provoke a real change of the state of the measured entity. In the following, we shall turn such a statement upside down and adopt an opposite semantic view: observations are not only detections of what pre-existed in a physical system, but processes that in general can also provoke changes, and this remains true also outside of the mi- croscopic domain. In other terms, our didactical point will be to show that observation is a much wider concept as is usually understood, which also includes the idea of * Electronic address: autoricerca@gmail.com transformation. Because of the nature of this essay, which on one hand goes over some (not sufficiently well) known results of the Geneva-Brussels school (and particularly those of Piron and Aerts) and on the other hand presents these same results with sometimes a slightly different perspective, it is clearly not very practical to always distinguish in the text the pieces of reasoning which are literally borrowed from Piron, Aerts and collaborators, and those instead that, although inspired by those reasoning, are presented here in a slightly different form or perspective, without compromising the readability and flow of the text. Therefore, we encourage the reader who will be fasci- nated by the ideas presented in this work, to go back to the original sources, to which we will often refer to in the text, to also appreciate the different expositional style, didactical choices, and pieces of explanations that were provided by their creators. But there are also – we believe – a few interesting orig- inal ideas and examples in our analysis, which as far as we can judge have not yet been made fully explicit by the above authors, like for instance our characterization of quantum measurements as observations of truly rela- tional properties between the observer and the observed entities, as a simple explanation for their ephemeral na- ture. That said, we might add that if we had had the chance to have access to the conceptual clarification presented in this article, when as young students we approached for the first time quantum physics, surely we would have found the content of this theory far less mysterious, and its conceptual understanding much less problematic. arXiv:1109.3536v3 [quant-ph] 18 Jun 2012